How to tell if a command exists in the shell

Reference:

Http://www.cnblogs.com/tuzkee/p/3755230.html

https://segmentfault.com/q/1010000000156870

Http://stackoverflow.com/questions/592620/check-if-a-program-exists-from-a-bash-script

Avoid using which, which can be implemented with the following commands:

$command-v Foo >/dev/null2>&1 | | {echo >&2  "I require Foo but It ' s not installed. Aborting. "; exit 1;} $ type foo >/dev/null 2>& 1 | | {echo >&2  "I require Foo but It ' s not installed. Aborting. "; exit 1;} $ hash foo 2>/dev/null | | {echo >&2  "I require Foo but It ' s not installed. Aborting. "; exit 1;}         

If your hash bang is  /bin/sh  then You should care on what POSIX says.  type  and  Hash ' exit codes aren ' t terribly well defined by POSIX, and  Hash  is Seen to exit successfully if the command doesn ' t exist (haven ' t seen this with  type  yet).   command 's exit status is a defined by POSIX, so that one was probably the safest to use.

If your script uses bash though, POSIX rules don ' t really matter anymore and both type and hash become perfectly safe to Use. Now have a to search just the and have the side-effect that the command's location would be type -P PATH hash hashed (f Or faster lookup next time you use it), which are usually a good thing since you probably check for its existence in order To actually use it.

As a simple example, here's a function that runs gdate if it exists, otherwise date :

gnudate() {    if hash gdate 2>/dev/null; then        gdate "[email protected]"    else        date "[email protected]"    fi}

In summary:

Where bash is your Shell/hashbang, consistently use hash (for commands) or type (to consider built-ins & keywords).

When writing a POSIX script, use command -v .

The first thing to note is that you should not use which to judge for the following reasons:

1, which non-shell built-in commands, with more overhead than built-in commands, and non-built-in commands depend on the implementation of the Platform, the implementation of different platforms may be different.

# type Typetype is a shell builtin# type commandcommand are a shell builtin# type Whichwhich is hashed (/usr/bin/which)

2, many system which does not set the return value when exiting, even if the command to find does not exist, which also returns 0

# which ls/usr/bin/ls# Echo $?0# which Aaano AAA in/usr/bin/bin/usr/sbin/sbin/usr/local/bin/usr/local/bin/usr/local /sbin/usr/ccs/bin/usr/openwin/bin/usr/dt/bin # echo $?0

3, many systems of which implementation, have secretly done some "outsiders" things

So, instead of using which, you can use the following method:

$ command-v foo >/dev/null 2>&1 | | {echo >&2 "I require foo but it's not installed.  Aborting. "; Exit 1; }$ type foo >/dev/null 2>&1 | | {echo >&2 "I require foo but it's not installed.  Aborting. "; Exit 1; }$ hash Foo 2>/dev/null | | {echo >&2 "I require foo but it's not installed.  Aborting. "; Exit 1; }

The sharp original, can be viewed here:

http://stackoverflow.com/questions/592620/how-to-check-if-a-program-exists-from-a-bash-script/677212#677212

How to tell if a command exists in the shell