夜‧貓

type - Indicate how a command name is interpreted.

1. An executable program- like all those files we saw in /usr/bin.

    date, cal, df, free, file, less, cp, mkdir, rm, ln, which, man, cat, nano, vi, ping, rmdir, su, sudo, pager, find, grep, gzip, bzip2, xz, ps, clear, apt-get, tar

    -->total 29 commands.

2. A command built into the shell itself - "shell builtins".

    exit, pwd, cd, type, help, command, echo, kill

    -->total  8 commands.

3. A shell function - miniature shell scripts incorporated into the environment.

    -->NULL!   

4. An alias - can be define, built from other command.

    ls

    -->total 1 command.