variable. If you type the command ls, the shell will first look in
/usr/local/bin; ls isn't there, so it will try /usr/bin; when that fails,
it will check /bin. There it will discover /bin/ls, stop its search, and
execute the program /bin/ls. If /usr/bin/X11/ls existed (it doesn't, but
pretend), it would be ignored.

You can see which ls the shell is going to use with the type command.
type ls will give you the answer /bin/ls. Try it yourself.

Try asking where type itself resides:

$ type type

type is a shell builtin

type isn't actually a program; it's a function provided by the shell.
However, you use it just like an external program.

There are a number of commands like this; type man builtins to read the
man page describing them. In general, you don't need to know whether a
command is a builtin or a real program; however, builtins will not show up
in the output of ps or top because they aren't separate processes. They're
just part of the shell.

Configuration Files

Many applications on Linux systems allow you to alter how they behave at
certain times by altering files containing configuration information.
These configuration files may contain application start-up information,
run-time settings and application shutdown settings. In general, a