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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.1‐2008, 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.1‐2008, 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.1‐2008 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.1‐2008, 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.1‐2008) 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.1‐2008, Chapter 8, Environment
Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2008, exec(1p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 COMMAND(1P)
Pages that refer to this page: type(1p)