NAME
sh —
command interpreter (shell)
SYNOPSIS
sh |
[-abCEeFfhIiLmnpquVvXx]
[+abCEeFfhIiLmnpquVvXx]
[-o option_name]
[+o option_name]
[command_file [argument ...]] |
sh |
-c [-s]
[-abCEeFfhIiLmnpquVvXx]
[+abCEeFfhIiLmnpquVvXx]
[-o option_name]
[+o option_name]
command_string
[command_name [argument ...]] |
sh |
-s
[-abCEeFfhIiLmnpquVvXx]
[+abCEeFfhIiLmnpquVvXx]
[-o option_name]
[+o option_name]
[argument ...] |
DESCRIPTION
sh is the standard command interpreter for the system. The
current version of
sh is in the process of being changed to
conform more closely to the POSIX 1003.2 and 1003.2a specifications for the
shell. This version has many features which make it appear similar in some
respects to the Korn shell, but it is not a Korn shell clone (see
ksh(1)). This man page is not
intended to be a tutorial or a complete specification of the shell.
Overview
The shell is a command that reads lines from either a file or the terminal,
interprets them, and generally executes other commands. A shell is the program
that is running when a user logs into the system. (Users can select which
shell is executed for them at login with the
chsh(1) command). The shell
implements a language that has flow control constructs, a macro facility that
provides a variety of features in addition to data storage, along with built
in history and line editing capabilities. It incorporates many features to aid
interactive use and has the advantage that the interpretative language is
common to both interactive and non-interactive use (shell scripts). That is,
commands can be typed directly to the running shell or can be put into a file
and the file can be executed directly by the shell.
Invocation
If no arguments are present and if the standard input, and standard error
output, of the shell are connected to a terminal (or terminals, or if the
-i flag is set), and the
-c option is not
present, the shell is considered an interactive shell. An interactive shell
generally prompts before each command and handles programming and command
errors differently (as described below). When first starting, the shell
inspects argument 0, and if it begins with a dash ‘-’, the shell
is also considered a login shell. This is normally done automatically by the
system when the user first logs in. A login shell first reads commands from
the files
/etc/profile and
.profile if
they exist. If the environment variable
ENV
is set on
entry to a shell, or is set in the
.profile of a login
shell, and either the shell is interactive, or the
posix
option is not set, the shell next reads commands from the file named in
ENV
. Therefore, a user should place commands that are
to be executed only at login time in the
.profile file, and
commands that are executed for every shell inside the
ENV
file. To set the
ENV
variable to some file, place the following line in your
.profile of your home directory
ENV=$HOME/.shinit; export ENV
substituting for “.shinit” any filename you wish. Since the
ENV
file can be read for every invocation of the
shell, including shell scripts and non-interactive shells, the following
paradigm is useful for restricting commands in the
ENV
file to interactive invocations. Place commands within the
“
case” and “
esac”
below (these commands are described later):
case $- in
*i*)
#
commands for interactive use only
...
esac
If command line arguments besides the options have been specified, and neither
-c nor
-s was given, then the shell treats
the first argument as the name of a file from which to read commands (a shell
script). This also becomes $0 and the remaining arguments are set as the
positional parameters of the shell ($1, $2, etc). Otherwise, if
-c was given, then the first argument, which must exist, is
taken to be a string of
sh commands to execute. Then if any
additional arguments follow the command string, those arguments become $0, $1,
... Otherwise, if additional arguments were given (which implies that
-s was set) those arguments become $1, $2, ... If $0 has not
been set by the preceding processing, it will be set to argv[0] as passed to
the shell, which will usually be the name of the shell itself. If
-s was given, or if neither
-c nor any
additional (non-option) arguments were present, the shell reads commands from
its standard input.
Argument List Processing
Currently, all of the single letter options that can meaningfully be set using
the
set built-in, have a corresponding name that can be used
as an argument to the
-o option. The set
-o name is provided next to the single letter option in the
description below. Some options have only a long name, they are described
after the flag options, they are used with
-o or
+o only, either on the command line, or with the
set built-in command. Other options described are for the
command line only. Specifying a dash “-” turns the option on,
while using a plus “+” disables the option. The following options
can be set from the command line and, unless otherwise stated, with the
set built-in (described later).
-
-
- -a
allexport
- Automatically export any variable to which a value is
assigned while this flag is set.
-
-
- -b
notify
- Enable asynchronous notification of background job
completion. (Not implemented.)
-
-
- -C
noclobber
- Don't overwrite existing files with
“>”.
-
-
- -c
- Read commands from the command_string
operand instead of, or in addition to, from the standard input. Special
parameter 0 will be set from the command_name
operand if given, and the positional parameters ($1, $2, etc.) set from
the remaining argument operands, if any. -c is only
available at invocation, it cannot be set, and there is
no form using “+”.
-
-
- -E
emacs
- Enable the built-in emacs style command line editor
(disables -V if it has been set). (See the
Command Line Editing
section below.)
-
-
- -e
errexit
- If not interactive, exit immediately if any untested
command fails. If interactive, and an untested command fails, cease all
processing of the current command and return to prompt for a new command.
The exit status of a command is considered to be explicitly tested if the
command is used to control an if,
elif, while, or
until, or if the command is the left hand operand of an
“&&” or “||” operator, or if it is a
pipeline (or simple command) preceded by the “!” operator.
With pipelines, only the status of the entire pipeline (indicated by the
last command it contains) is tested when -e is set to
determine if the shell should exit.
-
-
- -F
fork
- Cause the shell to always use
fork(2) instead of attempting
vfork(2) when it needs to
create a new process. This should normally have no visible effect, but can
slow execution. The sh can be compiled to always use
fork(2) in which case altering
the -F flag has no effect.
-
-
- -f
noglob
- Disable pathname expansion.
-
-
- -h
trackall
- Functions defined while this option is set will have paths
bound to commands to be executed by the function at the time of the
definition. When off when a function is defined, the file system is
searched for commands each time the function is invoked. (Not
implemented.)
-
-
- -I
ignoreeof
- Ignore EOFs from input when interactive. (After a large
number of consecutive EOFs the shell will exit anyway.)
-
-
- -i
interactive
- Force the shell to behave interactively.
-
-
- -L
local_lineno
- When set, before a function is defined, causes the variable
LINENO
when used within the function, to refer to
the line number defined such that first line of the function is line 1.
When reset, LINENO
in a function refers to the
line number within the file within which the definition of the function
occurs. This option defaults to “on” in this shell. For more
details see the section LINENO
below.
-
-
- -m
monitor
- Turn on job control (set automatically when
interactive).
-
-
- -n
noexec
- Read and parse commands, but do not execute them. This is
useful for checking the syntax of shell scripts. If -n
becomes set in an interactive shell, it will automatically be cleared just
before the next time the command line prompt (
PS1
)
is written.
-
-
- -p
nopriv
- Do not attempt to reset effective UID if it does not match
UID. This is not set by default to help avoid incorrect usage by setuid
root programs via system(3)
or popen(3).
-
-
- -q
quietprofile
- If the -v or -x options
have been set, do not apply them when reading initialization files, these
being /etc/profile, .profile, and the
file specified by the
ENV
environment
variable.
-
-
- -s
stdin
- Read commands from standard input (set automatically if
neither -c nor file arguments are present). If after
processing a command_string with the -c option, the
shell has not exited, and the -s option is set, it will
continue reading more commands from standard input. This option has no
effect when set or reset after the shell has already started reading from
the command_file, or from standard input. Note that the
-s flag being set does not cause the shell to be
interactive.
-
-
- -u
nounset
- Write a message to standard error when attempting to obtain
a value from a variable that is not set, and if the shell is not
interactive, exit immediately. For interactive shells, instead return
immediately to the command prompt and read the next command. Note that
expansions (described later, see
Word Expansions below) using the
‘+’, ‘-’, ‘=’, or ‘?’
operators test if the variable is set, before attempting to obtain its
value, and hence are unaffected by -u.
-
-
- -V
vi
- Enable the built-in
vi(1) command line editor
(disables -E if it has been set). (See the
Command Line Editing
section below.)
-
-
- -v
verbose
- The shell writes its input to standard error as it is read.
Useful for debugging.
-
-
- -X
Xtrace
- Cause output from the xtrace
(-x) option to be sent to standard error as it exists
when the -X option is enabled (regardless of its
previous state.) For example:
will arrange for tracing output to be sent to the file named, instead of
wherever it was previously being sent, until the X option is set again, or
cleared.
Each change (set or clear) to -X is also performed upon
-x, but not the converse.
-
-
- -x
xtrace
- Write each command to standard error (preceded by the
expanded value of “$PS4”) before it is executed. Unless
-X is set, “standard error” means that which
existed immediately before any redirections to be applied to the command
are performed. Useful for debugging.
-
-
-
cdprint
- Make an interactive shell always print the new directory
name when changed by the cd command. In a
non-interactive shell this option has no effect.
-
-
-
nolog
- Prevent the entry of function definitions into the command
history (see fc in the
Built-ins section.) (Not
implemented.)
-
-
-
pipefail
- If set, the way the exit status of a pipeline is determined
is altered. See Pipelines below for
the details.
-
-
-
posix
- Enables closer adherence to the POSIX shell standard. This
option will default set at shell startup if the environment variable
POSIXLY_CORRECT
is present. That can be overridden
(set or reset) by the -o option on the command line.
Currently this option controls whether (!posix) or not (posix) the file
given by the ENV
variable is read at startup by a
non-interactive shell. It also controls whether file descriptors greater
than 2 opened using the exec built-in command are passed
on to utilities executed (“yes” in posix mode), whether a
colon (:) terminates the user name in tilde (~) expansions other than in
assignment statements (“no” in posix mode), and whether the
shell treats an empty brace-list compound statement as a syntax error
(expected by POSIX) or permits it. Such statements “{ }” can
be useful when defining dummy functions. Lastly, in posix mode, only one
“!” is permitted before a pipeline.
-
-
-
promptcmds
- Allows command substitutions (as well as parameter and
arithmetic expansions, which are always performed) upon the prompt strings
PS1
, PS2
, and
PS4
each time, before they are output. This option
should not be set until after the prompts have been set (or verified) to
avoid accidentally importing unwanted command substitutions from the
environment.
-
-
-
tabcomplete
- Enables filename completion in the command line editor.
Typing a tab character will extend the current input word to match a
filename. If more than one filename matches it is only extended to be the
common prefix. Typing a second tab character will list all the matching
names. One of the editing modes, either -E or
-V, must be enabled for this to work.
Lexical Structure
The shell reads input in terms of lines from a file and breaks it up into words
at whitespace (blanks and tabs), and at certain sequences of characters that
are special to the shell called “operators”. There are two types
of operators: control operators and redirection operators (their meaning is
discussed later). The following is a list of operators:
- Control operators:
-
& && ( ) ; ;; ;& | ||
<newline>
- Redirection operators:
-
< > >| << >> <&
>& <<- <>
Quoting
Quoting is used to remove the special meaning of certain characters or words to
the shell, such as operators, whitespace, or keywords. There are four types of
quoting: matched single quotes, matched double quotes, backslash, and dollar
preceding matched single quotes (enhanced C style strings.)
