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SET(1P) POSIX Programmer's Manual SET(1P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

NAME
set - set or unset options and positional parameters

SYNOPSIS
set [-abCefmnuvx][-h][-o option][argument...]

set [+abCefmnuvx][+h][+o option][argument...]

set -- [argument...]

set -o

set +o

DESCRIPTION
If no options or arguments are specified, set shall write the names and values of all shell variables in the collation
sequence of the current locale. Each name shall start on a separate line, using the format:

"%s=%s\n", <name>, <value>

The value string shall be written with appropriate quoting; see the description of shell quoting in Quoting . The output
shall be suitable for reinput to the shell, setting or resetting, as far as possible, the variables that are currently
set; read-only variables cannot be reset.

When options are specified, they shall set or unset attributes of the shell, as described below. When arguments are spec-
ified, they cause positional parameters to be set or unset, as described below. Setting or unsetting attributes and posi-
tional parameters are not necessarily related actions, but they can be combined in a single invocation of set.

The set special built-in shall support the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax
Guidelines except that options can be specified with either a leading hyphen (meaning enable the option) or plus sign
(meaning disable it) unless otherwise specified.

Implementations shall support the options in the following list in both their hyphen and plus-sign forms. These options
can also be specified as options to sh.

-a When this option is on, the export attribute shall be set for each variable to which an assignment is performed;
see the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.21, Variable Assignment. If the assignment pre-
cedes a utility name in a command, the export attribute shall not persist in the current execution environment
after the utility completes, with the exception that preceding one of the special built-in utilities causes the
export attribute to persist after the built-in has completed. If the assignment does not precede a utility name in
the command, or if the assignment is a result of the operation of the getopts or read utilities, the export
attribute shall persist until the variable is unset.

-b This option shall be supported if the implementation supports the User Portability Utilities option. It shall
cause the shell to notify the user asynchronously of background job completions. The following message is written
to standard error:

"[%d]%c %s%s\n", <job-number>, <current>, <status>, <job-name>

where the fields shall be as follows:

<current>
The character '+' identifies the job that would be used as a default for the fg or bg utilities; this job can also
be specified using the job_id "%+" or "%%" . The character '-' identifies the job that would become the default if
the current default job were to exit; this job can also be specified using the job_id "%-" . For other jobs, this
field is a <space>. At most one job can be identified with '+' and at most one job can be identified with '-' . If
there is any suspended job, then the current job shall be a suspended job. If there are at least two suspended
jobs, then the previous job also shall be a suspended job.

<job-number>
A number that can be used to identify the process group to the wait, fg, bg, and kill utilities. Using these util-
ities, the job can be identified by prefixing the job number with '%' .

<status>
Unspecified.

<job-name>
Unspecified.

When the shell notifies the user a job has been completed, it may remove the job's process ID from the list of those
known in the current shell execution environment; see Asynchronous Lists . Asynchronous notification shall not be enabled
by default.

-C (Uppercase C.) Prevent existing files from being overwritten by the shell's '>' redirection operator (see Redi-
recting Output ); the ">|" redirection operator shall override this noclobber option for an individual file.

-e When this option is on, if a simple command fails for any of the reasons listed in Consequences of Shell Errors or
returns an exit status value >0, and is not part of the compound list following a while, until, or if keyword, and
is not a part of an AND or OR list, and is not a pipeline preceded by the ! reserved word, then the shell shall
immediately exit.

-f The shell shall disable pathname expansion.

-h Locate and remember utilities invoked by functions as those functions are defined (the utilities are normally
located when the function is executed).

-m This option shall be supported if the implementation supports the User Portability Utilities option. All jobs
shall be run in their own process groups. Immediately before the shell issues a prompt after completion of the
background job, a message reporting the exit status of the background job shall be written to standard error. If a
foreground job stops, the shell shall write a message to standard error to that effect, formatted as described by
the jobs utility. In addition, if a job changes status other than exiting (for example, if it stops for input or
output or is stopped by a SIGSTOP signal), the shell shall write a similar message immediately prior to writing
the next prompt. This option is enabled by default for interactive shells.

-n The shell shall read commands but does not execute them; this can be used to check for shell script syntax errors.
An interactive shell may ignore this option.

-o Write the current settings of the options to standard output in an unspecified format.

+o Write the current option settings to standard output in a format that is suitable for reinput to the shell as com-
mands that achieve the same options settings.

-o option

This option is supported if the system supports the User Portability Utilities option. It shall set various
options, many of which shall be equivalent to the single option letters. The following values of option shall be
supported:

allexport
Equivalent to -a.

errexit
Equivalent to -e.

ignoreeof
Prevent an interactive shell from exiting on end-of-file. This setting prevents accidental logouts when <con-
trol>-D is entered. A user shall explicitly exit to leave the interactive shell.

monitor
Equivalent to -m. This option is supported if the system supports the User Portability Utilities option.

noclobber
Equivalent to -C (uppercase C).

noglob
Equivalent to -f.

noexec
Equivalent to -n.

nolog
Prevent the entry of function definitions into the command history; see Command History List .

notify
Equivalent to -b.

nounset
Equivalent to -u.

verbose
Equivalent to -v.

vi
Allow shell command line editing using the built-in vi editor. Enabling vi mode shall disable any other command
line editing mode provided as an implementation extension.

It need not be possible to set vi mode on for certain block-mode terminals.

xtrace
Equivalent to -x.

-u The shell shall write a message to standard error when it tries to expand a variable that is not set and immedi-
ately exit. An interactive shell shall not exit.

