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Overview

The shell is a command that reads lines from either a file or the terminal, interprets them, and generally executes other commands. It is the program that is started when a user logs into the system, although a user can select a different shell 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, which can be executed directly by the shell.

Invocation

If no arguments are present and if the standard input of the shell is connected to a terminal (or if the -i option is set), 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 then .profile in a user's home directory, 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, the shell then subjects its value to parameter expansion and arithmetic expansion and reads commands from the named file. 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. The user can set the ENV variable to some file by placing the following line in the file .profile in the home directory, substituting for .shrc the filename desired:

The first non-option argument specified on the command line will be treated as the name of a file from which to read commands (a shell script), and the remaining arguments are set as the positional parameters of the shell ($1, $2, etc.). Otherwise, the shell reads commands from its standard input.

Unlike older versions of sh the ENV script is only sourced on invocation of interactive shells. This closes a well-known, and sometimes easily exploitable security hole related to poorly thought out ENV scripts.

Argument List Processing

All of the single letter options to sh have a corresponding long name, with the exception of -c and -/+o. These long names are provided next to the single letter options in the descriptions below. The long name for an option may be specified as an argument to the -/+o option of sh. Once the shell is running, the long name for an option may be specified as an argument to the -/+o option of the set built-in command (described later in the section called Built-in Commands). Introducing an option with a dash (‘-’) enables the option, while using a plus (‘+’) disables the option. A “--” or plain ‘-’ will stop option processing and will force the remaining words on the command line to be treated as arguments. The -/+o and -c options do not have long names. They take arguments and are described after the single letter options.

-a allexport

Flag variables for export when assignments are made to them.

-b notify

Enable asynchronous notification of background job completion. (UNIMPLEMENTED)

-C noclobber

Do not overwrite existing files with ‘>’.

-E emacs

Enable the built-in emacs(1) command line editor (disables the -V option if it has been set; set automatically when interactive on terminals).

-e errexit

Exit immediately if any untested command fails in non-interactive mode. The exit status of a command is considered to be explicitly tested if the command is part of the list used to control an if, elif, while, or until; if the command is the left hand operand of an “&&” or “||” operator; or if the command is a pipeline preceded by the ! keyword. If a shell function is executed and its exit status is explicitly tested, all commands of the function are considered to be tested as well.

It is recommended to check for failures explicitly instead of relying on -e because it tends to behave in unexpected ways, particularly in larger scripts.

-f noglob

Disable pathname expansion.

-h trackall

A do-nothing option for POSIX compliance.

-I ignoreeof

Ignore EOF's from input when in interactive mode.

-i interactive

Force the shell to behave interactively.

-m monitor

Turn on job control (set automatically when interactive). A new process group is created for each pipeline (called a job). It is possible to suspend jobs or to have them run in the foreground or in the background. In a non-interactive shell, this option can be set even if no terminal is available and is useful to place processes in separate process groups.

-n noexec

If not interactive, read commands but do not execute them. This is useful for checking the syntax of shell scripts.

-P physical

Change the default for the cd and pwd commands from -L (logical directory layout) to -P (physical directory layout).

-p privileged

Turn on privileged mode. This mode is enabled on startup if either the effective user or group ID is not equal to the real user or group ID. Turning this mode off sets the effective user and group IDs to the real user and group IDs. When this mode is enabled for interactive shells, the file /etc/suid_profile is sourced instead of ~/.profile after /etc/profile is sourced, and the contents of the ENV variable are ignored.

-s stdin

Read commands from standard input (set automatically if no file arguments are present). This option has no effect when set after the shell has already started running (i.e., when set with the set command).

-T trapsasync

When waiting for a child, execute traps immediately. If this option is not set, traps are executed after the child exits, as specified in IEEE Std 1003.2 (“POSIX.2”). This nonstandard option is useful for putting guarding shells around children that block signals. The surrounding shell may kill the child or it may just return control to the tty and leave the child alone, like this:

sh -T -c "trap 'exit 1' 2 ; some-blocking-program"

-u nounset

Write a message to standard error when attempting to expand a variable, a positional parameter or the special parameter ! that is not set, and if the shell is not interactive, exit immediately.

-V vi

Enable the built-in vi(1) command line editor (disables -E if it has been set).

-v verbose

The shell writes its input to standard error as it is read. Useful for debugging.

-x xtrace

Write each command (preceded by the value of the PS4 variable subjected to parameter expansion and arithmetic expansion) to standard error before it is executed. Useful for debugging.

nolog

Another do-nothing option for POSIX compliance. It only has a long name.

The -c option causes the commands to be read from the string operand instead of from the standard input. Keep in mind that this option only accepts a single string as its argument, hence multi-word strings must be quoted.