Backslash
An unquoted backslash preserves the literal meaning of the following character,
with the exception of ⟨newline⟩. An unquoted backslash preceding
a ⟨newline⟩ is treated as a line continuation, the two
characters are simply removed.
Single Quotes
Enclosing characters in single quotes preserves the literal meaning of all the
characters (except single quotes, making it impossible to put single quotes in
a single-quoted string).
Double Quotes
Enclosing characters within double quotes preserves the literal meaning of all
characters except dollar sign ($), backquote (`), and backslash (\). The
backslash inside double quotes is historically weird, and serves to quote only
the following characters (and these not in all contexts):
$ ` " \ <newline>,
where a backslash newline is a line continuation as above. Otherwise it remains
literal.
Dollar Single Quotes
($'...')
Note: this form of
quoting is still somewhat experimental, and yet to be included in the POSIX
standard. This implementation is based upon the current proposals for
standardization, and is subject to change should the eventual adopted text
differ.
Enclosing characters in a matched pair of single quotes, with the first
immediately preceded by an unquoted dollar sign ($) provides a quoting
mechanism similar to single quotes, except that within the sequence of
characters, any backslash (\), is an escape character, which causes the
following character to be treated specially. Only a subset of the characters
that can occur in the string are defined after a backslash, others are
reserved for future definition, and currently generate a syntax error if used.
The escape sequences are modeled after the similar sequences in strings in the
C programming language, with some extensions.
The following characters are treated literally when following the escape
character (backslash):
\ ' "
The sequence “\\” allows the escape character (backslash) to appear
in the string literally. “\'” allows a single quote character into
the string, such an escaped single quote does not terminate the quoted string.
“\"” is for compatibility with C strings, the double quote
has no special meaning in a shell C-style string, and does not need to be
escaped, but may be.
A newline following the escape character is treated as a line continuation, like
the same sequence in a double quoted string, or when not quoted – the
two characters, escape and newline, are removed from the input string.
The following characters, when escaped, are converted in a manner similar to the
way they would be in a string in the C language:
a b e f n r t v
An escaped ‘a’ generates an alert (or ‘BEL’) character,
that is, control-G, or 0x07. In a similar way, ‘b’ is backspace
(0x08), ‘e’ (an extension to C) is escape (0x1B), ‘f’
is form feed (0x0C), ‘n’ is newline (or line feed, 0x0A),
‘r’ is return (0x0D), ‘t’ is horizontal tab (0x09),
and ‘v’ is vertical tab (0x13).
In addition to those there are 5 forms that need additional data, which is
obtained from the subsequent characters. An escape (\) followed by one, two or
three, octal digits (‘0’..‘7’) is processed to form an
8 bit character value. If only one or two digits are present, the following
character must be something other than an octal digit. It is safest to always
use all 3 digits, with leading zeros if needed. If all three digits are
present, the first must be one of ‘0’..‘3’.
An escape followed by ‘x’ (lower case only) can be followed by one
or two hexadecimal digits (‘0’..‘9’,
‘A’..‘F’, or ‘a’..‘f’.) As
with octal, if only one hex digit is present, the following character must be
something other than a hex digit, so always giving 2 hex digits is best.
However, unlike octal, it is unspecified in the standard how many hex digits
can be consumed. This
sh takes at most two, but other shells
will continue consuming characters as long as they remain valid hex digits.
Consequently, users should ensure that the character following the hex escape
sequence is something other than a hex digit. One way to achieve this is to
end the $'...' string immediately after the final hex digit, and then,
immediately start another, so
$'\x33'$'4...'
always gives the character with value 0x33 (‘3’), followed by the
character ‘4’, whereas
$'\x334'
in some other shells would be the hex value 0x334 (10, or more, bits).
There are two escape sequences beginning with ‘\u’ or
‘\U’. The former is followed by from 1 to 4 hex digits, the latter
by from 1 to 8 hex digits. Leading zeros can be used to pad the sequences to
the maximum permitted length, to avoid any possible ambiguity problem with the
following character, and because there are some shells that insist on exactly
4 (or 8) hex digits. These sequences are evaluated to form the value of a
Unicode code point, which is then encoded into UTF-8 form, and entered into
the string. (The code point should be converted to the appropriate code point
value for the corresponding character in the character set given by the
current locale, or perhaps the locale in use when the shell was started, but
is not... currently.) Not all values that are possible to write are valid,
values that specify (known) invalid Unicode code points will be rejected, or
simply produce ‘?’.
Lastly, as another addition to what is available in C, the escape character
(backslash), followed by ‘c’ (lower case only) followed by one
additional character, which must be an alphabetic character (a letter), or one
of the following:
@ [ \ ] ^ _ ?
Other than ‘\c?’ the value obtained is the least significant 5 bits
of the ASCII value of the character following the ‘\c’ escape
sequence. That is what is commonly known as the “control”
character obtained from the given character. The escape sequence
‘\c?’ yields the ASCII DEL character (0x7F). Note that to obtain
the ASCII FS character (0x1C) this way, (that is control-\) the trailing
‘\’ must be escaped itself, and so for this one case, the full
escape sequence is “\c\\”. The sequence “\c\X” where
‘X’ is some character other than ‘\’ is reserved for
future use, its meaning is unspecified. In this
sh an error
is generated.
If any of the preceding escape sequences generate the value ‘\0’ (a
NUL character) that character, and all that follow in the same $'...' string,
are omitted from the resulting word.
After the $'...' string has had any included escape sequences converted, it is
treated as if it had been a single quoted string.
Reserved Words
Reserved words are words that have special meaning to the shell and are
recognized at the beginning of a line and after a control operator. The
following are reserved words:
! |
{ |
} |
case |
do |
done |
elif |
else |
esac |
fi |
for |
if |
in |
then |
until |
while |
Their meanings are discussed later.
Aliases
An alias is a name and corresponding value set using the
alias
built-in command. Whenever a reserved word (see above) may occur, and after
checking for reserved words, the shell checks the word to see if it matches an
alias. If it does, it replaces it in the input stream with its value. For
example, if there is an alias called “lf” with the value “ls
-F”, then the input:
lf foobar ⟨return⟩
would become
ls -F foobar
⟨return⟩
Aliases provide a convenient way for naive users to create shorthands for
commands without having to learn how to create functions with arguments. They
can also be used to create lexically obscure code. This use is strongly
discouraged.
Commands
The shell interprets the words it reads according to a language, the
specification of which is outside the scope of this man page (refer to the BNF
in the POSIX 1003.2 document). Essentially though, a line is read and if the
first word of the line (or after a control operator) is not a reserved word,
then the shell has recognized a simple command. Otherwise, a complex command
or some other special construct may have been recognized.
Simple Commands
If a simple command has been recognized, the shell performs the following
actions:
- Leading words of the form “name=value” are
stripped off, the value is expanded, as described below, and the results
are assigned to the environment of the simple command. Redirection
operators and their arguments (as described below) are stripped off and
saved for processing in step 3 below.
- The remaining words are expanded as described in the
Word Expansions section below.
The first remaining word is considered the command name and the command is
located. Any remaining words are considered the arguments of the command.
If no command name resulted, then the “name=value” variable
assignments recognized in item 1 affect the current shell.
- Redirections are performed, from first to last, in the
order given, as described in the next section.
Redirections
Redirections are used to change where a command reads its input or sends its
output. In general, redirections open, close, or duplicate an existing
reference to a file. The overall format used for redirection is:
[n]redir-op
file
where
redir-op is one of the redirection operators
mentioned previously. The following is a list of the possible redirections.
The [n] is an optional number, as in ‘3’ (not ‘[3]’),
that refers to a file descriptor. If present it must occur immediately before
the redirection operator, with no intervening white space, and becomes a part
of that operator.
-
-
- [n]>
file
- Redirect standard output (or n) to
file.
-
-
- [n]>|
file
- The same, but override the -C
option.
-
-
- [n]>>
file
- Append standard output (or n) to
file.
-
-
- [n]<
file
- Redirect standard input (or n) from
file.
-
-
- [n1]<&n2
- Duplicate standard input (or n1) from
file descriptor n2. n2 is
expanded if not a digit string, the result must be a number.
-
-
- [n]<&-
- Close standard input (or n).
-
-
- [n1]>&n2
- Duplicate standard output (or n1) to
n2.
-
-
- [n]>&-
- Close standard output (or n).
-
-
- [n]<>
file
- Open file for reading and writing on
standard input (or n).
The following redirection is often called a “here-document”.
[n]<< delimiter
here-doc-text ...
delimiter
The “here-doc-text” starts immediately after the next unquoted
newline character following the here-doc redirection operator. If there is
more than one here-document redirection on the same line, then the text for
the first (from left to right) is read first, and subsequent here-doc-text for
later here-doc redirections follows immediately after, until all such
redirections have been processed.
All the text on successive lines up to the delimiter, which must appear on a
line by itself, with nothing other than an immediately following newline, is
saved away and made available to the command on standard input, or file
descriptor n if it is specified. If the delimiter as specified on the initial
line is quoted, then the here-doc-text is treated literally; otherwise, the
text is treated much like a double quoted string, except that
‘"’ characters have no special meaning, and are not escaped
by ‘\’, and is subjected to parameter expansion, command
substitution, and arithmetic expansion as described in the
Word Expansions section below. If
the operator is “<<−” instead of
“<<”, then leading tabs in all lines in the here-doc-text,
including before the end delimiter, are stripped. If the delimiter is not
quoted, lines in here-doc-text that end with an unquoted \ are joined to the
following line, the \ and following newline are simply removed while reading
the here-doc, which thus guarantees that neither of those lines can be the end
delimiter.
It is a syntax error for the end of the input file (or string) to be reached
before the delimiter is encountered.
Search and Execution
There are three types of commands: shell functions, built-in commands, and
normal programs -- and the command is searched for (by name) in that order. A
command that contains a slash ‘/’ in its name is always a normal
program. They each are executed in a different way.
When a shell function is executed, all of the shell positional parameters
(except $0, which remains unchanged) are set to the arguments of the shell
function. The variables which are explicitly placed in the environment of the
command (by placing assignments to them before the function name) are made
local to the function and are set to the values given, and exported for the
benefit of programs executed with the function. Then the command given in the
function definition is executed. The positional parameters, and local
variables, are restored to their original values when the command completes.
This all occurs within the current shell, and the function can alter
variables, or other settings, of the shell.
Shell built-ins are executed internally to the shell, without spawning a new
process.
Otherwise, if the command name doesn't match a function or built-in, the command
is searched for as a normal program in the file system (as described in the
next section). When a normal program is executed, the shell runs the program,
passing the arguments and the environment to the program. If the program is
not a normal executable file, and if it does not begin with the "magic
number" whose ASCII representation is "#!", so
execve(2) returns
ENOEXEC
then) the shell will interpret the program in
a sub-shell. The child shell will reinitialize itself in this case, so that
the effect will be as if a new shell had been invoked to handle the ad-hoc
shell script, except that the location of hashed commands located in the
parent shell will be remembered by the child.
Note that previous versions of this document and the source code itself
misleadingly and sporadically refer to a shell script without a magic number
as a "shell procedure".
