How can I check if a command exists in a shell script? [duplicate]

Question

I am writing my first shell script. In my script I would like to check if a certain command exists, and if not, install the executable. How would I check if this command exists?

if # Check that foobar command doesnt exist
then
    # Now install foobar
fi

Just happened to come across. I think this is the same question as: stackoverflow.com/questions/592620/… , but it gives much more details. — Jerry Tian
– Jerry Tian, Commented Oct 23, 2012 at 4:14
Also relevant is How to 'hash -r' and refresh all shells? and When to rehash executables in $PATH with bash? — jww
– jww, Commented Jan 30, 2019 at 5:11
This was not "already answered" because it asks about shell scripts, which minimally means sh while the referred question is specific to bash. — monokrome
– monokrome, Commented Mar 19, 2020 at 22:44

Ivan Tarasov · Accepted Answer · 2018-03-25 21:59:44Z

344

In general, that depends on your shell, but if you use bash, zsh, ksh or sh (as provided by dash), the following should work:

if ! type "$foobar_command_name" > /dev/null; then
  # install foobar here
fi

For a real installation script, you'd probably want to be sure that type doesn't return successfully in the case when there is an alias foobar. In bash you could do something like this:

if ! foobar_loc="$(type -p "$foobar_command_name")" || [[ -z $foobar_loc ]]; then
  # install foobar here
fi

edited Mar 25, 2018 at 21:59

answered Sep 22, 2011 at 23:59

Ivan Tarasov

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10 Comments

Michael Durrant Over a year ago

I like this answer. I tried not to be too swayed by ivants' image which I like even more ;)

Andrew Over a year ago

hmm...when I change it to say if ! type "foo" > /dev/null; then I get the output on the screen "myscript.sh: line 12: type: foo: not found", however, it still seems to work because when I say if ! type "ls" > /dev/null; there is no output and the if statement does not get executed (since it returns true). How can I silence the output when the command doesnt exist?

Ivan Tarasov Over a year ago

Andrew, try if ! type "foo" > /dev/null 2>&1;

Iulius Curt Over a year ago

> /dev/null 2>&1 is the same as &> /dev/null

sherrellbc Over a year ago

The man page for type is very poor on my distro (Arch Linux). The help text is better, albeit only slightly. type: usage: type [-afptP] name [name ...] . The man page shows "options" as "none".

|

xuhdev · Accepted Answer · 2016-10-11 17:45:14Z

62

Five ways, 4 for bash and 1 addition for zsh:

type foobar &> /dev/null
hash foobar &> /dev/null
command -v foobar &> /dev/null
which foobar &> /dev/null
(( $+commands[foobar] )) (zsh only)

You can put any of them to your if clause. According to my tests (https://www.topbug.net/blog/2016/10/11/speed-test-check-the-existence-of-a-command-in-bash-and-zsh/), the 1st and 3rd method are recommended in bash and the 5th method is recommended in zsh in terms of speed.

answered Oct 11, 2016 at 17:45

xuhdev

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7 Comments

jww Over a year ago

Check if a program exists from a Bash script specifically advises against using which.

Tom Hale Over a year ago

Also echo =foobar in zsh. Note that your zsh solution only returns true/false, but doesn't print the pathname if existant. Also echo $commands[foo] won't return non-zero if foo doesn't exist. For a function that works in both shells, see here

xuhdev Over a year ago

@Raptor if examines exit code.

xeruf Over a year ago

the third option seems to be the only one with same behaviour in zsh and bash. For the others, zsh writes to stdout on error while bash writes to stderr.

xuhdev Over a year ago

@tubbo (( $+commands[foobar] )) works for me. Could you describe a scenario in which it fails?

|

Peter Mortensen · Accepted Answer · 2020-01-01 01:09:50Z

42

Try using type:

type foobar

For example:

$ type ls
ls is aliased to `ls --color=auto'

$ type foobar
-bash: type: foobar: not found

This is preferable to which for a few reasons:

The default which implementations only support the -a option that shows all options, so you have to find an alternative version to support aliases
type will tell you exactly what you are looking at (be it a Bash function or an alias or a proper binary).
type doesn't require a subprocess
type cannot be masked by a binary (for example, on a Linux box, if you create a program called which which appears in path before the real which, things hit the fan. type, on the other hand, is a shell built-in (yes, a subordinate inadvertently did this once).

edited Jan 1, 2020 at 1:09

Peter Mortensen

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answered Sep 22, 2011 at 23:36

Foo Bah

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2 Comments

Andrew Over a year ago

Could you put this in the form of an if/else statement (one that doesn't output to the console)?

