When you type a command into the shell, it has to find the program on your hard
disk before executing it. If the shell had to look all over the disk, it would
be very slow; instead, it looks in a list of directories contained in the PATH
environment variable. This list of directories makes up the shell's search
path; when you enter a command, it goes through each one in turn looking for
the program you asked to run.
You may need to change the PATH variable if you install programs yourself
in a non-standard location. The value of PATH is a colon-separated
list of directories. The default value on Debian systems is as follows:
This value is defined in the file /etc/profile and applies to all users.
You can easily change the value, just as you can change any environment 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
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.