which

Understanding the which Command

The which command is used to locate the full path of executables. It searches your system’s PATH environment variable, which contains a list of directories where the shell looks for executable files when you type a command. If the command is found, which prints its full path; otherwise, it returns nothing.

Basic Usage: Finding Single Executables

The most straightforward use of which is to find the location of a single command. For example, to find the location of the ls command, you’d simply type:

which ls

This will output a path similar to /bin/ls or /usr/bin/ls, depending on your system’s configuration. If ls isn’t found in your PATH, nothing will be printed.

Handling Multiple Commands

which can handle multiple commands as arguments. Let’s find the locations of ls, grep, and ping:

which ls grep ping

This will output the paths for each command, each on a new line. If one of the commands isn’t found, it will simply be omitted from the output.

Dealing with Aliases and Functions

which cleverly distinguishes between commands and aliases/functions. If you have an alias set for a command, which will reveal the alias, not the underlying executable. For example:

alias la='ls -la'
which la

This will likely show the alias definition, not the path to /bin/ls. To find the actual executable, you’d need to use the underlying command: which ls.

Checking for Executable Existence

The absence of output from which indicates that the command isn’t found in your PATH. You can use this to check if a specific command is installed and accessible:

which my_custom_command

If nothing is printed, my_custom_command is either not installed or not in your PATH.

Combining which with Other Commands

The output of which can be readily used with other commands. For instance, you might check the permissions of an executable found using which:

ls -l $(which ls)

This utilizes command substitution ($(...)) to feed the output of which ls as an argument to ls -l, displaying detailed information about the ls executable’s permissions and other attributes.

Advanced Scenario: Finding Executables in Specific Directories

While which primarily searches the PATH, you can manually specify directories to search. This is less common but can be useful in specific circumstances. (Note that this is not the intended use of which.) One approach is to use find:

find /usr/local/bin -name "my_program" -executable -print

This finds the executable my_program within /usr/local/bin. Remember to replace /usr/local/bin with the appropriate directory. This differs from which because which only searches the directories listed in the PATH environment variable.