The -/+o option takes as its only argument the long name of an option to be enabled or disabled. For example, the following two invocations of sh both enable the built-in emacs(1) command line editor:

set -E
set -o emacs

If used without an argument, the -o option displays the current option settings in a human-readable format. If +o is used without an argument, the current option settings are output in a format suitable for re-input into the shell.

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 called “operators”, which are special to the shell. There are two types of operators: control operators and redirection operators (their meaning is discussed later). The following is a list of valid operators:

Control operators:

&	&&	(	)	\n
;;	;&	;	|	||

Redirection operators:

<	>	<<	>>	<>
<&	>&	<<-	>|

The character ‘#’ introduces a comment if used at the beginning of a word. The word starting with ‘#’ and the rest of the line are ignored.

ASCII NUL characters (character code 0) are not allowed in shell input.

Quoting

Quoting is used to remove the special meaning of certain characters or words to the shell, such as operators, whitespace, keywords, or alias names.

There are four types of quoting: matched single quotes, dollar-single quotes, matched double quotes, and backslash.

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).

Dollar-Single Quotes

Enclosing characters between $' and ' preserves the literal meaning of all characters except backslashes and single quotes. A backslash introduces a C-style escape sequence:

\a

Alert (ring the terminal bell)

\b

Backspace

\cc

The control character denoted by ^c in stty(1). If c is a backslash, it must be doubled.

\e

The ESC character (ASCII 0x1b)

\f

Formfeed

\n

Newline

\r

Carriage return

\t

Horizontal tab

\v

Vertical tab

\\

Literal backslash

\'

Literal single-quote

\"

Literal double-quote

\nnn

The byte whose octal value is nnn (one to three digits)

\xnn

The byte whose hexadecimal value is nn (one or more digits only the last two of which are used)

\unnnn

The Unicode code point nnnn (four hexadecimal digits)

\Unnnnnnnn

The Unicode code point nnnnnnnn (eight hexadecimal digits)

The sequences for Unicode code points are currently only useful with UTF-8 locales. They reject code point 0 and UTF-16 surrogates.

If an escape sequence would produce a byte with value 0, that byte and the rest of the string until the matching single-quote are ignored.

Any other string starting with a backslash is an error.

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. It remains literal unless it precedes the following characters, which it serves to quote:

$

`

"

\

\n

Backslash

A backslash preserves the literal meaning of the following character, with the exception of the newline character (‘\n’). A backslash preceding a newline is treated as a line continuation.

Keywords

Keywords or 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 keywords:

Aliases

An alias is a name and corresponding value set using the alias built-in command. Wherever the command word of a simple command may occur, and after checking for keywords if a keyword may occur, 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

would become

Aliases are also recognized after an alias whose value ends with a space or tab. For example, if there is also an alias called “nohup” with the value “nohup ”, then the input

would become

Aliases provide a convenient way for naive users to create shorthands for commands without having to learn how to create functions with arguments. Using aliases in scripts is discouraged because the command that defines them must be executed before the code that uses them is parsed. This is fragile and not portable.

An alias name may be escaped in a command line, so that it is not replaced by its alias value, by using quoting characters within or adjacent to the alias name. This is most often done by prefixing an alias name with a backslash to execute a function, built-in, or normal program with the same name. See the Quoting subsection.

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 IEEE Std 1003.2 (“POSIX.2”) document). Essentially though, a line is read and if the first word of the line (or after a control operator) is not a keyword, 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:

1. Leading words of the form “name=value” are stripped off and assigned to the environment of the simple command (they do not affect expansions). Redirection operators and their arguments (as described below) are stripped off and saved for processing.
2. The remaining words are expanded as described in the section called Word Expansions, and the first remaining word is considered the command name and the command is located. The remaining words are considered the arguments of the command. If no command name resulted, then the “name=value” variable assignments recognized in 1) affect the current shell.
3. Redirections are performed 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:

The redir-op is one of the redirection operators mentioned previously. The following gives some examples of how these operators can be used. Note that stdin and stdout are commonly used abbreviations for standard input and standard output respectively.

The following redirection is often called a “here-document”.

[n]<< delimiter
here-doc-text
...
delimiter

All the text on successive lines up to the delimiter 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 subjected to parameter expansion, command substitution, and arithmetic expansion (as described in the section on Word Expansions). If the operator is “<<-” instead of “<<”, then leading tabs in the here-doc-text are stripped.

Search and Execution

There are three types of commands: shell functions, built-in commands, and normal programs. The command is searched for (by name) in that order. The three types of commands are all 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. Then the command given in the function definition is executed. The positional parameters are restored to their original values when the command completes. This all occurs within the current shell.

