which

What it does

How to write it

which [OPTIONS] PROGRAM [PROGRAM ...]

Most used

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$ run as a normal user# run as root (or with sudo)

1. 1Find where a program is

   $which passwd
   /usr/bin/passwd

   The program file that runs when you type passwd.

2. 2See all copies of a program in $PATH

   $which -a python3
   /usr/bin/python3
   /usr/local/bin/python3

   The first one wins. Two copies often explain "wrong version" problems.

3. 3Find the package of a program

   $rpm -qf $(which ss)
   iproute-6.x.x.el10.x86_64

   Now you know which package to update or reinstall.

4. 4Check if a tool exists in a script

   $which ansible >/dev/null 2>&1 && echo "ansible installed"

   The exit code is 0 when found, 1 when not found.

5. 5Show the alias too (RHEL default)

   $which ll
   alias ll='ls -l --color=auto'
   	/usr/bin/ls

   Because of the RHEL alias, which also shows aliases in an interactive shell.

6. 6Use the plain program without the alias

   $/usr/bin/which --skip-alias ll

   In a script the alias does not exist; this shows what a script would find.

7. 7Check the path of a privileged program in a review

   $which -a sudo

   Only /usr/bin/sudo should appear. Another sudo earlier in $PATH is a red flag.

8. 8Find which shell a user would get

   $which bash zsh fish
   /usr/bin/bash

   Only installed shells are printed; the others give an error message.

9. 9Get the path for a cron job or a systemd unit

   $which podman
   /usr/bin/podman

   cron and systemd use a short $PATH. Write the full path in ExecStart= and crontab lines.

10. 10Check many tools at once before a deployment

    $for t in git curl jq rsync; do which $t >/dev/null || echo "missing: $t"; done

    Prints only the tools that are not installed.

11. 11Compare with type

    $which cd; type cd

    which finds no file for cd (it is a built-in); type explains what it is.

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