What's what – which, whereis, whatis, and file  

There may be several files with the same name. Knowing which executable is being invoked and whether a file is compiled code or a script is useful information.

How to do it...

The which, whereis, file, and whatis commands report information about files and directories.

• which: The which command reports the location of a command:

$ which ls
        /bin/ls

• We often use commands without knowing the directory where the executable file is stored. Depending on how your PATH variable is defined, you may use a command from /bin, /usr/local/bin, or /opt/PACKAGENAME/bin.
• When we type a command, the terminal looks for the command in a set of directories and executes the first executable file it finds. The directories to search are specified in the PATH environment variable:

        $ echo $PATH
        /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

• We can add directories to be searched and export the new PATH. To add /opt/bin to PATH, use the following command...