Shell built-in commands are executed internally to the shell, without spawning a new process. There are two kinds of built-in commands: regular and special. Assignments before special builtins persist after they finish executing and assignment errors, redirection errors and certain operand errors cause a script to be aborted. Special builtins cannot be overridden with a function. Both regular and special builtins can affect the shell in ways normal programs cannot.

Otherwise, if the command name does not match a function or built-in command, 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 (i.e., if it does not begin with the “magic number” whose ASCII representation is “#!”, resulting in an ENOEXEC return value from execve(2)) but appears to be a text file, the shell will run a new instance of sh to interpret it.

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:

1. Command names containing a slash are simply executed without performing any searches.
2. The shell searches each entry in the PATH variable 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.

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 for normal or 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 is terminated by a signal, its exit status is greater than 128. The signal name can be found by passing the exit status to kill -l.

If there is no command word, the exit status is the exit status of the last command substitution executed, or zero if the command does not contain any command substitutions.

Complex Commands

Complex commands are combinations of simple commands with control operators or keywords, together creating a larger complex command. More generally, a command is one of the following:

• simple command
• pipeline
• list or compound-list
• compound command
• function definition

Unless otherwise stated, the exit status of a command is that of the last simple command executed by the command, or zero if no simple command was executed.

Pipelines

A pipeline is a sequence of one or more commands separated by the control operator ‘|’. The standard output of all but the last command is connected to the standard input of the next command. The standard output of the last command is inherited from the shell, as usual.

The format for a pipeline is:

The standard output of command1 is connected to the standard input of command2. The standard input, standard output, or both of a command is considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command.

Note that unlike some other shells, sh executes each process in a pipeline with more than one command in a subshell environment and as a child of the sh process.

If the pipeline is not in the background (discussed later), the shell waits for all commands to complete.

If the keyword ! does not precede the pipeline, the exit status is the exit status of the last command specified in the pipeline. Otherwise, the exit status is the logical NOT of the exit status of the last command. That is, if the last command returns zero, the exit status is 1; if the last command returns greater than zero, the exit status is zero.

Because pipeline assignment of standard input or standard output or both takes place before redirection, it can be modified by redirection. For example:

sends both the standard output and standard error of command1 to the standard input of command2.

A ‘;’ or newline terminator causes the preceding AND-OR-list (described below in the section called Short-Circuit List Operators) to be executed sequentially; an ‘&’ causes asynchronous execution of the preceding AND-OR-list.

Background Commands (&)

If a command is terminated by the control operator ampersand (‘&’), the shell executes the command in a subshell environment (see Grouping Commands Together below) and asynchronously; the shell does not wait for the command to finish before executing the next command.

The format for running a command in background is:

If the shell is not interactive, the standard input of an asynchronous command is set to /dev/null.

The exit status is zero.

Lists (Generally Speaking)

A list is a sequence of zero or more commands separated by newlines, semicolons, or ampersands, and optionally terminated by one of these three characters. 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 onto the next command; otherwise it waits for the command to terminate before proceeding to the next one.

Short-Circuit List Operators

“&&” and “||” are AND-OR list operators. “&&” executes the first command, and then executes the second command if the exit status of the first command is zero. “||” is similar, but executes the second command if the exit status of the first command is nonzero. “&&” and “||” both have the same priority.

Flow-Control Constructs (if, while, for, case)

The syntax of the if command is:

if list
then list
[elif list
then list] ...
[else list]
fi

The exit status is that of selected then or else list, or zero if no list was selected.

The syntax of the while command is:

while list
do list
done

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 exit status is that of the last execution of the second list, or zero if it was never executed.

The syntax of the for command is:

for variable [in word ...]
do list
done

If in and the following words are omitted, in "$@" is used instead. The words are expanded, and then the list is executed repeatedly with the variable set to each word in turn. The do and done commands may be replaced with ‘{’ and ‘}’.

The syntax of the break and continue commands is:

The break command terminates the num innermost for or while loops. The continue command continues with the next iteration of the innermost loop. These are implemented as special built-in commands.

The syntax of the case command is:

case word in
pattern) list ;;
...
esac

The pattern can actually be one or more patterns (see Shell Patterns described later), separated by ‘|’ characters. Tilde expansion, parameter expansion, command substitution, arithmetic expansion and quote removal are applied to the word. Then, each pattern is expanded in turn using tilde expansion, parameter expansion, command substitution and arithmetic expansion and the expanded form of the word is checked against it. If a match is found, the corresponding list is executed. If the selected list is terminated by the control operator ‘;&’ instead of ‘;;’, execution continues with the next list, continuing until a list terminated with ‘;;’ or the end of the case command.

