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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 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 [ -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 omitting  com-
       mand.

       On  systems  supporting the User Portability Utilities option, the com-
       mand 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 environment (see Shell  Execution  Environment  ),  to
              invoke command_name, but do not invoke command_name.

               * Utilities,  regular built-in utilities, command_names includ-
                 ing a slash character, and any  implementation-defined  func-
                 tions 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 current 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 categories command_name falls
              and shall include the information stated:

               * Utilities, regular built-in utilities,  and  any  implementa-
                 tion-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  com-
       mand:

       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 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 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 speci-
              fied by command_name could not be found.

       Otherwise, the exit status of command shall be that of the simple  com-
       mand 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  func-
       tions 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 get-
       conf  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 dis-
       tinguish "failure to find a utility" from "invoked utility exited  with
       an  error  indication". The value 127 was chosen because it is not com-
       monly 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 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
       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 direc-
           tory 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
       implementation extensions may be present that would  allow  user  func-
       tions  to exist when it is invoked; this capability 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  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
       IEEE Std 1003.1-2001)  when PATH is unset. At the very least, this fea-
       ture 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 addi-
       tion  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 applica-
       tion parsing reasons.

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, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications  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)
 

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