commandposix - Online in the Cloud

PROGRAM:

NAME

command — execute a simple command

SYNOPSIS

command [−p] command_name [argument...]

command [−p][−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 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.

OPTIONS

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.

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 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.

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 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.

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="$(command −p getconf PATH):$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.

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 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.

FUTURE DIRECTIONS

None.

Use commandposix online using onworks.net services