man-pages/man1p/set.1p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SET" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" set 
.SH NAME
set \- set or unset options and positional parameters
.SH SYNOPSIS
.LP
\fBset\fP \fB[\fP\fB-abCefmnuvx\fP\fB][\fP\fB-h\fP\fB][\fP\fB-o\fP
\fIoption\fP\fB][\fP\fIargument\fP\fB...\fP\fB]\fP\fB
.br
.sp
set\fP \fB[\fP\fB+abCefmnuvx\fP\fB][\fP\fB+h\fP\fB][\fP\fB+o\fP
\fIoption\fP\fB][\fP\fIargument\fP\fB...]\fP\fB
.br
.sp
set --\fP \fB[\fP\fIargument\fP\fB...\fP\fB]\fP\fB
.br
.sp
set -o
.br
.sp
set +o
.br
\fP
.SH DESCRIPTION
.LP
If no \fIoption\fPs or \fIargument\fPs are specified, \fIset\fP shall
write the names and values of all shell variables in
the collation sequence of the current locale. Each \fIname\fP shall
start on a separate line, using the format:
.sp
.RS
.nf

\fB"%s=%s\\n", <\fP\fIname\fP\fB>, <\fP\fIvalue\fP\fB>
\fP
.fi
.RE
.LP
The \fIvalue\fP string shall be written with appropriate quoting;
see the description of shell quoting in \fIQuoting\fP . 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.
.LP
When options are specified, they shall set or unset attributes of
the shell, as described below. When \fIargument\fPs are
specified, they cause positional parameters to be set or unset, as
described below. Setting or unsetting attributes and positional
parameters are not necessarily related actions, but they can be combined
in a single invocation of \fIset\fP.
.LP
The \fIset\fP 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.
.LP
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 \fIsh\fP.
.TP 7
\fB-a\fP
When this option is on, the \fIexport\fP 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 precedes a utility name in a command,
the \fIexport\fP 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 \fIexport\fP 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 \fIgetopts\fP or
\fIread\fP utilities, the \fIexport\fP attribute shall persist until
the variable is
unset.
.TP 7
\fB-b\fP
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: 
.sp
.RS
.nf

