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 .