Provided by: manpages-posix_2.16-1_all 
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 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 environment (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 implementation-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 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 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 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 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 _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 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 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 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 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 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
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 .