34

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?

44

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?

  • 1
    Also look at thsi – souravc Oct 19 '14 at 3:48
  • @souravc That's funny, I don't have a manpage for command on my machine. – Seth Oct 19 '14 at 3:49
  • 2
    try sudo apt-get install manpages-posix. It is not installed by default. look here – souravc Oct 19 '14 at 3:53
  • @souravc Very good! I will edit that link into the answer. – Seth Oct 19 '14 at 3:58
  • 1
    @Nori-chan Bash is Ubuntu's default shell. It is the thing that interprets the commands you enter and decides what to do and how to do it with the information you enter. By learning the Un*x command-line you are (in a way) learning bash :) – Seth Oct 19 '14 at 4:06
14

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 of some of your disk), and an internal shell function of the same name exist, 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.

  • 1
    What's worth adding more explicitly is that although command (name) ignoes shell functions, command -v (name) does not. command -v (name) >/dev/null is supposed to be the portable way of checking whether the command with that name exists, regardless of whether it's a shell builtin, function, or external utility. – hvd Aug 18 '14 at 11:49
  • 1
    command -v is the posix alternative to which, type, etc, stackoverflow.com/questions/762631/… – Javier López Aug 18 '14 at 13:23
  • For a real-world example, the Android AOSP build environment (at some point after v4.2.2) defines a shell function named 'make' that will spit out information about whether or not a build succeeds and how long it took. Other shell functions in the AOSP build environment use command make to invoke the actual-factual make program. Unfortunately, I have other shell scripts that broke when the AOSP environment started adding shit to what the make program outputs and it was irritating as hell figuring out where that extra output was mysteriously coming from. – Michael Burr Mar 20 '17 at 23:20
6

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
3

command is useful, for example, if you want to check for the existence of a particular command. which includes aliases into the lookup so it is not suitable for this purpose because you don't want a random alias to be considered as the command in question.

In other words, you can have a small function in a shell script like this:

exists() {
  command -v "$1" >/dev/null 2>&1
}

And then test for an available command (here, dialog), like so:

if ! exists dialog ; then
   echo "This script requires 'dialog'."
   echo "Install it with 'sudo apt-get install dialog', then try again!"
   exit 1
fi
2

It lets you run a shell command ignoring any shell functions.

http://ss64.com/bash/command.html

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