How does the 'which' command determine the location of an executable file?

The which command determines the location of an executable file by searching through the directories listed in the PATH environment variable. It checks each directory in the order they are listed and returns the first match it finds for the specified command.

For example, when you run:

which gcc

It will look through the directories in your PATH (like /usr/bin, /bin, etc.) and return the path to the gcc executable if it exists in one of those directories. If the command is not found in any of the directories, it will not return any output.