Path Search
When locating a command, the shell first looks to see if it has a shell function
by that name. Then it looks for a built-in command by that name. If a built-in
command is not found, one of two things happen:
- Command names containing a slash are simply executed
without performing any searches.
- Otherwise, the shell searches each entry in
PATH
in turn for the command. The value of the
PATH
variable should be a series of entries
separated by colons. Each entry consists of a directory name. The current
directory may be indicated implicitly by an empty directory name, or
explicitly by a single period. If a directory searched contains an
executable file with the same name as the command given, the search
terminates, and that program is executed.
Command Exit Status
Each command has an exit status that can influence the behavior of other shell
commands. The paradigm is that a command exits with zero in normal cases, or
to indicate success, and non-zero for failure, error, or a false indication.
The man page for each command should indicate the various exit codes and what
they mean. Additionally, the built-in commands return exit codes, as does an
executed shell function.
If a command consists entirely of variable assignments then the exit status of
the command is that of the last command substitution if any, otherwise 0.
If redirections are present, and any fail to be correctly performed, any command
present is not executed, and an exit status of 2 is returned.
Complex Commands
Complex commands are combinations of simple commands with control operators or
reserved words, together creating a larger complex command. Overall, a shell
program is a:
-
-
- list
- Which is a sequence of one or more AND-OR lists.
-
-
- AND-OR list
- is a sequence of one or more pipelines.
-
-
- pipeline
- is a sequence of one or more commands.
-
-
- command
- is one of a simple command, a compound command, or a
function definition.
-
-
- simple command
- has been explained above, and is the basic building
block.
-
-
- compound command
- provides mechanisms to group lists to achieve different
effects.
-
-
- function definition
- allows new simple commands to be created as groupings of
existing commands.
Unless otherwise stated, the exit status of a list is that of the last simple
command executed by the list.
Pipelines
A pipeline is a sequence of one or more commands separated by the control
operator ‘|’, and optionally preceded by the “!”
reserved word. Note that ‘|’ is an operator, and so is recognized
anywhere it appears unquoted, it does not require surrounding white space or
other syntax elements. On the other hand “!” being a reserved
word, must be separated from adjacent words by white space (or other
operators, perhaps redirects) and is only recognized as the reserved word when
it appears in a command word position (such as at the beginning of a
pipeline.)
The standard output of all but the last command in the sequence is connected to
the standard input of the next command. The standard output of the last
command is inherited from the shell, as usual, as is the standard input of the
first command.
The format for a pipeline is:
[!] command1 [| command2 ...]
The standard output of command1 is connected to the standard input of command2.
The standard input, standard output, or both of each command is considered to
be assigned by the pipeline before any redirection specified by redirection
operators that are part of the command are performed.
If the pipeline is not in the background (discussed later), the shell waits for
all commands to complete.
The commands in a pipeline can either be simple commands, or one of the compound
commands described below. The simplest case of a pipeline is a single simple
command.
If the
pipefail option is set when the pipeline completes and
its status is collected, the pipeline status is the status of the last
(rightmost) command in the pipeline to exit with non-zero exit status, or
zero, if, and only if, all commands in the pipeline exited with a status of
zero. If the
pipefail option is not set, which is the
default state, the pipeline status is the exit status of the last command in
the pipeline, and the exit status of any other commands in the pipeline is
ignored.
If the reserved word ! precedes the pipeline, the exit status becomes the
logical NOT of the pipeline status as determined above. That is, if the
pipeline status is zero, the exit status is 1; if the pipeline status is other
than zero, the exit status is zero. If there is no ! reserved word, the
pipeline status becomes the exit status.
Because pipeline assignment of standard input or standard output or both takes
place before redirection, it can be modified by redirection. For example:
$ command1 2>&1 | command2
sends both the standard output and standard error of command1 to the standard
input of command2.
Note that unlike some other shells, each process in the pipeline is a child of
the invoking shell (unless it is a shell built-in, in which case it executes
in the current shell -- but any effect it has on the environment is wiped).
A pipeline is a simple case of an AND-OR-list (described below.) A ; or
⟨newline⟩ terminator causes the preceding pipeline, or more
generally, the preceding AND-OR-list to be executed sequentially; that is, the
shell executes the commands, and waits for them to finish before proceeding to
following commands. An & terminator causes asynchronous (background)
execution of the preceding AND-OR-list (see the next paragraph below). The
exit status of an asynchronous AND-OR-list is zero. The actual status of the
commands, after they have completed, can be obtained using the
wait built-in command described later.
Background Commands --
&
If a command, pipeline, or AND-OR-list is terminated by the control operator
ampersand (&), the shell executes the command asynchronously -- that is,
the shell does not wait for the command to finish before executing the next
command.
The format for running a command in background is:
command1 & [command2 &
...]
If the shell is not interactive, the standard input of an asynchronous command
is set to
/dev/null. The process identifier of the most
recent command started in the background can be obtained from the value of the
special parameter “!” (see
Special Parameters) provided it
is accessed before the next asynchronous command is started.
Lists -- Generally Speaking
A list is a sequence of one or more commands separated by newlines, semicolons,
or ampersands, and optionally terminated by one of these three characters. A
shell program, which includes the commands given to an interactive shell, is a
list. Each command in such a list is executed when it is fully parsed. Another
use of a list is as a complete-command, which is parsed in its entirety, and
then later the commands in the list are executed only if there were no parsing
errors.
The commands in a list are executed in the order they are written. If command is
followed by an ampersand, the shell starts the command and immediately
proceeds to the next command; otherwise it waits for the command to terminate
before proceeding to the next one. A newline is equivalent to a
‘;’ when no other operator is present, and the command being input
could syntactically correctly be terminated at the point where the newline is
encountered, otherwise it is just whitespace.
AND-OR Lists
(Short-Circuit List Operators)
“&&” and “||” are AND-OR list operators. After
executing the commands that precede the “&&” the
subsequent command is executed if and only if the exit status of the preceding
command(s) is zero. “||” is similar, but executes the subsequent
command if and only if the exit status of the preceding command is nonzero. If
a command is not executed, the exit status remains unchanged and the following
AND-OR list operator (if any) uses that status. “&&” and
“||” both have the same priority. Note that these operators are
left-associative, so
true || echo bar && echo
baz
writes “baz” and nothing else. This is not the way it works in C.
Flow-Control
Constructs -- if, while, until, for, case
These commands are instances of compound commands. The syntax of the
if command is
if list
then list
[ elif list
then list ] ...
[ else list ]
fi
The first list is executed, and if the exit status of that list is zero, the
list following the
then is executed. Otherwise the list
after an
elif (if any) is executed and the process repeats.
When no more
elif reserved words, and accompanying lists,
appear, the list after the
else reserved word, if any, is
executed.
The syntax of the
while command is
The two lists are executed repeatedly while the exit status of the first list is
zero. The
until command is similar, but has the word
until in place of
while, which causes it
to repeat until the exit status of the first list is zero.
The syntax of the
for command is
for variable [ in word ... ]
do list
done
The words are expanded, or "$@" if “in” (and the following
words) is not present, and then the list is executed repeatedly with the
variable set to each word in turn. If “in” appears after the
variable, but no words are present, the list is not executed, and the exit
status is zero.
do and
done may be
replaced with ‘
{’ and
‘
}’, but doing so is non-standard and not
recommended.
The syntax of the
break and
continue
commands is
break [ num ]
continue [ num ]
break terminates the
num innermost
for,
while, or
until
loops.
continue breaks execution of the
num-1 innermost
for,
while, or
until loops, and then continues
with the next iteration of the enclosing loop. These are implemented as
special built-in commands. The parameter
num, if given,
must be an unsigned positive integer (greater than zero). If not given, 1 is
used.
The syntax of the
case command is
case word in
[(] pattern ) [ list ] ;&
[(] pattern ) [ list ] ;;
...
esac
The pattern can actually be one or more patterns (see
Shell Patterns described later),
separated by “|” characters.
Word is expanded and matched against each pattern in turn, from first to last,
with each pattern being expanded just before the match is attempted. When a
match is found, pattern comparisons cease, and the associated
“list”, if given, is evaluated. If the list is terminated with
“;&” execution then falls through to the following list, if
any, without evaluating its pattern, or attempting a match. When a list
terminated with “;;” has been executed, or when
esac is reached, execution of the
case
statement is complete. The exit status is that of the last command executed
from the last list evaluated, if any, or zero otherwise.
Grouping Commands Together
Commands may be grouped by writing either
(list)
or
{ list; }
These also form compound commands.
Note that while parentheses are operators, and do not require any extra syntax,
braces are reserved words, so the opening brace must be followed by white
space (or some other operator), and the closing brace must occur in a position
where a new command word might otherwise appear.
The first of these executes the commands in a sub-shell. Built-in commands
grouped into a (list) will not affect the current shell. The second form does
not fork another shell so is slightly more efficient, and allows for commands
which do affect the current shell. Grouping commands together this way allows
you to redirect their output as though they were one program:
{ echo -n "hello " ; echo "world" ; } > greeting
Note that “}” must follow a control operator (here, “;”)
so that it is recognized as a reserved word and not as another command
argument.
Functions
The syntax of a function definition is
name () command [redirect...]
A function definition is an executable statement; when executed it installs a
function named name and returns an exit status of zero. The command is
normally a list enclosed between “{” and “}”. The
standard syntax also allows the command to be any of the other compound
commands, including a sub-shell, all of which are supported. As an extension,
this shell also allows a simple command (or even another function definition)
to be used, though users should be aware this is non-standard syntax. This
means that
l() ls $@
works to make “l” an alternative name for the
ls
command.
If the optional redirect, (see
Redirections), which may be of any of
the normal forms, is given, it is applied each time the function is called.
This means that a simple “Hello World” function might be written
(in the extended syntax) as:
hello() cat <<EOF
Hello World!
EOF
To be correctly standards conforming this should be re-written as:
hello() { cat; } <<EOF
Hello World!
EOF
Note the distinction between those forms, and
hello() { cat <<EOF
Hello World!
EOF
}
which reads and processes the
here document each time the
shell executes the function, and which applies that input only to the cat
command, not to any other commands that might appear in the function.
Variables may be declared to be local to a function by using the
local command. This should usually appear as the first
statement of a function, though is an executable command which can be used
anywhere in a function. See
Built-ins
below for its definition.
The function completes after having executed
command with
exit status set to the status returned by
command. If
command is a compound-command it can use the
return command (see
Built-ins below) to finish before
completing all of
command.
Variables and Parameters
The shell maintains a set of parameters. A parameter denoted by a name is called
a variable. When starting up, the shell turns all the environment variables
into shell variables, and exports them. New variables can be set using the
form
name=value
Variables set by the user must have a name consisting solely of alphabetics,
numerics, and underscores - the first of which must not be numeric. A
parameter can also be denoted by a number or a special character as explained
below.
Positional Parameters
A positional parameter is a parameter denoted by a number (n > 0). The shell
sets these initially to the values of its command line arguments that follow
the name of the shell script. The
set built-in can also be
used to set or reset them, and
shift can be used to
manipulate the list.
To refer to the 10th (and later) positional parameters, the form ${n} must be
used. Without the braces, a digit following “$” can only refer to
one of the first 9 positional parameters, or the special parameter
“0”. The word “$10” is treated identically to
“${1}0”.