M-Razavi Over a year ago

And on some custom Linux, there isn't ''which'' command.

Mmmh mmh · Accepted Answer · 2020-03-30 11:15:29Z

33

The question doesn't specify a shell, so for those using fish (friendly interactive shell):

if command -v foo > /dev/null
  echo exists
else
  echo does not exist
end

For basic POSIX compatibility, we use the -v flag which is an alias for --search or -s.

edited Mar 30, 2020 at 11:15

Mmmh mmh

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answered Nov 22, 2015 at 21:52

Dennis

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5 Comments

IC_ Over a year ago

doesn't work for aliases

Dennis Over a year ago

@Herrgott that's because command -v/-s searches in the $PATH. Aliases in fish are functions, and functions are searched for in the $fish_function_path. Many ways to check if an alias exists, e.g. if alias | grep -w 'myalias' >/dev/null; echo ok; end.

Aaron Franke Over a year ago

Doesn't this execute the command too?

Dennis Over a year ago

@AaronFranke it only searches for the executable, doesn't execute it.

Brett Cannon Over a year ago

You can use --query and avoid the redirect: fishshell.com/docs/current/cmds/command.html. While not POSIX-compliant, command is built into Fish.

Community · Accepted Answer · 2017-05-23 12:10:44Z

22

Check if a program exists from a Bash script covers this very well. In any shell script, you're best off running command -v $command_name for testing if $command_name can be run. In bash you can use hash $command_name, which also hashes the result of any path lookup, or type -P $binary_name if you only want to see binaries (not functions etc.)

edited May 23, 2017 at 12:10

CommunityBot

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answered Aug 1, 2013 at 10:24

Morten

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Comments

Tom Hale · Accepted Answer · 2018-09-01 11:21:39Z

3

A function which works in both bash and zsh:

# Return the first pathname in $PATH for name in $1
function cmd_path () {
  if [[ $ZSH_VERSION ]]; then
    whence -cp "$1" 2> /dev/null
  else  # bash
     type -P "$1"  # No output if not in $PATH
  fi
}

Non-zero is returned if the command is not found in $PATH.

answered Sep 1, 2018 at 11:21

Tom Hale

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Comments

Kashyap · Accepted Answer · 2011-09-22 23:44:43Z

1

which <cmd>

also see options which supports for aliases if applicable to your case.

Example

$ which foobar
which: no foobar in (/usr/local/bin:/usr/bin:/cygdrive/c/Program Files (x86)/PC Connectivity Solution:/cygdrive/c/Windows/system32/System32/WindowsPowerShell/v1.0:/cygdrive/d/Program Files (x86)/Graphviz 2.28/bin:/cygdrive/d/Program Files (x86)/GNU/GnuPG
$ if [ $? -eq 0 ]; then echo "foobar is found in PATH"; else echo "foobar is NOT found in PATH, of course it does not mean it is not installed."; fi
foobar is NOT found in PATH, of course it does not mean it is not installed.
$

PS: Note that not everything that's installed may be in PATH. Usually to check whether something is "installed" or not one would use installation related commands relevant to the OS. E.g. rpm -qa | grep -i "foobar" for RHEL.

edited Sep 22, 2011 at 23:44

answered Sep 22, 2011 at 23:38

Kashyap

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5 Comments

Foo Bah Over a year ago

which has other pitfalls as well

Kashyap Over a year ago

I was just starting to read man type to see how it's better.. may be you can save me some time by posting it here.. :)

Henk Langeveld Over a year ago

The real trouble with which is that it is an external command, and won't be able to deal with the specifics of the current shell session internals, as it can have no knowledge of them.

jww Over a year ago

Check if a program exists from a Bash script specifically advises against using which.

monokrome Over a year ago

it's best to not use which here, other answers are safer

mgild · Accepted Answer · 2016-11-10 03:03:52Z

1

A function I have in an install script made for exactly this

function assertInstalled() {
    for var in "$@"; do
        if ! which $var &> /dev/null; then
            echo "Install $var!"
            exit 1
        fi
    done
}

example call:

assertInstalled zsh vim wget python pip git cmake fc-cache

answered Nov 10, 2016 at 3:03

mgild

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4 Comments

Alvin Over a year ago

heads up: this crashes zsh as is

mgild Over a year ago

would only crash if you are calling this outside a script or sourcing the script. Change exit 1 to return 1 or some other error code if you are doing so.

jww Over a year ago

Check if a program exists from a Bash script specifically advises against using which.

J. M. Becker Over a year ago

I used this but changed which to command -v

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