\fB"[%d]%c %s%s\\n", <\fP\fIjob-number\fP\fB>, <\fP\fIcurrent\fP\fB>, <status>, <job-name>
\fP
.fi
.RE
.LP
where the fields shall be as follows:
.TP 7
<\fIcurrent\fP>
.RS
The character \fB'+'\fP identifies the job that would be used as a
default for the \fIfg\fP or \fIbg\fP utilities; this job can also
be specified
using the \fIjob_id\fP \fB"%+"\fP or \fB"%%"\fP . The character \fB'-'\fP
identifies the job that would become the
default if the current default job were to exit; this job can also
be specified using the \fIjob_id\fP \fB"%-"\fP . For other
jobs, this field is a <space>. At most one job can be identified with
\fB'+'\fP and at most one job can be identified with
\fB'-'\fP . 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.
.RE
.TP 7
<\fIjob-number\fP>
.RS
A number that can be used to identify the process group to the \fIwait\fP,
\fIfg\fP, \fIbg\fP, and \fIkill\fP utilities. Using these utilities,
the job can be identified by prefixing the job number
with \fB'%'\fP .
.RE
.TP 7
<\fIstatus\fP>
.RS
Unspecified.
.RE
.TP 7
<\fIjob-name\fP>
.RS
Unspecified.
.RE
.sp
.LP
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 \fIAsynchronous Lists\fP
\&. Asynchronous
notification shall not be enabled by default.
.TP 7
\fB-C\fP
(Uppercase C.) Prevent existing files from being overwritten by the
shell's \fB'>'\fP redirection operator (see \fIRedirecting Output\fP
); the \fB">|"\fP redirection operator shall override this
\fInoclobber\fP option for an individual file.
.TP 7
\fB-e\fP
When this option is on, if a simple command fails for any of the reasons
listed in \fIConsequences of Shell Errors\fP or returns an exit status
value >0, and is not part of
the compound list following a \fBwhile\fP, \fBuntil\fP, or \fBif\fP
keyword, and is not a part of an AND or OR list, and is not
a pipeline preceded by the \fB!\fP reserved word, then the shell shall
immediately exit.
.TP 7
\fB-f\fP
The shell shall disable pathname expansion.
.TP 7
\fB-h\fP
Locate and remember utilities invoked by functions as those functions
are defined (the utilities are normally located when the
function is executed). 
.TP 7
\fB-m\fP
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 \fIjobs\fP 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.
.TP 7
\fB-n\fP
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.
.TP 7
\fB-o\fP
Write the current settings of the options to standard output in an
unspecified format.
.TP 7
\fB+o\fP
Write the current option settings to standard output in a format that
is suitable for reinput to the shell as commands that
achieve the same options settings.
.TP 7
\fB-o\ \fP \fIoption\fP
.sp
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 \fIoption\fP shall be supported: 
.TP 7
\fIallexport\fP
.RS
Equivalent to \fB-a\fP.
.RE
.TP 7
\fIerrexit\fP
.RS
Equivalent to \fB-e\fP.
.RE
.TP 7
\fIignoreeof\fP
.RS
Prevent an interactive shell from exiting on end-of-file. This setting
prevents accidental logouts when <control>-D is
entered. A user shall explicitly \fIexit\fP to leave the interactive
shell.
.RE
.TP 7
\fImonitor\fP
.RS
Equivalent to \fB-m\fP. This option is supported if the system supports
the User Portability Utilities option.
.RE
.TP 7
\fInoclobber\fP
.RS
Equivalent to \fB-C\fP (uppercase C).
.RE
.TP 7
\fInoglob\fP
.RS
Equivalent to \fB-f\fP.
.RE
.TP 7
\fInoexec\fP
.RS
Equivalent to \fB-n\fP.
.RE
.TP 7
\fInolog\fP
.RS
Prevent the entry of function definitions into the command history;
see \fICommand History List\fP
\&.
.RE
.TP 7
\fInotify\fP
.RS
Equivalent to \fB-b\fP.
.RE
.TP 7
\fInounset\fP
.RS
Equivalent to \fB-u\fP.
.RE
.TP 7
\fIverbose\fP
.RS
Equivalent to \fB-v\fP.
.RE
.TP 7
\fIvi\fP
.RS
Allow shell command line editing using the built-in \fIvi\fP editor.
Enabling \fIvi\fP mode shall disable any other command line editing
mode provided as an implementation
extension. 
.LP
It need not be possible to set \fIvi\fP mode on for certain block-mode
terminals.
.RE
.TP 7
\fIxtrace\fP
.RS
Equivalent to \fB-x\fP.
.RE
.sp
.TP 7
\fB-u\fP
The shell shall write a message to standard error when it tries to
expand a variable that is not set and immediately exit. An
interactive shell shall not exit.
.TP 7
\fB-v\fP
The shell shall write its input to standard error as it is read.
.TP 7
\fB-x\fP
The shell shall write to standard error a trace for each command after
it expands the command and before it executes it. It is
unspecified whether the command that turns tracing off is traced.
.sp
.LP
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 \fIsh\fP.
.LP
The remaining arguments shall be assigned in order to the positional
parameters. The special parameter \fB'#'\fP shall be set
to reflect the number of positional parameters. All positional parameters
shall be unset before any new values are assigned.
.LP
The special argument \fB"--"\fP immediately following the \fIset\fP
command name can be used to delimit the arguments if the
first argument begins with \fB'+'\fP or \fB'-'\fP , or to prevent
inadvertent listing of all shell variables when there are
no arguments. The command \fIset\fP \fB--\fP without \fIargument\fP
shall unset all positional parameters and set the special
parameter \fB'#'\fP to zero.
.SH OPTIONS
.LP
See the DESCRIPTION.
.SH OPERANDS
.LP
See the DESCRIPTION.
.SH STDIN
.LP
Not used.
.SH INPUT FILES
.LP
None.
.SH ENVIRONMENT VARIABLES
.LP
None.
.SH ASYNCHRONOUS EVENTS
.LP
Default.
.SH STDOUT
.LP
See the DESCRIPTION.
.SH STDERR
.LP
The standard error shall be used only for diagnostic messages.
.SH OUTPUT FILES
.LP
None.
.SH EXTENDED DESCRIPTION
.LP
None.
.SH EXIT STATUS
.LP
Zero.
.SH CONSEQUENCES OF ERRORS
.LP
Default.
.LP
\fIThe following sections are informative.\fP
.SH APPLICATION USAGE
.LP
None.
.SH EXAMPLES
.LP
Write out all variables and their values:
.sp
.RS
.nf

\fBset
\fP
.fi
.RE
.LP
Set $1, $2, and $3 and set \fB"$#"\fP to 3:
.sp
.RS
.nf

\fBset c a b
\fP
.fi
.RE
.LP
Turn on the \fB-x\fP and \fB-v\fP options:
.sp
.RS
.nf

\fBset -xv
\fP
.fi
.RE
.LP
Unset all positional parameters:
.sp
.RS
.nf

\fBset --
\fP
.fi
.RE
.LP
Set $1 to the value of \fIx\fP, even if it begins with \fB'-'\fP or
\fB'+'\fP :
.sp
.RS
.nf

\fBset -- "$x"
\fP
.fi
.RE
.LP
Set the positional parameters to the expansion of \fIx\fP, even if
\fIx\fP expands with a leading \fB'-'\fP or \fB'+'\fP
:
.sp
.RS
.nf

\fBset -- $x
\fP
.fi
.RE
.SH RATIONALE
.LP
The \fIset\fP -- form is listed specifically in the SYNOPSIS even
though this usage is implied by the Utility Syntax
Guidelines. The explanation of this feature removes any ambiguity
about whether the \fIset\fP -- form might be misinterpreted as
being equivalent to \fIset\fP without any options or arguments. The
functionality of this form has been adopted from the
KornShell. In System V, \fIset\fP -- only unsets parameters if there
is at least one argument; the only way to unset all
parameters is to use \fIshift\fP. 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:
.sp
.RS
.nf