Special Parameters
A special parameter is a parameter denoted by one of the following special
characters. The value of the parameter is listed next to its character.
-
-
- *
- Expands to the positional parameters, starting from one.
When the expansion occurs within a double-quoted string it expands to a
single field with the value of each parameter separated by the first
character of the
IFS
variable, or by a
⟨space⟩ if IFS
is unset.
-
-
- @
- Expands to the positional parameters, starting from one.
When the expansion occurs within double quotes, each positional parameter
expands as a separate argument. If there are no positional parameters, the
expansion of @ generates zero arguments, even when @ is double-quoted.
What this basically means, for example, is if $1 is “abc” and
$2 is “def ghi”, then “$@” expands to the two
arguments:
"abc" "def ghi"
-
-
- #
- Expands to the number of positional parameters.
-
-
- ?
- Expands to the exit status of the most recent
pipeline.
-
-
- - (Hyphen, or minus.)
- Expands to the current option flags (the single-letter
option names concatenated into a string) as specified on invocation, by
the set built-in command, or implicitly by the shell.
-
-
- $
- Expands to the process ID of the invoked shell. A sub-shell
retains the same value of $ as its parent.
-
-
- !
- Expands to the process ID of the most recent background
command executed from the current shell. For a pipeline, the process ID is
that of the last command in the pipeline. If no background commands have
yet been started by the shell, then “!” will be unset. Once
set, the value of “!” will be retained until another
background command is started.
-
-
- 0 (Zero.)
- Expands to the name of the shell or shell script.
Word Expansions
This section describes the various expansions that are performed on words. Not
all expansions are performed on every word, as explained later.
Tilde expansions, parameter expansions, command substitutions, arithmetic
expansions, and quote removals that occur within a single word expand to a
single field. It is only field splitting or pathname expansion that can create
multiple fields from a single word. The single exception to this rule is the
expansion of the special parameter @ within double quotes, as was described
above.
The order of word expansion is:
- Tilde Expansion, Parameter Expansion, Command
Substitution, Arithmetic Expansion (these all occur at the same
time).
- Field Splitting is performed on fields generated by step
(1) unless the
IFS
variable is null.
- Pathname Expansion (unless set -f is in
effect).
- Quote Removal.
The $ character is used to introduce parameter expansion, command substitution,
or arithmetic evaluation.
Tilde
Expansion (substituting a user's home directory)
A word beginning with an unquoted tilde character (~) is subjected to tilde
expansion. Provided all of the subsequent characters in the word are unquoted
up to an unquoted slash (/) or when in an assignment or not in posix mode, an
unquoted colon (:), or if neither of those appear, the end of the word, they
are treated as a user name and are replaced with the pathname of the named
user's home directory. If the user name is missing (as in
~/foobar), the tilde is replaced with the value of the
HOME variable (the current user's home directory).
In variable assignments, an unquoted tilde immediately after the assignment
operator (=), and each unquoted tilde immediately after an unquoted colon in
the value to be assigned is also subject to tilde expansion as just stated.
Parameter Expansion
The format for parameter expansion is as follows:
${expression}
where expression consists of all characters until the matching “}”.
Any “}” escaped by a backslash or within a quoted string, and
characters in embedded arithmetic expansions, command substitutions, and
variable expansions, are not examined in determining the matching
“}”.
The simplest form for parameter expansion is:
${parameter}
The value, if any, of parameter is substituted.
The parameter name or symbol can be enclosed in braces, which are optional in
this simple case, except for positional parameters with more than one digit or
when parameter is followed by a character that could be interpreted as part of
the name. If a parameter expansion occurs inside double quotes:
- Pathname expansion is not performed on the results of the
expansion.
- Field splitting is not performed on the results of the
expansion, with the exception of the special rules for @.
In addition, a parameter expansion where braces are used, can be modified by
using one of the following formats. If the
“
:
” is omitted in the following modifiers,
then the test in the expansion applies only to unset parameters, not null
ones.
-
-
- ${parameter:−word}
- Use Default Values. If parameter is unset or null, the
expansion of word is substituted; otherwise, the value of parameter is
substituted.
-
-
- ${parameter:=word}
- Assign Default Values. If parameter is unset or null, the
expansion of word is assigned to parameter. In all cases, the final value
of parameter is substituted. Only variables, not positional parameters or
special parameters, can be assigned in this way.
-
-
- ${parameter:?[word]}
- Indicate Error if Null or Unset. If parameter is unset or
null, the expansion of word (or a message indicating it is unset if word
is omitted) is written to standard error and a non-interactive shell exits
with a nonzero exit status. An interactive shell will not exit, but any
associated command(s) will not be executed. If the parameter is set, its
value is substituted.
-
-
- ${parameter:+word}
- Use Alternative Value. If parameter is unset or null, null
is substituted; otherwise, the expansion of word is substituted. The value
of parameter is not used in this expansion.
-
-
- ${#parameter}
- String Length. The length in characters of the value of
parameter.
The following four varieties of parameter expansion provide for substring
processing. In each case, pattern matching notation (see
Shell Patterns), rather than regular
expression notation, is used to evaluate the patterns. If parameter is * or @,
the result of the expansion is unspecified. Enclosing the full parameter
expansion string in double quotes does not cause the following four varieties
of pattern characters to be quoted, whereas quoting characters within the
braces has this effect.
-
-
- ${parameter%word}
- Remove Smallest Suffix Pattern. The word is expanded to
produce a pattern. The parameter expansion then results in parameter, with
the smallest portion of the suffix matched by the pattern deleted. If the
word is to start with a ‘%’ character, it must be quoted.
-
-
- ${parameter%%word}
- Remove Largest Suffix Pattern. The word is expanded to
produce a pattern. The parameter expansion then results in parameter, with
the largest portion of the suffix matched by the pattern deleted. The
“%%” pattern operator only produces different results from the
“%” operator when the pattern contains at least one unquoted
‘*’.
-
-
- ${parameter#word}
- Remove Smallest Prefix Pattern. The word is expanded to
produce a pattern. The parameter expansion then results in parameter, with
the smallest portion of the prefix matched by the pattern deleted. If the
word is to start with a ‘#’ character, it must be quoted.
-
-
- ${parameter##word}
- Remove Largest Prefix Pattern. The word is expanded to
produce a pattern. The parameter expansion then results in parameter, with
the largest portion of the prefix matched by the pattern deleted. This has
the same relationship with the ‘#’ pattern operator as
“%%” has with “%”.
Command Substitution
Command substitution allows the output of a command to be substituted in place
of the command (and surrounding syntax). Command substitution occurs when the
command is enclosed as follows:
$(command)
or (“backquoted” version):
`command`
The shell expands the command substitution by executing command in a sub-shell
environment and replacing the command substitution with the standard output of
the command, removing sequences of one or more ⟨newline⟩s at the
end of the substitution. (Embedded ⟨newline⟩s before the end of
the output are not removed; however, during field splitting, they may be
translated into ⟨space⟩s, depending on the value of
IFS
and any quoting that is in effect.)
Arithmetic Expansion
Arithmetic expansion provides a mechanism for evaluating an arithmetic
expression and substituting its value. The format for arithmetic expansion is
as follows:
$((expression))
The expression in an arithmetic expansion is treated as if it were in double
quotes, except that a double quote character inside the expression is just a
normal character (it quotes nothing.) The shell expands all tokens in the
expression for parameter expansion, command substitution, and quote removal
(the only quoting character is the backslash ‘\’, and only when
followed by another ‘\’, a dollar sign ‘$’, a
backquote ‘`’ or a newline.)
Next, the shell evaluates the expanded result as an arithmetic expression and
substitutes the calculated value of that expression.
Arithmetic expressions use a syntax similar to that of the C language, and are
evaluated using the ‘
intmax_t
’ data type
(this is an extension to POSIX, which requires only
‘
long
’ arithmetic.) Shell variables may be
referenced by name inside an arithmetic expression, without needing a
“$” sign. Variables that are not set, or which have an empty (null
string) value, used this way evaluate as zero (that is, “x” in
arithmetic, as an R-Value, is evaluated as “${x:-0}”) unless the
sh -u flag is set, in which case a
reference to an unset variable is an error. Note that unset variables used in
the ${var} form expand to a null string, which might result in syntax errors.
Referencing the value of a variable which is not numeric is an error.
All of the C expression operators applicable to integers are supported, and
operate as they would in a C expression. Use white space, or parentheses, to
disambiguate confusing syntax, otherwise, as in C, the longest sequence of
consecutive characters which make a valid token (operator, variable name, or
number) is taken to be that token, even if the token designated cannot be used
and a different interpretation could produce a successful parse. This means,
as an example, that “a+++++b” is parsed as the gibberish sequence
“a ++ ++ + b”, rather than as the valid alternative “a ++ +
++ b”. Similarly, separate the ‘,’ operator from numbers
with white space to avoid the possibility of confusion with the decimal
indicator in some locales (though fractional, or floating-point, numbers are
not supported in this implementation.)
It should not be necessary to state that the C operators which operate on, or
produce, pointer types, are not supported. Those include unary “*”
and “&” and the struct and array referencing binary operators:
“.”, “->” and “[”.
White Space
Splitting (Field Splitting)
After parameter expansion, command substitution, and arithmetic expansion the
shell scans the results of expansions and substitutions that did not occur in
double quotes, and “$@” even if it did, for field splitting and
multiple fields can result.
The shell treats each character of the
IFS
as a
delimiter and uses the delimiters to split the results of parameter expansion
and command substitution into fields.
Non-whitespace characters in
IFS
are treated strictly as
parameter separators. So adjacent non-whitespace
IFS
characters will produce empty parameters. On the other hand, any sequence of
whitespace characters that occur in
IFS
(known as
IFS
whitespace) can occur, leading and trailing
IFS
whitespace, and any
IFS
whitespace surrounding a non whitespace
IFS
delimiter,
is removed. Any sequence of
IFS
whitespace characters
without a non-whitespace
IFS
delimiter acts as a
single field separator.
If
IFS
is unset it is assumed to contain space, tab, and
newline, all of which are
IFS
whitespace characters.
If
IFS
is set to a null string, there are no
delimiters, and no field splitting occurs.
Pathname Expansion
(File Name Generation)
Unless the
-f flag is set, file name generation is performed
after word splitting is complete. Each word is viewed as a series of patterns,
separated by slashes. The process of expansion replaces the word with the
names of all existing files whose names can be formed by replacing each
pattern with a string that matches the specified pattern. There are two
restrictions on this: first, a pattern cannot match a string containing a
slash, and second, a pattern cannot match a string starting with a period
unless the first character of the pattern is a period. The next section
describes the patterns used for both Pathname Expansion and the
case command.
Shell Patterns
A pattern consists of normal characters, which match themselves, and
meta-characters. The meta-characters are “!”, “*”,
“?”, and “[”. These characters lose their special
meanings if they are quoted. When command or variable substitution is
performed and the dollar sign or backquotes are not double-quoted, the value
of the variable or the output of the command is scanned for these characters
and they are turned into meta-characters.
