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NAME
command — execute a simple command
SYNOPSIS
command [−p] command_name [argument...]
command [−p][−v|−V] command_name
DESCRIPTION
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.1‐2008, 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 Execution
Environment), 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 and Execution), 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 Section 2.12,
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, Command Search 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.1‐2008, 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.1‐2008 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.1‐2008, 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, Special Built-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.1‐2008) 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 ALSO
Section 2.9.1.1, Command Search and Execution, Section 2.12, Shell Execution Environment, Section 2.14,
Special Built-In Utilities, sh, type
The Base Definitions volume of POSIX.1‐2008, Chapter 8, Environment Variables, Section 12.2, Utility
Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, exec
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
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