Grouping Commands Together

Commands may be grouped by writing either

(list)

or

{ list; }

The first form executes the commands in a subshell environment. A subshell environment has its own copy of:

1. The current working directory as set by cd.
2. The file creation mask as set by umask.
3. Resource limits as set by ulimit.
4. References to open files.
5. Traps as set by trap.
6. Known jobs.
7. Positional parameters and variables.
8. Shell options.
9. Shell functions.
10. Shell aliases.

These are copied from the parent shell environment, except that trapped (but not ignored) signals are reset to the default action and known jobs are cleared. Any changes do not affect the parent shell environment.

A subshell environment may be implemented as a child process or differently. If job control is enabled in an interactive shell, commands grouped in parentheses can be suspended and continued as a unit.

For compatibility with other shells, two open parentheses in sequence should be separated by whitespace.

The second form never forks another shell, so it is slightly more efficient. Grouping commands together this way allows the user to redirect their output as though they were one program:

{ echo -n "hello"; echo " world"; } > greeting

Functions

The syntax of a function definition is

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 ‘}’.

Variables may be declared to be local to a function by using the local command. This should appear as the first statement of a function, and the syntax is:

The local command is implemented as a built-in command. The exit status is zero unless the command is not in a function or a variable name is invalid.

When a variable is made local, it inherits the initial value and exported and readonly flags from the variable with the same name in the surrounding scope, if there is one. Otherwise, the variable is initially unset. The shell uses dynamic scoping, so that if the variable x is made 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.

The only special parameter that can be made local is ‘-’. Making ‘-’ local causes any shell options (including those that only have long names) that are changed via the set command inside the function to be restored to their original values when the function returns.

The syntax of the return command is

It terminates the current executional scope, returning from the closest nested function or sourced script; if no function or sourced script is being executed, it exits the shell instance. The return command is implemented as a special built-in command.

Variables and Parameters

The shell maintains a set of parameters. A parameter denoted by a name (consisting solely of alphabetics, numerics, and underscores, and starting with an alphabetic or an underscore) is called a variable. When starting up, the shell turns all environment variables with valid names into shell variables. New variables can be set using the form

A parameter can also be denoted by a number or a special character as explained below.

Assignments are expanded differently from other words: tilde expansion is also performed after the equals sign and after any colon and usernames are also terminated by colons, and field splitting and pathname expansion are not performed.

This special expansion applies not only to assignments that form a simple command by themselves or precede a command word, but also to words passed to the export, local or readonly built-in commands that have this form. For this, the builtin's name must be literal (not the result of an expansion) and may optionally be preceded by one or more literal instances of command without options.

Positional Parameters

A positional parameter is a parameter denoted by a number greater than zero. 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 command can also be used to set or reset them.

Special Parameters

Special parameters are parameters denoted by a single special character or the digit zero. They are shown in the following list, exactly as they would appear in input typed by the user or in the source of a shell script.

$*

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) 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 subshell 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 this parameter is referenced, the shell will remember the process ID and its exit status until the wait built-in command reports completion of the process.

$0

(zero) Expands to the name of the shell script if passed on the command line, the name operand if given (with -c) or otherwise argument 0 passed to the shell.

Special Variables

The following variables are set by the shell or have special meaning to it:

CDPATH

The search path used with the cd built-in.

EDITOR

The fallback editor used with the fc built-in. If not set, the default editor is ed(1).

FCEDIT

The default editor used with the fc built-in.

HISTSIZE

The number of previous commands that are accessible.

HOME

The user's home directory, used in tilde expansion and as a default directory for the cd built-in.

IFS

Input Field Separators. This is initialized at startup to ⟨space⟩, ⟨tab⟩, and ⟨newline⟩ in that order. This value also applies if IFS is unset, but not if it is set to the empty string. See the White Space Splitting section for more details.

LINENO

The current line number in the script or function.

MAIL

The name of a mail file, that will be checked for the arrival of new mail. Overridden by MAILPATH.

MAILPATH

A colon (‘:’) separated list of file names, for the shell to check for incoming mail. This variable overrides the MAIL setting. There is a maximum of 10 mailboxes that can be monitored at once.

OPTIND

The index of the next argument to be processed by getopts. This is initialized to 1 at startup.

PATH

The default search path for executables. See the Path Search section for details.

PPID

The parent process ID of the invoked shell. This is set at startup unless this variable is in the environment. A later change of parent process ID is not reflected. A subshell retains the same value of PPID.