An asterisk (“*”) matches any string of characters. A question mark
(“?”) matches any single character. A left bracket
(“[”) introduces a character class. The end of the character class
is indicated by a right bracket (“]”); if this “]” is
missing then the “[” matches a “[” rather than
introducing a character class. A character class matches any of the characters
between the square brackets. A named class of characters (see
wctype(3)) may be specified by
surrounding the name with (“[:”) and (“:]”). For
example, (“[[:alpha:]]”) is a shell pattern that matches a single
letter. A range of characters may be specified using a minus sign
(“−”). The character class may be complemented by making an
exclamation mark (“!”) the first character of the character class.
To include a “]” in a character class, make it the first character
listed (after the “!”, if any). To include a
“−”, make it the first (after !) or last character listed.
If both “]” and “−” are to be included, the
“]” must be first (after !) and the “−” last, in
the character class.
Built-ins
This section lists the built-in commands which are built-in because they need to
perform some operation that can't be performed by a separate process. Or just
because they traditionally are. In addition to these, there are several other
commands that may be built in for efficiency (e.g.
printf(1),
echo(1),
test(1), etc).
-
-
- : [arg
...]
- A null command that returns a 0 (true) exit value. Any
arguments or redirects are evaluated, then ignored.
-
-
- . file
- The dot command reads and executes the commands from the
specified file in the current shell environment. The
file does not need to be executable and is looked up from the directories
listed in the
PATH
variable if its name does not
contain a directory separator (‘/’). The return command (see
Built-ins below) can be used for a
premature return from the sourced file.
The POSIX standard has been unclear on how loop control keywords (break and
continue) behave across a dot command boundary. This implementation allows
them to control loops surrounding the dot command, but obviously such
behavior should not be relied on. It is now permitted by the standard, but
not required.
-
-
- alias
[name[=string
...]]
- If name=string is specified, the
shell defines the alias name with value
string. If just name is
specified, the value of the alias name is printed.
With no arguments, the alias built-in prints the names
and values of all defined aliases (see unalias).
-
-
- bg
[job] ...
- Continue the specified jobs (or the current job if no jobs
are given) in the background.
-
-
- command
[-p]
[-v]
[-V] command
[arg ...]
- Execute the specified command but ignore shell functions
when searching for it. (This is useful when you have a shell function with
the same name as a command.)
-
-
- -p
- search for command using a
PATH
that guarantees to find all the standard utilities.
-
-
- -V
- Do not execute the command but search for the command
and print the resolution of the command search. This is the same as
the type built-in.
-
-
- -v
- Do not execute the command but search for the command
and print the absolute pathname of utilities, the name for built-ins
or the expansion of aliases.
-
-
- cd
[-P]
[directory
[replace]]
- Switch to the specified directory (default
$HOME
). If replace is
specified, then the new directory name is generated by replacing the first
occurrence of the string directory in the current
directory name with replace. Otherwise if
directory is ‘-’, then the current
working directory is changed to the previous current working directory as
set in OLDPWD
. Otherwise if an entry for
CDPATH
appears in the environment of the
cd command or the shell variable
CDPATH
is set and the directory name does not
begin with a slash, and its first (or only) component isn't dot or dot
dot, then the directories listed in CDPATH
will be
searched for the specified directory. The format of
CDPATH
is the same as that of
PATH
.
The -P option instructs the shell to update
PWD
with the specified physical directory path and
change to that directory. This is the default.
When the directory changes, the variable OLDPWD
is
set to the working directory before the change.
Some shells also support a -L option, which instructs the
shell to update PWD
with the logical path and to
change the current directory accordingly. This is not supported.
In an interactive shell, the cd command will print out the
name of the directory that it actually switched to if this is different
from the name that the user gave, or always if the
cdprint option is set. The destination may be different
either because the CDPATH
mechanism was used or if
the replace argument was used.
-
-
- eval string
...
- Concatenate all the arguments with spaces. Then re-parse
and execute the command.
-
-
- exec
[command arg ...]
- Unless command is omitted, the shell process is replaced
with the specified program (which must be a real program, not a shell
built-in or function). Any redirections on the exec
command are marked as permanent, so that they are not undone when the
exec command finishes. When the posix
option is not set, file descriptors created via such redirections are
marked close-on-exec (see
open(2)
O_CLOEXEC
or
fcntl(2)
F_SETFD /
FD_CLOEXEC
),
unless the descriptors they point to refer to the standard input, output,
or error (file descriptors 0, 1, 2). Traditionally Bourne-like shells
(except ksh(1)), made those
file descriptors available to exec'ed processes. This behavior is required
by the POSIX standard, so when the posix option is set,
this shell also acts that way. To be assured the close-on-exec setting is
off, redirect the descriptor to (or from) itself, either when invoking a
command for which the descriptor is wanted open, or by using
exec (perhaps the same exec as opened
it, after the open) to leave the descriptor open in the shell and pass it
to all commands invoked subsequently. Alternatively, see the
fdflags command below, which can set, or clear, this,
and other, file descriptor flags.
-
-
- exit
[exitstatus]
- Terminate the shell process. If
exitstatus is given it is used as the exit status of
the shell; otherwise the exit status of the preceding command (the current
value of $?) is used.
-
-
- export
[-npx] name
...
-
- export -p
[-x]
- With no options, but one or more names, the specified names
are exported so that they will appear in the environment of subsequent
commands. With -n the specified names are un-exported.
Variables can also be un-exported using the unset built in command. With
-x (exclude) the specified names are marked not to be
exported, and any that had been exported, will be un-exported. Later
attempts to export the variable will be refused. Note this does not
prevent explicitly exporting a variable to a single command, script or
function by preceding that command invocation by a variable assignment to
that variable, provided the variable is not also read-only. That is
export -x FOO; # FOO will now not be
exported by default
FOO=some_value my_command
still passes the value (FOO=some_value) to my_command
through the environment.
The shell allows the value of a variable to be set at the same time it is
exported by writing
export name=value
With no arguments the export command lists the names of all exported
variables, or if -x was given, all variables marked not
for export. With the -p option specified the output will
be formatted suitably for non-interactive use.
The export built-in exits with status 0, unless an invalid
option, or option combination, is given, or unless an attempt is made to
export a variable which has been marked as unavailable for export, in
which cases it exits with status 1.
Note that there is no restriction upon exporting, or un-exporting, read-only
variables. The no-export flag can be reset by unsetting the variable and
creating it again – provided it is not also read-only.
-
-
- fc
[-e editor]
[first
[last]]
-
- fc -l
[-nr]
[first
[last]]
-
- fc -s
[old=new]
[first]
- The fc built-in lists, or edits and
re-executes, commands previously entered to an interactive shell.
-
-
- -e
editor
- Use the editor named by editor to edit the commands.
The editor string is a command name, subject to search via the
PATH
variable. The value in the
FCEDIT
variable is used as a default when
-e is not specified. If
FCEDIT
is null or unset, the value of the
EDITOR
variable is used. If
EDITOR
is null or unset,
ed(1) is used as the
editor.
-
-
- -l
(ell)
- List the commands rather than invoking an editor on
them. The commands are written in the sequence indicated by the first
and last operands, as affected by -r, with each
command preceded by the command number.
-
-
- -n
- Suppress command numbers when listing with -l.
-
-
- -r
- Reverse the order of the commands listed (with
-l) or edited (with neither -l nor
-s).
-
-
- -s
- Re-execute the command without invoking an editor.
-
-
- first
-
- last
- Select the commands to list or edit. The number of
previous commands that can be accessed are determined by the value of
the
HISTSIZE
variable. The value of first or
last or both are one of the following:
-
-
- [+]number
- A positive number representing a command number;
command numbers can be displayed with the -l
option.
-
-
- -number
- A negative decimal number representing the command
that was executed number of commands previously. For example, -1
is the immediately previous command.
-
-
- string
- A string indicating the most recently entered command
that begins with that string. If the old=new operand is not also
specified with -s, the string form of the first
operand cannot contain an embedded equal sign.
The following environment variables affect the execution of fc:
-
-
FCEDIT
- Name of the editor to use.
-
-
HISTSIZE
- The number of previous commands that are
accessible.
-
-
- fg
[job]
- Move the specified job or the current job to the
foreground. A foreground job can interact with the user via standard
input, and receive signals from the terminal.
-
-
- fdflags
[-v] [fd
...]
-
- fdflags
[-v] -s
flags fd [...]
- Get or set file descriptor flags. The -v
argument enables verbose printing, printing flags that are also off, and
the flags of the file descriptor being set after setting. The
-s flag interprets the flags
argument as a comma separated list of file descriptor flags, each preceded
with a “+” or a “−” indicating to set or
clear the respective flag. Valid flags are: append,
async, sync,
nonblock, fsync,
dsync, rsync,
direct, nosigpipe, and
cloexec. Unique abbreviations of these names, of at
least 2 characters, may be used on input. See
fcntl(2) and
open(2) for more
information.
-
-
- getopts
optstring var
- The POSIX getopts command, not to be
confused with the Bell Labs -derived
getopt(1).
The first argument should be a series of letters, each of which may be
optionally followed by a colon to indicate that the option requires an
argument. The variable specified is set to the parsed option.
The getopts command deprecates the older
getopt(1) utility due to its
handling of arguments containing whitespace.
The getopts built-in may be used to obtain options and
their arguments from a list of parameters. When invoked,
getopts places the value of the next option from the
option string in the list in the shell variable specified by
var and its index in the shell variable
OPTIND
. When the shell is invoked,
OPTIND
is initialized to 1. For each option that
requires an argument, the getopts built-in will place it
in the shell variable OPTARG
. If an option is not
allowed for in the optstring, then
OPTARG
will be unset.
optstring is a string of recognized option letters
(see getopt(3)). If a letter
is followed by a colon, the option is expected to have an argument which
may or may not be separated from it by whitespace. If an option character
is not found where expected, getopts will set the
variable var to a “?”;
getopts will then unset OPTARG
and write output to standard error. By specifying a colon as the first
character of optstring all errors will be ignored.
A nonzero value is returned when the last option is reached. If there are no
remaining arguments, getopts will set
var to the special option, “--”,
otherwise, it will set var to “?”.
The following code fragment shows how one might process the arguments for a
command that can take the options [a] and
[b], and the option [c],
which requires an argument.
while getopts abc: f
do
case $f in
a | b) flag=$f;;
c) carg=$OPTARG;;
\?) echo $USAGE; exit 1;;
esac
done
shift $((OPTIND - 1))
This code will accept any of the following as equivalent:
cmd -acarg file file
cmd -a -c arg file file
cmd -carg -a file file
cmd -a -carg -- file file
-
-
- hash -rv
command ...
- The shell maintains a hash table which remembers the
locations of commands. With no arguments whatsoever, the
hash command prints out the contents of this table.
Entries which have not been looked at since the last cd
command are marked with an asterisk; it is possible for these entries to
be invalid.
With arguments, the hash command removes the specified
commands from the hash table (unless they are functions) and then locates
them. With the -v option, hash prints the locations of
the commands as it finds them. The -r option causes the
hash command to delete all the entries in the hash table except for
functions.
-
-
- inputrc
file
- Read the file to set key bindings as
defined by editrc(5).
-
-
- jobid
[-g|-j|-p]
[job]
- With no flags, print the process identifiers of the
processes in the job. If the job argument is
omitted, the current job is used. Any of the ways to select a job may be
used for job, including the ‘%’ forms,
or the process id of the job leader (“$!” if the job was
created in the background.)