-v The shell shall write its input to standard error as it is read.

-x The shell shall write to standard error a trace for each command after it expands the command and before it exe-
cutes it. It is unspecified whether the command that turns tracing off is traced.

The default for all these options shall be off (unset) unless stated otherwise in the description of the option or unless
the shell was invoked with them on; see sh.

The remaining arguments shall be assigned in order to the positional parameters. The special parameter '#' shall be set
to reflect the number of positional parameters. All positional parameters shall be unset before any new values are
assigned.

The special argument "--" immediately following the set command name can be used to delimit the arguments if the first
argument begins with '+' or '-', or to prevent inadvertent listing of all shell variables when there are no arguments.
The command set -- without argument shall unset all positional parameters and set the special parameter '#' to zero.

OPTIONS
See the DESCRIPTION.

OPERANDS
See the DESCRIPTION.

STDIN
Not used.

INPUT FILES
None.

ENVIRONMENT VARIABLES
None.

ASYNCHRONOUS EVENTS
Default.

STDOUT
See the DESCRIPTION.

STDERR
The standard error shall be used only for diagnostic messages.

OUTPUT FILES
None.

EXTENDED DESCRIPTION
None.

EXIT STATUS
Zero.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

APPLICATION USAGE
None.

EXAMPLES
Write out all variables and their values:

set

Set $1, $2, and $3 and set "$#" to 3:

set c a b

Turn on the -x and -v options:

set -xv

Unset all positional parameters:

set --

Set $1 to the value of x, even if it begins with '-' or '+' :

set -- "$x"

Set the positional parameters to the expansion of x, even if x expands with a leading '-' or '+' :

set -- $x

RATIONALE
The set -- form is listed specifically in the SYNOPSIS even though this usage is implied by the Utility Syntax Guide-
lines. The explanation of this feature removes any ambiguity about whether the set -- form might be misinterpreted as
being equivalent to set without any options or arguments. The functionality of this form has been adopted from the Korn-
Shell. In System V, set -- only unsets parameters if there is at least one argument; the only way to unset all parameters
is to use shift. Using the KornShell version should not affect System V scripts because there should be no reason to
issue it without arguments deliberately; if it were issued as, for example:

set -- "$@"

and there were in fact no arguments resulting from "$@", unsetting the parameters would have no result.

The set + form in early proposals was omitted as being an unnecessary duplication of set alone and not widespread histor-
ical practice.

The noclobber option was changed to allow set -C as well as the set -o noclobber option. The single-letter version was
added so that the historical "$-" paradigm would not be broken; see Special Parameters .

The -h flag is related to command name hashing and is only required on XSI-conformant systems.

The following set flags were omitted intentionally with the following rationale:

-k The -k flag was originally added by the author of the Bourne shell to make it easier for users of pre-release ver-
sions of the shell. In early versions of the Bourne shell the construct set name= value had to be used to assign
values to shell variables. The problem with -k is that the behavior affects parsing, virtually precluding writing
any compilers. To explain the behavior of -k, it is necessary to describe the parsing algorithm, which is imple-
mentation-defined. For example:

set -k; echo name=value

and:

set -k
echo name=value

behave differently. The interaction with functions is even more complex. What is more, the -k flag is never needed,
since the command line could have been reordered.

-t The -t flag is hard to specify and almost never used. The only known use could be done with here-documents. More-
over, the behavior with ksh and sh differs. The reference page says that it exits after reading and executing one
command. What is one command? If the input is date; date, sh executes both date commands while ksh does only the
first.

Consideration was given to rewriting set to simplify its confusing syntax. A specific suggestion was that the unset util-
ity should be used to unset options instead of using the non- getopt() -able + option syntax. However, the conclusion was
reached that the historical practice of using + option was satisfactory and that there was no compelling reason to modify
such widespread historical practice.

The -o option was adopted from the KornShell to address user needs. In addition to its generally friendly interface, -o
is needed to provide the vi command line editing mode, for which historical practice yields no single-letter option name.
(Although it might have been possible to invent such a letter, it was recognized that other editing modes would be devel-
oped and -o provides ample name space for describing such extensions.)

Historical implementations are inconsistent in the format used for -o option status reporting. The +o format without an
option-argument was added to allow portable access to the options that can be saved and then later restored using, for
instance, a dot script.

Historically, sh did trace the command set +x, but ksh did not.

The ignoreeof setting prevents accidental logouts when the end-of-file character (typically <control>-D) is entered. A
user shall explicitly exit to leave the interactive shell.

The set -m option was added to apply only to the UPE because it applies primarily to interactive use, not shell script
applications.

The ability to do asynchronous notification became available in the 1988 version of the KornShell. To have it occur, the
user had to issue the command:

trap "jobs -n" CLD

The C shell provides two different levels of an asynchronous notification capability. The environment variable notify is
analogous to what is done in set -b or set -o notify. When set, it notifies the user immediately of background job com-
pletions. When unset, this capability is turned off.

The other notification ability comes through the built-in utility notify. The syntax is:

notify [%job ... ]

By issuing notify with no operands, it causes the C shell to notify the user asynchronously when the state of the current
job changes. If given operands, notify asynchronously informs the user of changes in the states of the specified jobs.

To add asynchronous notification to the POSIX shell, neither the KornShell extensions to trap, nor the C shell notify
environment variable seemed appropriate ( notify is not a proper POSIX environment variable name).

The set -b option was selected as a compromise.

The notify built-in was considered to have more functionality than was required for simple asynchronous notification.

FUTURE DIRECTIONS
None.