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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 |
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COMMAND(1P) POSIX Programmer's Manual COMMAND(1P)
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.
command — execute a simple command
command [-p] command_name [argument...]
command [-p][-v|-V] command_name
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.
The command utility shall conform to the Base Definitions volume
of POSIX.1‐2017, 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.
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.
Not used.
None.
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.1‐2017, 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.
Default.
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>
The standard error shall be used only for diagnostic messages.
None.
None.
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.
Default.
The following sections are informative.
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.1‐2017
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.
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.1‐2017, 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.
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.1‐2017) 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.
None.
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.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, exec(1p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 2017 COMMAND(1P)
Pages that refer to this page: type(1p)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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