If one of the flags is given, then instead of the list of process
identifiers, the jobid command prints:
- With -g
the process group, if one was created for this job, or nothing
otherwise (the job is in the same process group as the shell.)
- With -j
the job identifier (using ‘%n’ notation, where n is a
number) is printed.
- With -p
only the process id of the process group leader is printed.
These flags are mutually exclusive.
jobid exits with status 2 if there is an argument error,
status 1, if with -g the job had no separate process
group, or with -p there is no process group leader
(should not happen), and otherwise exits with status 0.
-
-
- jobs
[-l|-p]
[job...]
- Without job arguments, this command lists out all the
background processes which are children of the current shell process. With
job arguments, the listed jobs are shown instead. Without flags, the
output contains the job identifier (see
Job Control below), an indicator
character if the job is the current or previous job, the current status of
the job (running, suspended, or terminated successfully, unsuccessfully,
or by a signal) and a (usually abbreviated) command string.
With the -l flag the output is in a longer form, with the
process identifiers of each process (run from the top level, as in a
pipeline), and the status of each process, rather than the job status.
With the -p flag, the output contains only the process
identifier of the lead process.
In an interactive shell, each job shown as completed in the output from the
jobs command is implicitly waited for, and is removed from the jobs table,
never to be seen again. In an interactive shell, when a background job
terminates, the jobs command (with that job as an
argument) is implicitly run just before outputting the next PS1 command
prompt, after the job terminated. This indicates that the job finished,
shows its status, and cleans up the job table entry for that job.
Non-interactive shells need to execute wait commands to
clean up terminated background jobs.
-
-
- local
[-INx]
[variable |
-] ...
- Define local variables for a function. Local variables have
their attributes, and values, as they were before the
local declaration, restored when the function
terminates.
With the -N flag, variables made local, are unset
initially inside the function. Unless the -x flag is
also given, such variables are also unexported. The -I
flag, which is the default in this shell, causes the initial value and
exported attribute of local variables to be inherited from the variable
with the same name in the surrounding scope, if there is one. If there is
not, the variable is initially unset, and not exported. The
-N and -I flags are mutually
exclusive, if both are given, the last specified applies. The read-only
and unexportable attributes are always inherited, if a variable with the
same name already exists.
The -x flag (lower case) causes the local variable to be
exported, while the function runs, unless it has the unexportable
attribute. This can also be accomplished by using the
export command, giving the same
variable names, after the local
command.
Making an existing read-only variable local is possible, but pointless. If
an attempt is made to assign an initial value to such a variable, the
local command fails, as does any later attempted
assignment. If the readonly command is applied to a
variable that has been declared local, the variable cannot be (further)
modified within the function, or any other functions it calls, however
when the function returns, the previous status (and value) of the variable
is returned.
Values may be given to local variables on the local
command line in a similar fashion as used for export and
readonly. These values are assigned immediately after
the initialization described above. Note that any variable references on
the command line will have been expanded before local is
executed, so expressions like
local -N X=${X}
are well defined, first $X is expanded, and then the command run is
local -N X=old-value-of-X
After arranging to preserve the old value and attributes, of X
(“old-value-of X”) local unsets
X
, unexports it, and then assigns the
“old-value-of-X” to X
.
The shell uses dynamic scoping, so that if you make the variable x local to
function f, which then calls function g, references to the variable x made
inside g will refer to the variable x declared inside f, not to the global
variable named x.
Another way to view this, is as if the shell just has one flat, global,
namespace, in which all variables exist. The local
command conceptually copies the variable(s) named to unnamed temporary
variables, and when the function ends, copies them back again. All
references to the variables reference the same global variables, but while
the function is active, after the local command has run,
the values and attributes of the variables might be altered, and later,
when the function completes, be restored.
Note that the parameters $1, $2, ... (see
Positional Parameters),
and $#, $* and $@ (see Special
Parameters), are always made local in all functions, and are reset
inside the function to represent the options and arguments passed to the
function. Note that $0 however retains the value it had outside the
function, as do all the other special parameters.
The only other special parameter that can be made local is “-”.
Making “-” local causes any shell options that are changed via
the set command inside the function to be restored to their original
values when the function returns. If -X option is
altered after “-” has been made local, then when the function
returns, the previous destination for xtrace output (as
of the time of the local command) will also be restored.
It is an error to use local outside the scope of a
function definition. When used inside a function, it exits with status 0,
unless an undefined option is used, or an attempt is made to assign a
value to a read-only variable.
Note that either -I or -N should always
be used, or variables made local should always be given a value, or
explicitly unset, as the default behavior (inheriting the earlier value,
or starting unset after local) differs amongst shell
implementations. Using “local -” is an extension not
implemented by most shells.
See the section LINENO below for details of
the effects of making the variable LINENO
local.
-
-
- pwd
[-LP]
- Print the current directory. If -L is
specified the cached value (initially set from
PWD
) is checked to see if it refers to the current
directory; if it does the value is printed. Otherwise the current
directory name is found using
getcwd(3). The environment
variable PWD
is set to the printed value.
The default is pwd -L, but note that the
built-in cd command doesn't support the
-L option and will cache (almost) the absolute path. If
cd is changed (as unlikely as that is),
pwd may be changed to default to pwd
-P.
If the current directory is renamed and replaced by a symlink to the same
directory, or the initial PWD
value followed a
symbolic link, then the cached value may not be the absolute path.
The built-in command may differ from the program of the same name because
the program will use PWD
and the built-in uses a
separately cached value.
-
-
- read
[-p prompt]
[-r]
variable
[...]
- The prompt is printed if the -p option is
specified and the standard input is a terminal. Then a line is read from
the standard input. The trailing newline is deleted from the line and the
line is split as described in the field splitting section of the
Word Expansions section above,
and the pieces are assigned to the variables in order. If there are more
pieces than variables, the remaining pieces (along with the characters in
IFS
that separated them) are assigned to the last
variable. If there are more variables than pieces, the remaining variables
are assigned the null string. The read built-in will
indicate success unless EOF is encountered on input, in which case failure
is returned.
By default, unless the -r option is specified, the
backslash “\” acts as an escape character, causing the
following character to be treated literally. If a backslash is followed by
a newline, the backslash and the newline will be deleted.
-
-
- readonly name
...
-
- readonly
[-p]
- The specified names are marked as read only, so that they
cannot be subsequently modified or unset. The shell allows the value of a
variable to be set at the same time it is marked read only by writing
readonly name=value
With no arguments the readonly command lists the names of all read only
variables. With the -p option specified the output will
be formatted suitably for non-interactive use.
-
-
- return
[n]
- Stop executing the current function or a dot command with
return value of n or the value of the last executed
command, if not specified. For portability, n should
be in the range from 0 to 255.
The POSIX standard says that the results of ‘return’ outside a
function or a dot command are unspecified. This implementation treats such
a return as a no-op with a return value of 0 (success, true). Use the exit
command instead, if you want to return from a script or exit your
shell.
-
-
- set [{
-options | +options | --
}] arg ...
- The set command performs four different
functions.
With no arguments, it lists the values of all shell variables.
With a single option of either ‘-o’ or
‘+o’ set outputs the
current values of the options. In the -o form, all
options are listed, with their current values. In the +o
form, the shell outputs a string that can later be used as a command to
reset all options to their current values.
If options are given, it sets the specified option flags, or clears them as
described in the Argument
List Processing section. In addition to the options listed there, when
the “option name” given to set
-o is default all of the options are
reset to the values they had immediately after sh
initialization, before any startup scripts, or other input, had been
processed. While this may be of use to users or scripts, its primary
purpose is for use in the output of “set
+o”, to avoid that command needing to list every
available option. There is no +o default.
The fourth use of the set command is to set the values of the shell's
positional parameters to the specified arguments. To change the positional
parameters without changing any options, use “--” as the first
argument to set. If no following arguments are present, the set command
will clear all the positional parameters (equivalent to executing
“shift $#”.) Otherwise the following arguments become
“$1”, “$2”, ..., and “$#” is set to
the number of arguments present.
-
-
- setvar
variable value
- Assigns value to variable. (In general it is better to
write variable=value rather than using setvar.
setvar is intended to be used in functions that assign
values to variables whose names are passed as parameters.)
-
-
- shift
[n]
- Shift the positional parameters n times. If n is omitted, 1
is assumed. Each shift sets the value of
$1 to the previous value of
$2, the value of $2 to the
previous value of $3, and so on, decreasing the
value of $# by one. The shift count must be less
than or equal to the number of positional parameters ( “$#”)
before the shift.
-
-
- times
- Prints two lines to standard output. Each line contains two
accumulated time values, expressed in minutes and seconds (including
fractions of a second.) The first value gives the user time consumed, the
second the system time.
The first output line gives the CPU and system times consumed by the shell
itself. The second line gives the accumulated times for children of this
shell (and their descendants) which have exited, and then been
successfully waited for by the relevant parent. See
times(3) for more
information.
times has no parameters, and exits with an exit status of
0 unless an attempt is made to give it an option.
-
-
- trap action
signal ...
-
- trap -
-
- trap
[-l]
-
- trap
[-p] signal ...
-
- trap N signal
...
-
Cause the shell to parse and execute action when any of the specified
signals are received. The signals are specified by signal number or as the
name of the signal. If signal is
0
or its equivalent, EXIT, the action is executed
when the shell exits. The action may be a null
(empty) string, which causes the specified signals to be ignored. With
action set to ‘-’ the specified signals
are set to their default actions. If the first
signal is specified in its numeric form, then
action can be omitted to achieve the same effect.
This archaic, but still standard, form should not be relied upon, use the
explicit ‘-’ action. If no signals are specified with an
action of ‘-’, all signals are reset.
When the shell forks off a sub-shell, it resets trapped (but not ignored)
signals to the default action. On non-interactive shells, the
trap command has no effect on signals that were ignored
on entry to the shell. On interactive shells, the trap
command will catch or reset signals ignored on entry.
Issuing trap with option -l will print a
list of valid signal names. trap without any arguments
causes it to write a list of signals and their associated non-default
actions to the standard output in a format that is suitable as an input to
the shell that achieves the same trapping results. With the
-p flag, trap prints the same information for the
signals specified, or if none are given, for all signals, including those
where the action is the default.
Examples:
trap
List trapped signals and their corresponding actions.
trap -l
Print a list of valid signals.
trap '' INT QUIT tstp 30
Ignore signals INT QUIT TSTP USR1.
trap date INT
Run the “date” command (print the date) upon receiving signal
INT.
trap HUP INT
Run the “HUP” command, or function, upon receiving signal INT.
trap 1 2
Reset the actions for signals 1 (HUP) and 2 (INT) to their defaults.
traps=$(trap -p)
# more commands ...
trap 'action' SIG
# more commands ...
eval "$traps"
Save the trap status, execute commands, changing some traps, and then reset
all traps to their values at the start of the sequence. The
-p option is required in the first command here, or any
signals that were previously untrapped (in their default states) and which
were altered during the intermediate code, would not be reset by the final
“eval”.
-
-
- type
[name ...]
