PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       command — execute a simple command

SYNOPSIS         top

       command [−p] command_name [argument...]

       command [−p][−v|−V] command_name

DESCRIPTION         top

       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         top

       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         top

       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         top

       Not used.

INPUT FILES         top

       None.

ENVIRONMENT VARIABLES         top

       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         top

       Default.

STDOUT         top

       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         top

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES         top

       None.

EXTENDED DESCRIPTION         top

       None.

EXIT STATUS         top

       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         top

       Default.

       The following sections are informative.

APPLICATION USAGE         top

       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         top

        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         top

       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         top

       None.

SEE ALSO         top

       Section 2.9.1.1, Command Search and Execution, Section 2.12, Shell
       Execution Environment, Section 2.14, Special Built-In Utilities,
       sh(1p), type(1p)

       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(1p)

COPYRIGHT         top

       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 .

       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                 2013                         COMMAND(1P)

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