PS1

The primary prompt string, which defaults to “$ ”, unless you are the superuser, in which case it defaults to “# ”. PS1 may include any of the following formatting sequences, which are replaced by the given information:

\H

This system's fully-qualified hostname (FQDN).

\h

This system's hostname.

\W

The final component of the current working directory.

\w

The entire path of the current working directory.

\$

Superuser status. “$ ” for normal users and “# ” for superusers.

\\

A literal backslash.

PS2

The secondary prompt string, which defaults to “> ”. PS2 may include any of the formatting sequences from PS1.

PS4

The prefix for the trace output (if -x is active). The default is “+ ”.

Word Expansions

This clause 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:

1. Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic Expansion (these all occur at the same time).
2. Field Splitting is performed on fields generated by step (1) unless the IFS variable is null.
3. Pathname Expansion (unless the -f option is in effect).
4. Quote Removal.

The ‘$’ character is used to introduce parameter expansion, command substitution, or arithmetic expansion.

Tilde Expansion (substituting a user's home directory)

A word beginning with an unquoted tilde character (‘~’) is subjected to tilde expansion. All the characters up to a slash (‘/’) or the end of the word are treated as a username and are replaced with the user's home directory. If the username is missing (as in ~/foobar), the tilde is replaced with the value of the HOME variable (the current user's home directory).

Parameter Expansion

The format for parameter expansion is as follows:

where expression consists of all characters until the matching ‘}’. Any ‘}’ escaped by a backslash or within a single-quoted or double-quoted string, and characters in embedded arithmetic expansions, command substitutions, and variable expansions, are not examined in determining the matching ‘}’. If the variants with ‘+’, ‘-’, ‘=’ or ‘?’ occur within a double-quoted string, as an extension there may be unquoted parts (via double-quotes inside the expansion); ‘}’ within such parts are also not examined in determining the matching ‘}’.

The simplest form for parameter expansion is:

The value, if any, of parameter is substituted.

The parameter name or symbol can be enclosed in braces, which are optional 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:

1. Field splitting is not performed on the results of the expansion, with the exception of the special parameter @.
2. Pathname expansion is not performed on the results of the expansion.

In addition, a parameter expansion can be modified by using one of the following formats.

${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. Quoting inside word does not prevent field splitting or pathname expansion. 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 the shell exits with a nonzero exit status. Otherwise, the value of parameter is substituted. An interactive shell need not exit.

${parameter:+word}

Use Alternate Value. If parameter is unset or null, null is substituted; otherwise, the expansion of word is substituted.

In the parameter expansions shown previously, use of the colon in the format results in a test for a parameter that is unset or null; omission of the colon results in a test for a parameter that is only unset.

The word inherits the type of quoting (unquoted, double-quoted or here-document) from the surroundings, with the exception that a backslash that quotes a closing brace is removed during quote removal.

${#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 one of the special parameters * 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.

${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.

${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.

${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.

Command Substitution

Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution occurs when the command is enclosed as follows:

or the backquoted version:

The shell expands the command substitution by executing command and replacing the command substitution with the standard output of the command, removing sequences of one or more newlines at the end of the substitution. Embedded newlines before the end of the output are not removed; however, during field splitting, they may be translated into spaces depending on the value of IFS and the quoting that is in effect. The command is executed in a subshell environment, except that the built-in commands jobid, jobs, and trap return information about the parent shell environment and times returns information about the same process if they are the only command in a command substitution.

If a command substitution of the $( form begins with a subshell, the $( and ( must be separated by whitespace to avoid ambiguity with arithmetic expansion.

Arithmetic Expansion

Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value. The format for arithmetic expansion is as follows:

The expression is treated as if it were in double-quotes, except that a double-quote inside the expression is not treated specially. The shell expands all tokens in the expression for parameter expansion, command substitution, arithmetic expansion and quote removal.

The allowed expressions are a subset of C expressions, summarized below.

The result of the expression is substituted in decimal.

White Space Splitting (Field Splitting)

In certain contexts, after parameter expansion, command substitution, and arithmetic expansion the shell scans the results of expansions and substitutions that did not occur in double-quotes for field splitting and multiple fields can result.

Characters in IFS that are whitespace (⟨space⟩, ⟨tab⟩, and ⟨newline⟩) are treated differently from other characters in IFS.

Whitespace in IFS at the beginning or end of a word is discarded.

Subsequently, a field is delimited by either

1. a non-whitespace character in IFS with any whitespace in IFS surrounding it, or
2. one or more whitespace characters in IFS.

If a word ends with a non-whitespace character in IFS, there is no empty field after this character.

If no field is delimited, the word is discarded. In particular, if a word consists solely of an unquoted substitution and the result of the substitution is null, it is removed by field splitting even if IFS is null.