- Interpret each name as a command and print the resolution
of the command search. Possible resolutions are: shell keyword, alias,
shell built-in, command, tracked alias and not found. For aliases the
alias expansion is printed; for commands and tracked aliases the complete
pathname of the command is printed.
-
-
- ulimit
[-H | -S]
[-a | -btfdscmlrpnv
[value]]
- Inquire about or set the hard or soft limits on processes
or set new limits. The choice between hard limit (which no process is
allowed to violate, and which may not be raised once it has been lowered)
and soft limit (which causes processes to be signaled but not necessarily
killed, and which may be raised) is made with these flags:
-
-
- -H
- set or inquire about hard limits
-
-
- -S
- set or inquire about soft limits. If neither
-H nor -S is specified, the soft
limit is displayed or both limits are set. If both are specified, the
last one wins.
The limit to be interrogated or set, then, is chosen by specifying any one
of these flags:
-
-
- -a
- show all the current limits
-
-
- -b
- show or set the limit on the socket buffer size of a
process (in bytes)
-
-
- -c
- show or set the limit on the largest core dump size
that can be produced (in 512-byte blocks)
-
-
- -d
- show or set the limit on the data segment size of a
process (in kilobytes)
-
-
- -f
- show or set the limit on the largest file that can be
created (in 512-byte blocks)
-
-
- -l
- show or set the limit on how much memory a process can
lock with mlock(2) (in
kilobytes)
-
-
- -m
- show or set the limit on the total physical memory that
can be in use by a process (in kilobytes)
-
-
- -n
- show or set the limit on the number of files a process
can have open at once
-
-
- -p
- show or set the limit on the number of processes this
user can have at one time
-
-
- -r
- show or set the limit on the number of threads this
user can have at one time
-
-
- -s
- show or set the limit on the stack size of a process
(in kilobytes)
-
-
- -t
- show or set the limit on CPU time (in seconds)
-
-
- -v
- show or set the limit on how large a process address
space can be
If none of these is specified, it is the limit on file size that is shown or
set. If value is specified, the limit is set to that number; otherwise the
current limit is displayed.
Limits of an arbitrary process can be displayed or set using the
sysctl(8) utility.
-
-
- umask
[-S]
[mask]
- Set the value of umask (see
umask(2)) to the specified
octal value. If the argument is omitted, the umask value is printed. With
-S a symbolic form is used instead of an octal
number.
-
-
- unalias
[-a]
[name]
- If name is specified, the shell
removes that alias. If -a is specified, all aliases are
removed.
-
-
- unset
[-efvx] name
...
- If -v is specified, the specified
variables are unset and unexported. Readonly variables cannot be unset. If
-f is specified, the specified functions are undefined.
If -e is given, the specified variables are unexported,
but otherwise unchanged, alternatively, if -x is given,
the exported status of the variable will be retained, even after it is
unset.
If no flags are provided -v is assumed. If
-f is given with one of the other flags, then the named
variables will be unset, or unexported, and functions of the same names
will be undefined. The -e and -x flags
both imply -v. If -e is given, the
-x flag is ignored.
The exit status is 0, unless an attempt was made to unset a readonly
variable, in which case the exit status is 1. It is not an error to unset
(or undefine) a variable (or function) that is not currently set (or
defined.)
-
-
- wait
[-n]
[-p var]
[job ...]
- Wait for the specified jobs to complete and return the exit
status of the last job in the parameter list, or 127 if that job is not a
current child of the shell.
If no job arguments are given, wait for all jobs to
complete and then return an exit status of zero (including when there were
no jobs, and so nothing exited.)
With the -n option, wait instead for any one of the given
jobs, or if none are given, any job, to complete,
and return the exit status of that job. If none of the given
job arguments is a current child of the shell, or if
no job arguments are given and the shell has no
unwaited for children, then the exit status will be 127.
The -p var option allows the process
(or job) identifier of the job for which the exit status is returned to be
obtained. The variable named (which must not be readonly) will be unset
initially, then if a job has exited and its status is being returned, set
to the identifier from the arg list (if given) of that job, or the lead
process identifier of the job to exit when used with -n
and no job arguments. Note that -p with neither
-n nor job arguments is useless,
as in that case no job status is returned, the variable named is simply
unset.
If the wait is interrupted by a signal, its exit status will be greater than
128, and var, if given, will remain unset.
Once waited upon, by specific process number or job-id, or by a
wait with no arguments, knowledge of the child is
removed from the system, and it cannot be waited upon again.
Note than when a list of jobs are given, more that one argument might refer
to the same job. In that case, if the final argument represents a job that
is also given earlier in the list, it is not defined whether the status
returned will be the exit status of the job, or 127 indicating that the
child no longer existed when the wait command reached the later argument
in the list. In this sh the exit status will be that
from the job. sh waits for each job exactly once,
regardless of how many times (or how many different ways) it is listed in
the arguments to wait. That is
is identical to
Job Control
Each process (or set of processes) started by
sh is created as
a “job” and added to the jobs table. When enabled by the
-m option (aka
-o
monitor) when the job is created,
sh
places each job (if run from the top level shell) into a process group of its
own, which allows control of the process(es), and its/their descendants, as a
unit. When the
-m option is off, or when started from a
sub-shell environment, jobs share the same process group as the parent shell.
The
-m option is enabled by default in interactive shells
with a terminal as standard input and standard error.
Jobs with separate process groups may be stopped, and then later resumed in the
foreground (with access to the terminal) or in the background (where
attempting to read from the terminal will result in the job stopping.) A list
of current jobs can be obtained using the
jobs built-in
command. Jobs are identified using either the process identifier of the lead
process of the job (the value available in the special parameter
‘$!’ if the job is started in the background), or using percent
notation. Each job is given a “job number” which is a small
integer, starting from 1, and can be referenced as ‘%n’ where n is
that number. Note that this applies to jobs both with and without their own
process groups. Job numbers are shown in the output from the
jobs command enclosed in brackets (‘[’ and
‘]’). Whenever the job table becomes empty, the numbers begin at
one again. In addition, there is the concept of a current, and a previous job,
identified by ‘%+’ (or ‘%%’ or even just
‘%’), and a previous job, identified by ‘%-’. Whenever
a background job is started, or a job is resumed in the background, it becomes
the current job. The job that was the current job (prepare for a big surprise
here, drum roll..., wait for it...) becomes the previous job. When the current
job terminates, the previous job is promoted to be the current job. In
addition the form “%string” finds the job for which the command
starts with “string” and the form “%?string” finds the
job which contains the “string” in its command somewhere. Both
forms require the result to be unambiguous. For this purpose the
“command” is that shown in the output from the
jobs command, not the original command line.
The
bg,
fg,
jobid,
jobs,
kill, and
wait
commands all accept job identifiers as arguments, in addition to process
identifiers (larger integers). See the
Built-ins section above, and
kill(1), for more details of those
commands. In addition, a job identifier (using one of the ‘%
forms’) issued as a command, without arguments, is interpreted as if it
had been given as the argument to the
fg command.
To cause a foreground process to stop, enter the terminal's
stop character (usually control-Z). To cause a background
process to stop, send it a
STOP
signal, using the kill
command. A useful function to define is
stop() { kill -s STOP "${@:-%%}"; }
The
fg command resumes a stopped job, placing it in the
foreground, and
bg resumes a stopped job in the background.
The
jobid command provides information about process
identifiers, job identifiers, and the process group identifier, for a job.
Whenever a sub-shell is created, the jobs table becomes invalid (the sub-shell
has no children.) However, to enable uses like
the table is only actually cleared in a sub-shell when needed to create the
first job there (built-in commands run in the foreground do not create jobs.)
Note that in this environment, there is no useful current job (%% actually
refers to the sub-shell itself, but is not accessible) but the job which is
the current job in the parent can be accessed as %-.
Command Line Editing
When
sh is being used interactively from a terminal, the
current command and the command history (see
fc in the
Built-ins section) can be edited using
emacs-mode or vi-mode command-line editing. The command
‘
set -o emacs
’ enables emacs-mode editing.
The command ‘
set -o vi
’ enables vi-mode
editing and places the current shell process into
vi
insert mode. (See the
Argument
List Processing section above.)
The
vi mode uses commands similar to a subset of those
described in the
vi(1) man page.
With vi-mode enabled,
sh can be switched between insert mode
and command mode. It's similar to
vi(1): pressing the
⟨ESC⟩ key will throw you into command VI command mode. Pressing
the ⟨return⟩ key while in command mode will pass the line to the
shell.
The
emacs mode uses commands similar to a subset available
in the
emacs editor. With emacs-mode enabled, special keys
can be used to modify the text in the buffer using the control key.
sh uses the
editline(3) library. See
editline(7) for a list of the
possible command bindings, and the default settings in
emacs and
vi modes. Also see
editrc(5) for the commands that
can be given to configure
editline(7) in the file named
by the
EDITRC
parameter, or a file used with the
inputrc built-in command, or using
editline(7)'s configuration
command line.
When command line editing is enabled, the
editline(7) functions control
printing of the
PS1
and
PS2
prompts when required. As, in this mode, the command line editor needs to keep
track of what characters are in what position on the command line, care needs
to be taken when setting the prompts. Normal printing characters are handled
automatically, however mode setting sequences, which do not actually display
on the terminal, need to be identified to
editline(7). This is done,
when needed, by choosing a character that is not needed anywhere in the
prompt, including in the mode setting sequences, any single character is
acceptable, and assigning it to the shell parameter
PSlit
. Then that character should be used, in pairs,
in the prompt string. Between each pair of
PSlit
characters are mode setting sequences, which affect the printing attributes of
the following (normal) characters of the prompt, but do not themselves appear
visibly, nor change the terminal's cursor position.
Each such sequence, that is
PSlit
character, mode
setting character sequence, and another
PSlit
character, must currently be followed by at least one following normal prompt
character, or it will be ignored. That is, a
PSlit
character cannot be the final character of
PS1
or
PS2
, nor may two
PSlit
delimited sequences appear adjacent to each other. Each sequence can contain
as many mode altering sequences as are required however. Only the first
character from
PSlit
will be used. When set
PSlit
should usually be set to a string containing
just one character, then it can simply be embedded in
PS1
(or
PS2
) as in
PS1="${PSlit}mset${PSlit}XYZ${PSlit}mclr${PSlit}ABC"
The prompt visible will be “XYZABC” with the “XYZ” part
shown according as defined by the mode setting characters “mset”,
and then cleared again by “mclr”. See
tput(1) for one method to generate
appropriate mode sequences. Note that both parts, XYZ and ABC, must each
contain at least one character.
If
PSlit
is unset, which is its initial state, or set to
a null string, no literal character will be defined, and all characters of the
prompt strings will be assumed to be visible characters (which includes spaces
etc.) To allow smooth use of prompts, without needing redefinition, when
editline(7) is disabled, the
character chosen should be one which will be ignored by the terminal if
received, as when
editline(7)
is not in use, the prompt strings are simply written to the terminal. For
example, setting:
PSlit="$(printf '\1')"
PS1="${PSlit}$(tput bold blink)${PSlit}\$${PSlit}$(tput sgr0)${PSlit} "
will arrange for the primary prompt to be a bold blinking dollar sign, if
supported by the current terminal, followed by an (ordinary) space, and, as
the SOH (Control-A) character ('\1') will not normally affect a terminal, this
same prompt will usually work with
editline(7) enabled or
disabled.
