Intro To 'which' Command In Linux

2023-05-09 - By Robert Elder

     I use the 'which' command to locate the full path of executables like this:

which ls

     and the output from this command is the following:

/usr/bin/ls

     Many common commands like 'ls', 'rm' or 'cp' are really just executable programs.  I can use the 'which' command to locate where these executable programs are stored:

which ls
which rm
which cp

/usr/bin/ls
/usr/bin/rm
/usr/bin/cp

ls -latr /usr/bin/ls /usr/bin/rm /usr/bin/cp

-rwxr-xr-x 1 root root  72056 Sep  5  2019 /usr/bin/rm
-rwxr-xr-x 1 root root 142144 Sep  5  2019 /usr/bin/ls
-rwxr-xr-x 1 root root 153976 Sep  5  2019 /usr/bin/cp

     Some commands like 'cd' or 'alias' are actually built into the shell itself, so the 'which' command doesn't show anything for them:

which cd
which alias

(no output)

     The 'which' command is useful for debugging which version of a command actually runs:

     For example, you could change the behavior of the 'ls' command by creating a simple script like this called 'ls' and changing the PATH variable.  First, add this to a file called 'ls' in the current directory:

#!/bin/bash
echo "This is my custom ls command!"

     then, give the 'ls' script execute permissions:

chmod u+x ls

     and verify these permissions on the 'ls' file using the real 'ls' command:

ls -l

total 4
-rwxrw-r-- 1 robert robert 37 May  9 22:54 ls

     and verify these permissions with the real 'ls' command:

     Now before we make the final change, take note of the current value of the 'PATH' variable:

echo $PATH

/home/robert/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

     And also take note of what the 'which' command shows for 'ls':

which ls

/usr/bin/ls

     Now, update the 'PATH' variable to also include the current directory:

PATH="/home/robert/important:$PATH"

     Verify that the 'PATH' variable was updated:

echo $PATH

/home/robert/important:/home/robert/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

     Now, typing 'ls' runs the script at this location instead of the regular 'ls' executable:

ls

     so the output will be this:

This is my custom ls command!

     And that's why the 'which' command is my favourite Linux command.

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