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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 dif-
       fer (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 [ -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 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 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  omit-
       ting command.

       On systems supporting the User Portability Utilities option, the command utility also shall provide information
       concerning how a command name is interpreted by the shell; see -v and -V.

OPTIONS
       The command utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, 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     (On systems supporting the User Portability Utilities option.) Write a string to  standard  output  that
              indicates  the  pathname or command that will be used by the shell, in the current shell execution envi-
              ronment (see 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 implementa-
                 tion-defined  functions  that  are  found using the PATH variable (as described in 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     (On systems supporting the User Portability Utilities option.) 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 cur-
              rent shell execution environment (see 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 cate-
              gories 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 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 IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for  the  prece-
              dence 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  exam-
              ple, 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 writ-
              ten 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  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 com-
       mand.

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 _CS_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 dis-
       tinction 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 Command Search and Execution allows for a standard utility  to  be  imple-
       mented  as  a regular built-in as long as it is found in the appropriate place in a PATH search.  So, for exam-
       ple, command -v true might yield /bin/true or some similar  pathname.  Other  implementation-defined  utilities
       that  are not defined by this volume of IEEE Std 1003.1-2001 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="$(command -p getconf _CS_PATH):$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 imple-
       mentation extensions may be present that would allow user functions to exist when it is invoked; this  capabil-
       ity  is  not  specified  by this volume of IEEE Std 1003.1-2001, 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 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  stan-
       dard 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 IEEE Std 1003.1-2001) 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  rea-
       sons.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Command  Search  and  Execution,  Shell Execution Environment, Special Built-In Utilities, sh, type, the System
       Interfaces volume of IEEE Std 1003.1-2001, exec

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 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 .



IEEE/The Open Group                  2003                          COMMAND(1P)