What is the 'command' command and what is it used for? [duplicate]
Recently I found command: command
which has no manual entry but help displays as follows:
$ help command
command: command [-pVv] command [arg ...]
Execute a simple command or display information about commands.
Runs COMMAND with ARGS suppressing shell function lookup, or display
information about the specified COMMANDs. Can be used to invoke commands
on disk when a function with the same name exists.
Options:
-p use a default value for PATH that is guaranteed to find all of
the standard utilities
-v print a description of COMMAND similar to the `type' builtin
-V print a more verbose description of each COMMAND
Exit Status:
Returns exit status of COMMAND, or failure if COMMAND is not found.
Is command -v
is alternative of which
?
What arguments are accepted by this command and how/when to use command
?
Solution 1:
command
is a bash builtin as we can see:
seth@host:~$ type command
command is a shell builtin
So we know command
is provided by our shell, bash. Digging into man bash
we can see what its use is:
(from man bash
):
command [-pVv] command [arg ...]
Run command with args suppressing the normal shell function
lookup. Only builtin commands or commands found in the PATH are
executed. If the -p option is given, the search for command is
performed using a default value for PATH that is guaranteed to
find all of the standard utilities. If either the -V or -v
option is supplied, a description of command is printed. The -v
option causes a single word indicating the command or file name
used to invoke command to be displayed; the -V option produces a
more verbose description. If the -V or -v option is supplied,
the exit status is 0 if command was found, and 1 if not. If
neither option is supplied and an error occurred or command
cannot be found, the exit status is 127. Otherwise, the exit
status of the command builtin is the exit status of command.
Essentially you would use command
to bypass "normal function lookup". For example, say you had a function in your .bashrc
:
function say_hello() {
echo 'Hello!'
}
Normally, when you run say_hello
in your terminal bash would find the function named say_hello
in your .bashrc
before it found, say, an application named say_hello
. Using:
command say_hello
makes bash bypass its normal function lookup and go straight to either builtins or your $PATH
. Note that this function lookup also include aliases. Using command
will bypass both functions and aliases.
If the -p
option is provided bash bypasses your custom $PATH
and uses its own default.
The -v
or -V
flags bash prints a description (short for -v
, long for -V
) of the command.
Note: As souravc pointed out in the comments an easier method for finding information about shell builtins can be found here: How to make `man` work for shell builtin commands and keywords?
Solution 2:
This is built-in command of the Bash shell.
The only advantage I see with this built-in is summarized in the following sentence of the help text:
Can be used to invoke commands on disk when a function with the same name exists.
So if you want to execute a program (a binary file saved somewhere on your disk), and an internal shell function of the same name exists, then you can invoke your program using this built-in.
And yes, command -v
will give the same kind of result as type
.
I've found it also under the Dash shell.
Solution 3:
It has two different uses:
One use is to ignore aliases and functions, and run the executable file found in PATH, even when an alias or a function with the same name exists.
As example, I'll use an alias for ls
that appends a /
to directory names:
$ alias ls='ls --classify'
$ ls -d .
./
$ command ls -d .
.
In an interactive shell, it may be more convenient to use a backslash before the command name as alternative, shorter syntax:
$ \ls -d .
.
The other use is to find the command that will be run when the commands name isn't used by using the option -v
. It seems to be the most portable/POSIX variant of which
.
$ command -v ls
alias ls='ls --classify'
$ command -v sed
/bin/sed