\fBset -- "$@"
\fP
.fi
.RE
.LP
and there were in fact no arguments resulting from \fB"$@"\fP , unsetting
the parameters would have no result.
.LP
The \fIset\fP + form in early proposals was omitted as being an unnecessary
duplication of \fIset\fP alone and not widespread
historical practice.
.LP
The \fInoclobber\fP option was changed to allow \fIset\fP \fB-C\fP
as well as the \fIset\fP \fB-o\fP \fInoclobber\fP
option. The single-letter version was added so that the historical
\fB"$-"\fP paradigm would not be broken; see \fISpecial Parameters\fP
\&.
.LP
The \fB-h\fP flag is related to command name hashing and is only required
on XSI-conformant systems.
.LP
The following \fIset\fP flags were omitted intentionally with the
following rationale:
.TP 7
\fB-k\fP
The \fB-k\fP flag was originally added by the author of the Bourne
shell to make it easier for users of pre-release versions
of the shell. In early versions of the Bourne shell the construct
\fIset\fP \fIname\fP= \fIvalue\fP had to be used to assign
values to shell variables. The problem with \fB-k\fP is that the behavior
affects parsing, virtually precluding writing any
compilers. To explain the behavior of \fB-k\fP, it is necessary to
describe the parsing algorithm, which is
implementation-defined. For example: 
.sp
.RS
.nf

\fBset -k; echo\fP \fIname\fP\fB=\fP\fIvalue\fP
.fi
.RE
.LP
and:
.sp
.RS
.nf

\fBset -k
echo\fP \fIname\fP\fB=\fP\fIvalue\fP
.fi
.RE
.LP
behave differently. The interaction with functions is even more complex.
What is more, the \fB-k\fP flag is never needed, since
the command line could have been reordered.
.TP 7
\fB-t\fP
The \fB-t\fP flag is hard to specify and almost never used. The only
known use could be done with here-documents. Moreover,
the behavior with \fIksh\fP and \fIsh\fP differs. The
reference page says that it exits after reading and executing one
command. What is one command? If the input is \fIdate\fP;
\fIdate\fP, \fIsh\fP executes both \fIdate\fP commands
while \fIksh\fP does only the first.
.sp
.LP
Consideration was given to rewriting \fIset\fP to simplify its confusing
syntax. A specific suggestion was that the \fIunset\fP utility should
be used to unset options instead of using the non- \fIgetopt\fP()
-able + \fIoption\fP syntax. However, the conclusion was reached that
the
historical practice of using + \fIoption\fP was satisfactory and that
there was no compelling reason to modify such widespread
historical practice.
.LP
The \fB-o\fP option was adopted from the KornShell to address user
needs. In addition to its generally friendly interface,
\fB-o\fP is needed to provide the \fIvi\fP 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 developed and \fB-o\fP provides ample
name space for describing such extensions.)
.LP
Historical implementations are inconsistent in the format used for
\fB-o\fP option status reporting. The \fB+o\fP 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.
.LP
Historically, \fIsh\fP did trace the command \fIset\fP \fB+x\fP, but
\fIksh\fP did not.
.LP
The \fIignoreeof\fP setting prevents accidental logouts when the end-of-file
character (typically <control>-D) is
entered. A user shall explicitly \fIexit\fP to leave the interactive
shell.
.LP
The \fIset\fP \fB-m\fP option was added to apply only to the UPE because
it applies primarily to interactive use, not shell
script applications.
.LP
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:
.sp
.RS
.nf

\fBtrap "jobs -n" CLD
\fP
.fi
.RE
.LP
The C shell provides two different levels of an asynchronous notification
capability. The environment variable \fInotify\fP is
analogous to what is done in \fIset\fP \fB-b\fP or \fIset\fP \fB-o\fP
\fInotify\fP. When set, it notifies the user immediately
of background job completions. When unset, this capability is turned
off.
.LP
The other notification ability comes through the built-in utility
\fInotify\fP. The
syntax is:
.sp
.RS
.nf

\fBnotify\fP \fB[\fP\fB%job ...\fP \fB]\fP
.fi
.RE
.LP
By issuing \fInotify\fP with no operands, it causes the C shell to
notify the user
asynchronously when the state of the current job changes. If given
operands, \fInotify\fP
asynchronously informs the user of changes in the states of the specified
jobs.
.LP
To add asynchronous notification to the POSIX shell, neither the KornShell
extensions to \fItrap\fP, nor the C shell \fInotify\fP environment
variable
seemed appropriate ( \fInotify\fP is not a proper POSIX environment
variable name).
.LP
The \fIset\fP \fB-b\fP option was selected as a compromise.
.LP
The \fInotify\fP built-in was considered to have more functionality
than was required
for simple asynchronous notification.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fISpecial Built-In Utilities\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .