COMMAND(1P) POSIX Programmer's Manual COMMAND(1P)
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not be implemented on Linux.
NAME
command — execute a simple command
SYNOPSIS
command [-p] command_name [argument...]
command [-p][-v|-V] command_nameDESCRIPTION
The command utility shall cause the shell to treat the arguments as a
simple command, suppressing the shell function lookup that is described
in Section 2.9.1.1, Command Search and Execution, item 1b.
If the command_name is the same as the name of one of the special built-
in utilities, the special properties in the enumerated list at the
beginning of Section 2.14, Special Built-In Utilities shall not occur. In
every other respect, if command_name is not the name of a function, the
effect of command (with no options) shall be the same as omitting
command.
When the -v or -V option is used, the command utility shall provide
information concerning how a command name is interpreted by the shell.
OPTIONS
The command utility shall conform to the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-p Perform the command search using a default value for PATH that
is guaranteed to find all of the standard utilities.
-v Write a string to standard output that indicates the pathname
or command that will be used by the shell, in the current shell
execution environment (see Section 2.12, Shell ExecutionEnvironment), to invoke command_name, but do not invoke
command_name.
* Utilities, regular built-in utilities, command_names
including a <slash> character, and any implementation-
defined functions that are found using the PATH variable
(as described in Section 2.9.1.1, Command Search andExecution), shall be written as absolute pathnames.
* Shell functions, special built-in utilities, regular built-
in utilities not associated with a PATH search, and shell
reserved words shall be written as just their names.
* An alias shall be written as a command line that represents
its alias definition.
* Otherwise, no output shall be written and the exit status
shall reflect that the name was not found.
-V Write a string to standard output that indicates how the name
given in the command_name operand will be interpreted by the
shell, in the current shell execution environment (see Section2.12, Shell Execution Environment), but do not invoke
command_name. Although the format of this string is
unspecified, it shall indicate in which of the following
categories command_name falls and shall include the information
stated:
* Utilities, regular built-in utilities, and any
implementation-defined functions that are found using the
PATH variable (as described in Section 2.9.1.1, CommandSearch and Execution), shall be identified as such and
include the absolute pathname in the string.
* Other shell functions shall be identified as functions.
* Aliases shall be identified as aliases and their
definitions included in the string.
* Special built-in utilities shall be identified as special
built-in utilities.
* Regular built-in utilities not associated with a PATH
search shall be identified as regular built-in utilities.
(The term ``regular'' need not be used.)
* Shell reserved words shall be identified as reserved words.
OPERANDS
The following operands shall be supported:
argument One of the strings treated as an argument to command_name.
command_name
The name of a utility or a special built-in utility.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of
command:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
POSIX.1‐2017, Section 8.2, Internationalization Variables for
the precedence of internationalization variables used to
determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all
the other internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments).
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error
and informative messages written to standard output.
NLSPATH Determine the location of message catalogs for the processing
of LC_MESSAGES.
PATH Determine the search path used during the command search
described in Section 2.9.1.1, Command Search and Execution,
except as described under the -p option.
ASYNCHRONOUS EVENTS
Default.
STDOUT
When the -v option is specified, standard output shall be formatted as:
"%s\n", <pathname or command>
When the -V option is specified, standard output shall be formatted as:
"%s\n", <unspecified>
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
When the -v or -V options are specified, the following exit values shall
be returned:
0 Successful completion.
>0 The command_name could not be found or an error occurred.
Otherwise, the following exit values shall be returned:
126 The utility specified by command_name was found but could not be
invoked.
127 An error occurred in the command utility or the utility specified
by command_name could not be found.
Otherwise, the exit status of command shall be that of the simple command
specified by the arguments to command.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.APPLICATION USAGE
The order for command search allows functions to override regular built-
ins and path searches. This utility is necessary to allow functions that
have the same name as a utility to call the utility (instead of a
recursive call to the function).
The system default path is available using getconf; however, since
getconf may need to have the PATH set up before it can be called itself,
the following can be used:
command -p getconf PATH
There are some advantages to suppressing the special characteristics of
special built-ins on occasion. For example:
command exec > unwritable-file
does not cause a non-interactive script to abort, so that the output
status can be checked by the script.
The command, env, nohup, time, and xargs utilities have been specified to
use exit code 127 if an error occurs so that applications can distinguish
``failure to find a utility'' from ``invoked utility exited with an error
indication''. The value 127 was chosen because it is not commonly used
for other meanings; most utilities use small values for ``normal error
conditions'' and the values above 128 can be confused with termination
due to receipt of a signal. The value 126 was chosen in a similar manner
to indicate that the utility could be found, but not invoked. Some
scripts produce meaningful error messages differentiating the 126 and 127
cases. The distinction between exit codes 126 and 127 is based on
KornShell practice that uses 127 when all attempts to exec the utility
fail with [ENOENT], and uses 126 when any attempt to exec the utility
fails for any other reason.
Since the -v and -V options of command produce output in relation to the
current shell execution environment, command is generally provided as a
shell regular built-in. If it is called in a subshell or separate utility
execution environment, such as one of the following:
(PATH=foo command -v)
nohup command -v
it does not necessarily produce correct results. For example, when called
with nohup or an exec function, in a separate utility execution
environment, most implementations are not able to identify aliases,
functions, or special built-ins.
Two types of regular built-ins could be encountered on a system and these
are described separately by command. The description of command search
in Section 2.9.1.1, Command Search and Execution allows for a standard
utility to be implemented as a regular built-in as long as it is found in
the appropriate place in a PATH search. So, for example, command -v true
might yield /bin/true or some similar pathname. Other implementation-
defined utilities that are not defined by this volume of POSIX.1‐2017
might exist only as built-ins and have no pathname associated with them.
These produce output identified as (regular) built-ins. Applications
encountering these are not able to count on execing them, using them with
nohup, overriding them with a different PATH, and so on.
EXAMPLES
1. Make a version of cd that always prints out the new working directory
exactly once:
cd() {
command cd "$@" >/dev/null
pwd
}
2. Start off a ``secure shell script'' in which the script avoids being
spoofed by its parent:
IFS='
'
# The preceding value should be <space><tab><newline>.
# Set IFS to its default value.
\unalias -a
# Unset all possible aliases.
# Note that unalias is escaped to prevent an alias
# being used for unalias.
unset -f command
# Ensure command is not a user function.
PATH="$(command -p getconf PATH):$PATH"
# Put on a reliable PATH prefix.
# ...
At this point, given correct permissions on the directories called by
PATH, the script has the ability to ensure that any utility it calls
is the intended one. It is being very cautious because it assumes
that implementation extensions may be present that would allow user
functions to exist when it is invoked; this capability is not
specified by this volume of POSIX.1‐2017, but it is not prohibited as
an extension. For example, the ENV variable precedes the invocation
of the script with a user start-up script. Such a script could define
functions to spoof the application.
RATIONALE
Since command is a regular built-in utility it is always found prior to
the PATH search.
There is nothing in the description of command that implies the command
line is parsed any differently from that of any other simple command. For
example:
command a | b ; c
is not parsed in any special way that causes '|' or ';' to be treated
other than a pipe operator or <semicolon> or that prevents function
lookup on b or c.
The command utility is somewhat similar to the Eighth Edition shell
builtin command, but since command also goes to the file system to search
for utilities, the name builtin would not be intuitive.
The command utility is most likely to be provided as a regular built-in.
It is not listed as a special built-in for the following reasons:
* The removal of exportable functions made the special precedence of a
special built-in unnecessary.
* A special built-in has special properties (see Section 2.14, SpecialBuilt-In Utilities) that were inappropriate for invoking other
utilities. For example, two commands such as:
date > unwritable-file
command date > unwritable-file
would have entirely different results; in a non-interactive script,
the former would continue to execute the next command, the latter
would abort. Introducing this semantic difference along with
suppressing functions was seen to be non-intuitive.
The -p option is present because it is useful to be able to ensure a safe
path search that finds all the standard utilities. This search might not
be identical to the one that occurs through one of the exec functions (as
defined in the System Interfaces volume of POSIX.1‐2017) when PATH is
unset. At the very least, this feature is required to allow the script to
access the correct version of getconf so that the value of the default
path can be accurately retrieved.
The command -v and -V options were added to satisfy requirements from
users that are currently accomplished by three different historical
utilities: type in the System V shell, whence in the KornShell, and which
in the C shell. Since there is no historical agreement on how and what to
accomplish here, the POSIX command utility was enhanced and the
historical utilities were left unmodified. The C shell which merely
conducts a path search. The KornShell whence is more elaborate—in
addition to the categories required by POSIX, it also reports on tracked
aliases, exported aliases, and undefined functions.
The output format of -V was left mostly unspecified because human users
are its only audience. Applications should not be written to care about
this information; they can use the output of -v to differentiate between
various types of commands, but the additional information that may be
emitted by the more verbose -V is not needed and should not be
arbitrarily constrained in its verbosity or localization for application
parsing reasons.
FUTURE DIRECTIONS
None.
SEE ALSOSection 2.9.1.1, Command Search and Execution, Section 2.12, ShellExecution Environment, Section 2.14, Special Built-In Utilities, sh, type
The Base Definitions volume of POSIX.1‐2017, Chapter 8, EnvironmentVariables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, execCOPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology --
Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
Any typographical or formatting errors that appear in this page are most
likely to have been introduced during the conversion of the source files
to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 COMMAND(1P)