Pathname Expansion (File Name Generation)

Unless the -f option 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 Pathname Expansion, the four varieties of parameter expansion for substring processing 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 back quotes 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 ‘]’; if the ‘]’ 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 locale-dependent range of characters may be specified using a minus sign. 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. The character class may be complemented by making an exclamation point (‘!’) the first character of the character class. A caret (‘^’) has the same effect but is non-standard.

To include a ‘]’ in a character class, make it the first character listed (after the ‘!’ or ‘^’, if any). To include a ‘-’, make it the first or last character listed.

Built-in Commands

This section lists the built-in commands.

:

A null command that returns a 0 (true) exit value.

. file

The commands in the specified file are read and executed by the shell. The return command may be used to return to the . command's caller. If file contains any ‘/’ characters, it is used as is. Otherwise, the shell searches the PATH for the file. If it is not found in the PATH, it is sought in the current working directory.

[

A built-in equivalent of test(1).

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 command prints the names and values of all defined aliases (see unalias). Alias values are written with appropriate quoting so that they are suitable for re-input to the shell. Also see the Aliases subsection.

bg [job ...]

Continue the specified jobs (or the current job if no jobs are given) in the background.

bind [-aeklrsv] [key [command]]

List or alter key bindings for the line editor. This command is documented in editrc(5).

break [num]

See the Flow-Control Constructs subsection.

builtin cmd [arg ...]

Execute the specified built-in command, cmd. This is useful when the user wishes to override a shell function with the same name as a built-in command.

cd [-L | -P] [-e] [directory]

 

cd -

Switch to the specified directory, to the directory specified in the HOME environment variable if no directory is specified or to the directory specified in the OLDPWD environment variable if directory is -. If directory does not begin with /, ., or .., then the directories listed in the CDPATH variable will be searched for the specified directory. If CDPATH is unset, the current directory is searched. The format of CDPATH is the same as that of PATH. In an interactive shell, the cd command will print out the name of the directory that it actually switched to if the CDPATH mechanism was used or if directory was -.

If the -P option is specified, .. is handled physically and symbolic links are resolved before .. components are processed. If the -L option is specified, .. is handled logically. This is the default.

The -e option causes cd to return exit status 1 if the full pathname of the new directory cannot be determined reliably or at all. Normally this is not considered an error, although a warning is printed.

If changing the directory fails, the exit status is greater than 1. If the directory is changed, the exit status is 0, or also 1 if -e was given.

chdir

A synonym for the cd built-in command.

command [-p] [utility [argument ...]]

 

command [-p] -v utility

 

command [-p] -V utility

The first form of invocation executes the specified utility, ignoring shell functions in the search. If utility is a special builtin, it is executed as if it were a regular builtin.

If the -p option is specified, the command search is performed using a default value of PATH that is guaranteed to find all of the standard utilities.

If the -v option is specified, utility is not executed but a description of its interpretation by the shell is printed. For ordinary commands the output is the path name; for shell built-in commands, shell functions and keywords only the name is written. Aliases are printed as “alias name=value”.

The -V option is identical to -v except for the output. It prints “utility is description” where description is either the path name to utility, a special shell builtin, a shell builtin, a shell function, a shell keyword or an alias for value.

continue [num]

See the Flow-Control Constructs subsection.

echo [-e | -n] [string ...]

Print a space-separated list of the arguments to the standard output and append a newline character.

-n

Suppress the output of the trailing newline.

-e

Process C-style backslash escape sequences. The echo command understands the following character escapes:

\a

Alert (ring the terminal bell)

\b

Backspace

\c

Suppress the trailing newline (this has the side-effect of truncating the line if it is not the last character)

\e

The ESC character (ASCII 0x1b)

\f

Formfeed

\n

Newline

\r

Carriage return

\t

Horizontal tab

\v

Vertical tab

\\

Literal backslash

\0nnn

(Zero) The character whose octal value is nnn

If string is not enclosed in quotes then the backslash itself must be escaped with a backslash to protect it from the shell. For example

$ echo -e "a\vb"
a
 b
$ echo -e a\\vb
a
 b
$ echo -e "a\\b"
a\b
$ echo -e a\\\\b
a\b

Only one of the -e and -n options may be specified.

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 command or function). Any redirections on the exec command are marked as permanent, so that they are not undone when the exec command finishes.

exit [exitstatus]

Terminate the shell process. If exitstatus is given it is used as the exit status of the shell. Otherwise, if the shell is executing an EXIT trap, the exit status of the last command before the trap is used; if the shell is executing a trap for a signal, the shell exits by resending the signal to itself. Otherwise, the exit status of the preceding command is used. The exit status should be an integer between 0 and 255.

export name ...

