4.3.1 The Set Builtin

This builtin is so complicated that it deserves its own section. set allows you to change the values of shell options and set the positional parameters, or to display the names and values of shell variables.

set

          set [--abefhkmnptuvxBCEHPT] [-o option] [argument ...]
          set [+abefhkmnptuvxBCEHPT] [+o option] [argument ...]

If no options or arguments are supplied, set displays the names and values of all shell variables and functions, sorted according to the current locale, in a format that may be reused as input for setting or resetting the currently-set variables. Read-only variables cannot be reset. In posix mode, only shell variables are listed.

When options are supplied, they set or unset shell attributes. Options, if specified, have the following meanings:

-a

Mark variables and function which are modified or created for export to the environment of subsequent commands.

-b

Cause the status of terminated background jobs to be reported immediately, rather than before printing the next primary prompt.

-e

Exit immediately if a pipeline (see Pipelines), which may consist of a single simple command (see Simple Commands), a subshell command enclosed in parentheses (see Command Grouping), or one of the commands executed as part of a command list enclosed by braces (see Command Grouping) returns a non-zero status. The shell does not exit if the command that fails is part of the command list immediately following a while or until keyword, part of the test in an if statement, part of any command executed in a && or || list except the command following the final && or ||, any command in a pipeline but the last, or if the command's return status is being inverted with !. A trap on ERR, if set, is executed before the shell exits.

This option applies to the shell environment and each subshell environment separately (see Command Execution Environment), and may cause subshells to exit before executing all the commands in the subshell.

-f

Disable filename expansion (globbing).

-h

Locate and remember (hash) commands as they are looked up for execution. This option is enabled by default.

-k

All arguments in the form of assignment statements are placed in the environment for a command, not just those that precede the command name.

-m

Job control is enabled (see Job Control).

-n

Read commands but do not execute them; this may be used to check a script for syntax errors. This option is ignored by interactive shells.

-o option-name

Set the option corresponding to option-name:

allexport

Same as -a.

braceexpand

Same as -B.

emacs

Use an emacs-style line editing interface (see Command Line Editing). This also affects the editing interface used for read -e.

errexit

Same as -e.

errtrace

Same as -E.

functrace

Same as -T.

hashall

Same as -h.

histexpand

Same as -H.

history

Enable command history, as described in Bash History Facilities. This option is on by default in interactive shells.

ignoreeof

An interactive shell will not exit upon reading EOF.

keyword

Same as -k.

monitor

Same as -m.

noclobber

Same as -C.

noexec

Same as -n.

noglob

Same as -f.

nolog

Currently ignored.

notify

Same as -b.

nounset

Same as -u.

onecmd

Same as -t.

physical

Same as -P.

pipefail

If set, the return value of a pipeline is the value of the last (rightmost) command to exit with a non-zero status, or zero if all commands in the pipeline exit successfully. This option is disabled by default.

posix

Change the behavior of Bash where the default operation differs from the posix standard to match the standard (see Bash POSIX Mode). This is intended to make Bash behave as a strict superset of that standard.

privileged

Same as -p.

verbose

Same as -v.

vi

Use a vi-style line editing interface. This also affects the editing interface used for read -e.

xtrace

Same as -x.

-p

Turn on privileged mode. In this mode, the $BASH_ENV and $ENV files are not processed, shell functions are not inherited from the environment, and the SHELLOPTS, BASHOPTS, CDPATH and GLOBIGNORE variables, if they appear in the environment, are ignored. If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, these actions are taken and the effective user id is set to the real user id. If the -p option is supplied at startup, the effective user id is not reset. Turning this option off causes the effective user and group ids to be set to the real user and group ids.

-t

Exit after reading and executing one command.

-u

Treat unset variables and parameters other than the special parameters ‘@’ or ‘*’ as an error when performing parameter expansion. An error message will be written to the standard error, and a non-interactive shell will exit.

-v

Print shell input lines as they are read.

-x

Print a trace of simple commands, for commands, case commands, select commands, and arithmetic for commands and their arguments or associated word lists after they are expanded and before they are executed. The value of the PS4 variable is expanded and the resultant value is printed before the command and its expanded arguments.

-B

The shell will perform brace expansion (see Brace Expansion). This option is on by default.

-C

Prevent output redirection using ‘>’, ‘>&’, and ‘<>’ from overwriting existing files.

-E

If set, any trap on ERR is inherited by shell functions, command substitutions, and commands executed in a subshell environment. The ERR trap is normally not inherited in such cases.

-H

Enable ‘!’ style history substitution (see History Interaction). This option is on by default for interactive shells.

-P

If set, do not follow symbolic links when performing commands such as cd which change the current directory. The physical directory is used instead. By default, Bash follows the logical chain of directories when performing commands which change the current directory.

For example, if /usr/sys is a symbolic link to /usr/local/sys then:

               $ cd /usr/sys; echo $PWD
               /usr/sys
               $ cd ..; pwd
               /usr

If set -P is on, then:

               $ cd /usr/sys; echo $PWD
               /usr/local/sys
               $ cd ..; pwd
               /usr/local

-T

If set, any trap on DEBUG and RETURN are inherited by shell functions, command substitutions, and commands executed in a subshell environment. The DEBUG and RETURN traps are normally not inherited in such cases.

--

If no arguments follow this option, then the positional parameters are unset. Otherwise, the positional parameters are set to the arguments, even if some of them begin with a ‘-’.

-

Signal the end of options, cause all remaining arguments to be assigned to the positional parameters. The -x and -v options are turned off. If there are no arguments, the positional parameters remain unchanged.

Using ‘+’ rather than ‘-’ causes these options to be turned off. The options can also be used upon invocation of the shell. The current set of options may be found in $-.

The remaining N arguments are positional parameters and are assigned, in order, to $1, $2, ... $N. The special parameter # is set to N.

The return status is always zero unless an invalid option is supplied.