ENVIRONMENT
-
-
CDPATH
- The search path used with the cd
built-in.
-
-
EDITRC
- Gives the name of the file containing commands for
editline(7). See
editrc(5) for possible
content and format. The file is processed, when in interactive mode with
command line editing enabled, whenever
EDITRC
is
set (even with no actual value change,) and if command line editing
changes from disabled to enabled, or the editor style used is changed.
(See the -E and -V options of the
set built-in command, described in
Built-ins above, which are documented
further above in Argument
List Processing.) If unset “$HOME/.editrc” is used.
-
-
HISTSIZE
- The number of lines in the history buffer for the
shell.
-
-
HOME
- Set automatically by
login(1) from the user's
login directory in the password file
(passwd(5)). This
environment variable also functions as the default argument for the
cd built-in.
-
-
HOSTNAME
- Set to the current hostname of the system, as returned by
gethostname(3). This is
obtained each time
HOSTNAME
is expanded, so
changes to the system's name are reflected without further action. If
unset, it returns nothing. Setting it does nothing except reverse the
effect of an earlier unset.
-
-
IFS
- Input Field Separators. This is normally set to
⟨space⟩, ⟨tab⟩, and ⟨newline⟩.
See the White Space
Splitting section for more details.
-
-
LANG
- The string used to specify localization information that
allows users to work with different culture-specific and language
conventions. See nls(7).
-
-
LINENO
- The current line number in the script or function. See the
section LINENO below for more
details.
-
-
MAIL
- The name of a mail file, that will be checked for the
arrival of new mail. Overridden by
MAILPATH
. The
check occurs just before PS1
is written,
immediately after reporting jobs which have changed status, in interactive
shells only. New mail is considered to have arrived if the monitored file
has increased in size since the last check.
-
-
MAILPATH
- A colon “:” separated list of file names, for
the shell to check for incoming mail. This environment setting overrides
the
MAIL
setting. There is a maximum of 10
mailboxes that can be monitored at once.
-
-
PATH
- The default search path for executables. See the
Path Search section above.
-
-
PPID
- The process identified of the parent process of the current
shell. This value is set at shell startup, ignoring any value in the
environment, and then made readonly.
-
-
PS1
- The primary prompt string, which defaults to “$
”, unless you are the superuser, in which case it defaults to
“# ”. This string is subject to parameter, arithmetic,
and if enabled by setting the promptcmds option, command
substitution before being output. During execution of commands used by
command substitution, execution tracing, the xtrace
(set -x) option is temporarily
disabled. If promptcmds is not set and the prompt string
uses command substitution, the prompt used will be an appropriate error
string. For other expansion errors, a message will be output, and the
unexpanded string will then be used as the prompt.
-
-
PS2
- The secondary prompt string, which defaults to “>
”. After expansion (as for
PS1
) it is
written whenever more input is required to complete the current
command.
-
-
PS4
- Output, after expansion like
PS1
,
before each line when execution trace (set
-x) is enabled. PS4
defaults to
“+ ”.
-
-
PSc
- Initialized by the shell, ignoring any value from the
environment, to a single character string, either ‘#’ or
‘$’, depending upon whether the current user is the superuser
or not. This is intended for use when building a custom
PS1
.
-
-
PSlit
- Defines the character which may be embedded in pairs, in
PS1
or PS2
to indicate to
editline(7) that the
characters between each pair of occurrences of the
PSlit
character will not appear in the visible
prompt, and will not cause the terminal's cursor to change position, but
rather set terminal attributes for the following prompt character(s) at
least one of which must be present. See
Command Line Editing above
for more information.
-
-
RANDOM
- Returns a different pseudo-random integer, in the range
[0,32767] each time it is accessed.
RANDOM
can be
assigned an integer value to seed the PRNG. If the value assigned is a
constant, then the sequence of values produces on subsequent references of
RANDOM
will repeat after the next time the same
constant is assigned. Note, this is not guaranteed to remain constant from
one version of the shell to another – the PRNG algorithm, or seeding
method is subject to change. If RANDOM
is assigned
an empty value (null string) then the next time
RANDOM
is accessed, it will be seeded from a more
genuinely random source. The sequence of pseudo-random numbers generated
will not be able to be generated again (except by luck, whether good or
bad, depends!) This is also how the initial seed is generated, if none has
been assigned before RANDOM
is first accessed
after shell initialization.
-
-
SECONDS
- Returns the number of seconds since the current shell was
started. Attempts to set this variable are ignored. If unset, it remains
unset, and returns nothing, unless set again.
-
-
START_TIME
- Initialized by the shell to the number of seconds since the
Epoch (see localtime(3))
when the shell was started. The value of
$((START_TIME +
SECONDS
))
represents the current time, if START_TIME
has not
been modified, and SECONDS
is not unset.
-
-
TERM
- The default terminal setting for the shell. This is
inherited by children of the shell, and is used in the history editing
modes.
-
-
ToD
- When referenced, uses the value of
ToD_FORMAT
(or “%T” if
ToD_FORMAT
is unset) as the format argument to
strftime(3) to encode the
current time of day, in the time zone defined by
TZ
if set, or current local time if not, and
returns the result. If unset ToD
returns nothing.
Setting it has no effect, other than to reverse the effect of an earlier
unset.
-
-
ToD_FORMAT
- Can be set to the
strftime(3) format string
to be used when expanding
ToD
. Initially
unset.
-
-
TZ
- If set, gives the time zone (see
localtime(3),
environ(7)) to use when
formatting
ToD
and if exported, other utilities
that deal with times. If unset, the system's local wall clock time zone is
used.
-
-
NETBSD_SHELL
- Unlike the variables previously mentioned, this variable is
somewhat strange, in that it cannot be set, inherited from the
environment, modified, or exported from the shell. If set, by the shell,
it indicates that the shell is the sh defined by this
manual page, and gives its version information. It can also give
information in additional space separated words, after the version string.
If the shell was built as part of a reproducible build, the relevant date
that was used for that build will be included. Finally, any non-standard
compilation options, which may affect features available, that were used
when building the shell will be listed.
NETBSD_SHELL
behaves like any other variable that
has the read-only and un-exportable attributes set.
LINENO
LINENO
is in many respects a normal shell variable,
containing an integer value. and can be expanded using any of the forms
mentioned above which can be used for any other variable.
LINENO
can be exported, made readonly, or unset, as with
any other variable, with similar effects. Note that while being readonly
prevents later attempts to set, or unset,
LINENO
, it
does not prevent its value changing. References to
LINENO
(when not unset) always obtain the current line
number. However,
LINENO
should normally not ever be
set or unset. In this shell setting
LINENO
reverses
the effect of an earlier
unset, but does not otherwise
affect the value obtained. If unset,
LINENO
should not
normally be set again, doing so is not portable. If
LINENO
is set or unset, different shells act
differently. The value of
LINENO
is never imported
from the environment when the shell is started, though if present there, as
with any other variable,
LINENO
will be exported by
this shell.
LINENO
is set automatically by the shell to be the
number of the source line on which it occurs. When exported,
LINENO
is exported with its value set to the line
number it would have had had it been referenced on the command line of the
command to which it is exported. Line numbers are counted from 1, which is the
first line the shell reads from any particular file. For this shell, standard
input, including in an interactive shell, the user's terminal, is just another
file and lines are counted there as well. However note that not all shells
count interactive lines this way, it is not wise to rely upon
LINENO
having a useful value, except in a script, or a
function.
The role of
LINENO
in functions is less clear. In some
shells,
LINENO
continues to refer to the line number
in the script which defines the function, in others lines count from one
within the function, always (and resume counting normally once the function
definition is complete) and others count in functions from one if the function
is defined interactively, but otherwise just reference the line number in the
script in which the function is defined. This shell gives the user the option
to choose. If the
-L flag (the
local_lineno option, see
Argument List Processing)
is set, when the function is defined, then the function defaults to counting
lines with one being the first line of the function. When the
-L flag is not set, the shell counts lines in a function
definition in the same continuous sequence as the lines that surround the
function definition. Further, if
LINENO
is made local
(see
Built-ins above) inside the function,
the function can decide which behavior it prefers. If
LINENO
is made local and inherited, and not given a
value, as in
local -I
LINENO
then from that point in the function,
LINENO
will give
the line number as if lines are counted in sequence with the lines that
surround the function definition (and any other function definitions in which
this is nested.) If
LINENO
is made local, and in that
same command, given a value, as
local
[-I|-N]
LINENO
=value
then
LINENO
will give the line number as if lines are
counted from one from the beginning of the function. The value nominally
assigned in this case is irrelevant, and ignored. For completeness, if lineno
is made local and unset, as in
local -N
LINENO
then
LINENO
is simply unset inside the function, and
gives no value at all.
Now for some technical details. The line on which
LINENO
occurs in a parameter expansion, is the line that contains the ‘$’
that begins the expansion of
LINENO
. In the case of
nested expansions, that ‘$’ is the one that actually has
LINENO
as its parameter. In an arithmetic expansion,
where no ‘$’ is used to evaluate
LINENO
but
LINENO
is simply referenced as a variable, then
the value is the line number of the line that contains the ‘L’ of
LINENO
. For functions line one of the function
definition (when relevant) is the line that contains the first character of
the function name in the definition. When exported, the line number of the
command is the line number where the first character of the word which becomes
the command name occurs.
When the shell opens a new file, for any reason, it counts lines from one in
that file, and then resumes its original counting once it resumes reading the
previous input stream. When handling a string passed to
eval
the line number starts at the line on which the string starts, and then if the
string contains internal newline characters, those characters increase the
line number. This means that references to
LINENO
in
such a case can produce values larger than would be produced by a reference on
the line after the
eval.
FILES
- $HOME/.profile
- /etc/profile
EXIT STATUS
Errors that are detected by the shell, such as a syntax error, will cause the
shell to exit with a non-zero exit status. If the shell is not an interactive
shell, the execution of the shell file will be aborted. Otherwise the shell
will return the exit status of the last command executed, or if the exit
built-in is used with a numeric argument, it will return the argument.
SEE ALSO
csh(1),
echo(1),
getopt(1),
ksh(1),
login(1),
printf(1),
test(1),
editline(3),
getopt(3),
editrc(5),
passwd(5),
editline(7),
environ(7),
nls(7),
sysctl(8)
HISTORY
A
sh command appeared in
Version 1
AT&T UNIX. It was replaced in
Version 7
AT&T UNIX with a version that introduced the basis of the current
syntax. That was, however, unmaintainable so we wrote this one.
BUGS
Setuid shell scripts should be avoided at all costs, as they are a significant
security risk.
The characters generated by filename completion should probably be quoted to
ensure that the filename is still valid after the input line has been
processed.
The
trap command cannot usefully be used, yet, within a
command substitution, to obtain the current trap values, as all command
substitutions are currently executed within a sub-shell environment, and in
sub-shells all non-ignored, non-default, traps are reset. As a workaround, it
is possible to redirect output from “trap” or “trap
-p” to a file, and then read the file later using the “.”
command.
Job control of compound statements (loops, etc) is a complete mess.
Many, many, more. (But less than there were...)