which

Find the path of a command

$ which [command]

$ which [command1] [command2]

$ which -a [command]

which [-a] command...

which searches for executables in the directories listed in the PATH environment variable and prints the path of the first match found.
This is useful for determining which version of a command will be executed when multiple versions exist, or for confirming a command is installed and in your PATH.
With -a, all matches are shown, revealing if multiple versions exist in different PATH directories. The first listed is the one that would be executed.

-a, --all

Print all matching executables in PATH, not just first

Which only searches PATH; it doesn't show shell builtins, aliases, or functions. Use type in bash or whence in zsh for comprehensive command lookup.
Some systems have different which implementations with varying options. The -a flag is common but not universal.
If a command isn't found, which typically exits with status 1 and produces no output (or an error message depending on implementation).

type(1), whereis(1), command(1), hash(1)