command [-p] command_name [argument...] command [-p][-v|-V] command_name
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.
The following options shall be supported:
"%s\n", <pathname or command>
When the -V option is specified, standard output shall be formatted as:
"%s\n", <unspecified>
Otherwise, the following exit values shall be returned:
Otherwise, the exit status of command shall be that of the simple command specified by the arguments to command.
The following sections are informative.
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.
cd() {
command cd "$@" >/dev/null
pwd
}
IFS=' ' # The preceding value should be <space><tab><newline>. # Set IFS to its default value. \unalias -a # Unset all possible aliases. # Note that unalias is escaped to prevent an alias # being used for unalias. unset -f command # Ensure command is not a user function. PATH="$(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.
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:
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.
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
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