Originální popis anglicky:
command - execute a simple command
Návod, kniha: POSIX Programmer's Manual
command [-p] command_name
[argument ...]
command [ -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
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.
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.
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
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.
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:
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 _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.
- 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.
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:
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.
None.
Command Search and Execution ,
Shell Execution Environment
,
Special Built-In Utilities ,
sh ,
type , the System
Interfaces volume of IEEE Std 1003.1-2001,
exec
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
.