 

export [-p]

The specified names are exported so that they will appear in the environment of subsequent commands. The only way to un-export a variable is to unset it. The shell allows the value of a variable to be set at the same time as it is exported by writing

export name=value

With no arguments the export command lists the names of all exported variables. If the -p option is specified, the exported variables are printed as “export name=value” lines, suitable for re-input to the shell.

false

A null command that returns a non-zero (false) exit value.

fc [-e editor] [first [last]]

 

fc -l [-nr] [first [last]]

 

fc -s [old=new] [first]

The fc built-in command 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:

[+]num

A positive number representing a command number; command numbers can be displayed with the -l option.

-num

A negative decimal number representing the command that was executed num 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 variables affect the execution of fc:

FCEDIT

Name of the editor to use for history editing.

HISTSIZE

The number of previous commands that are accessible.

fg [job]

Move the specified job or the current job to the foreground.

getopts optstring var

The POSIX getopts command. The getopts command deprecates the older getopt(1) command. The first argument should be a series of letters, each possibly followed by a colon which indicates that the option takes an argument. The specified variable is set to the parsed option. The index of the next argument is placed into the shell variable OPTIND. If an option takes an argument, it is placed into the shell variable OPTARG. If an invalid option is encountered, var is set to ‘?’. It returns a false value (1) when it encounters the end of the options. A new set of arguments may be parsed by assigning OPTIND=1.

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.

With arguments, the hash command removes each specified command from the hash table (unless they are functions) and then locates it. 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.

jobid [job]

Print the process IDs of the processes in the specified job. If the job argument is omitted, use the current job.

jobs [-lps] [job ...]

Print information about the specified jobs, or all jobs if no job argument is given. The information printed includes job ID, status and command name.

If the -l option is specified, the PID of each job is also printed. If the -p option is specified, only the process IDs for the process group leaders are printed, one per line. If the -s option is specified, only the PIDs of the job commands are printed, one per line.

kill

A built-in equivalent of kill(1) that additionally supports sending signals to jobs.

local [variable ...] [-]

See the Functions subsection.

printf

A built-in equivalent of printf(1).

pwd [-L | -P]

Print the path of the current directory. The built-in command may differ from the program of the same name because the built-in command remembers what the current directory is rather than recomputing it each time. This makes it faster. However, if the current directory is renamed, the built-in version of pwd(1) will continue to print the old name for the directory.

If the -P option is specified, symbolic links are resolved. If the -L option is specified, the shell's notion of the current directory is printed (symbolic links are not resolved). This is the default.

read [-p prompt] [-t timeout] [-er] 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 section on White Space Splitting (Field Splitting) 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.

Backslashes are treated specially, unless the -r option is specified. If a backslash is followed by a newline, the backslash and the newline will be deleted. If a backslash is followed by any other character, the backslash will be deleted and the following character will be treated as though it were not in IFS, even if it is.

If the -t option is specified and the timeout elapses before a complete line of input is supplied, the read command will return an exit status as if terminated by SIGALRM without assigning any values. The timeout value may optionally be followed by one of ‘s’, ‘m’ or ‘h’ to explicitly specify seconds, minutes or hours. If none is supplied, ‘s’ is assumed.

The -e option exists only for backward compatibility with older scripts.

The exit status is 0 on success, 1 on end of file, between 2 and 128 if an error occurs and greater than 128 if a trapped signal interrupts read.

readonly [-p] [name ...]

Each specified name is marked as read only, so that it cannot be subsequently modified or unset. The shell allows the value of a variable to be set at the same time as it is marked read only by using the following form:

readonly name=value

With no arguments the readonly command lists the names of all read only variables. If the -p option is specified, the read-only variables are printed as “readonly name=value” lines, suitable for re-input to the shell.

return [exitstatus]

See the Functions subsection.

set [-/+abCEefIimnpTuVvx] [-/+o longname] [-c string] [-- arg ...]

