Essential system commands

This category of commands refer to executable files that are almost definitely installed on your system, and not implemented in Bash itself. All of these commands are specified by the POSIX standard, and so systems that aim to implement that standard should have them available.

The ls command

The ls program prints human-readable listings of directories. By default, with no arguments, this list is just the names of each file in the current directory, printed horizontally for readability:

$ ls
books  documents  music

If we add the -a flag, we can include dot files in our output, files beginning with a period:

$ ls -a
.  ..  .bashrc  .profile  books  documents  music

Note that this output includes the...