command

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



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

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.1‐2017, 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 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.1‐2017, 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, 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.1‐2017) 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 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.1‐2017, Chapter 8, Environment
       Variables, Section 12.2, Utility Syntax Guidelines

       The System Interfaces volume of POSIX.1‐2017, exec

COPYRIGHT
       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)