The set command performs three different functions:

• With no arguments, it lists the values of all shell variables.
• If options are given, either in short form or using the long “-/+o longname” form, it sets or clears the specified options as described in the section called Argument List Processing.
• If the “--” option is specified, set will replace the shell's positional parameters with the subsequent arguments. If no arguments follow the “--” option, all the positional parameters will be cleared, which is equivalent to executing the command “shift $#”. The “--” flag may be omitted when specifying arguments to be used as positional replacement parameters. This is not recommended, because the first argument may begin with a dash (‘-’) or a plus (‘+’), which the set command will interpret as a request to enable or disable options.

setvar variable value

Assigns the specified value to the specified variable. The setvar command is intended to be used in functions that assign values to variables whose names are passed as parameters. In general it is better to write “variable=value” rather than using setvar.

shift [n]

Shift the positional parameters n times, or once if n is not specified. A shift sets the value of $1 to the value of $2, the value of $2 to the value of $3, and so on, decreasing the value of $# by one. For portability, shifting if there are zero positional parameters should be avoided, since the shell may abort.

test

A built-in equivalent of test(1).

times

Print the amount of time spent executing the shell process and its children. The first output line shows the user and system times for the shell process itself, the second one contains the user and system times for the children.

trap [action] signal ...

 

trap -l

Cause the shell to parse and execute action when any specified signal is received. The signals are specified by name or number. In addition, the pseudo-signal EXIT may be used to specify an action that is performed when the shell terminates. The action may be an empty string or a dash (‘-’); the former causes the specified signal to be ignored and the latter causes the default action to be taken. Omitting the action and using only signal numbers is another way to request the default action. In a subshell or utility environment, the shell resets trapped (but not ignored) signals to the default action. The trap command has no effect on signals that were ignored on entry to the shell.

Option -l causes the trap command to display a list of valid signal names.

true

A null command that returns a 0 (true) exit value.

type [name ...]

Interpret each name as a command and print the resolution of the command search. Possible resolutions are: shell keyword, alias, special shell builtin, shell builtin, 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 [-HSabcdfklmnopstuvw] [limit]

Set or display resource limits (see getrlimit(2)). If limit is specified, the named resource will be set; otherwise the current resource value will be displayed.

If -H is specified, the hard limits will be set or displayed. While everybody is allowed to reduce a hard limit, only the superuser can increase it. The -S option specifies the soft limits instead. When displaying limits, only one of -S or -H can be given. The default is to display the soft limits, and to set both the hard and the soft limits.

Option -a causes the ulimit command to display all resources. The parameter limit is not acceptable in this mode.

The remaining options specify which resource value is to be displayed or modified. They are mutually exclusive.

-b sbsize

The maximum size of socket buffer usage, in bytes.

-c coredumpsize

The maximal size of core dump files, in 512-byte blocks. Setting coredumpsize to 0 prevents core dump files from being created.

-d datasize

The maximal size of the data segment of a process, in kilobytes.

-f filesize

The maximal size of a file, in 512-byte blocks.

-k kqueues

The maximal number of kqueues (see kqueue(2)) for this user ID.

-l lockedmem

The maximal size of memory that can be locked by a process, in kilobytes.

-m memoryuse

The maximal resident set size of a process, in kilobytes.

-n nofiles

The maximal number of descriptors that could be opened by a process.

-o umtxp

The maximal number of process-shared locks (see pthread(3)) for this user ID.

-p pseudoterminals

The maximal number of pseudo-terminals for this user ID.

-s stacksize

The maximal size of the stack segment, in kilobytes.

-t time

The maximal amount of CPU time to be used by each process, in seconds.

-u userproc

The maximal number of simultaneous processes for this user ID.

-v virtualmem

The maximal virtual size of a process, in kilobytes.

-w swapuse

The maximum amount of swap space reserved or used for this user ID, in kilobytes.

umask [-S] [mask]

Set the file creation mask (see umask(2)) to the octal or symbolic (see chmod(1)) value specified by mask. If the argument is omitted, the current mask value is printed. If the -S option is specified, the output is symbolic, otherwise the output is octal.

unalias [-a] [name ...]

The specified alias names are removed. If -a is specified, all aliases are removed.

unset [-fv] name ...

The specified variables or functions are unset and unexported. If the -v option is specified or no options are given, the name arguments are treated as variable names. If the -f option is specified, the name arguments are treated as function names.

wait [job ...]

Wait for each specified job to complete and return the exit status of the last process in the last specified job. If any job specified is unknown to the shell, it is treated as if it were a known job that exited with exit status 127. If no operands are given, wait for all jobs to complete and return an exit status of zero.

Command Line Editing

When sh is being used interactively from a terminal, the current command and the command history (see fc in Built-in Commands) can be edited using vi-mode command line editing. This mode uses commands similar to a subset of those described in the vi(1) man page. The command “set -o vi” (or “set -V”) enables vi-mode editing and places sh into vi insert mode. With vi-mode enabled, sh can be switched between insert mode and command mode by typing ⟨ESC⟩. Hitting ⟨return⟩ while in command mode will pass the line to the shell.

Similarly, the “set -o emacs” (or “set -E”) command can be used to enable a subset of emacs-style command line editing features.