The Built-In Commands
The first set of commands examined in this chapter are built-in commands. A built-in command is part of the shell itself; it is not stored in a separate file on disk. Built-in commands are slightly more efficient than external programs because there is no overhead associated with reading and loading them from a file on disk. Unless you are looping thousands of times, it usually does not matter whether the command you use is built in or external. The built-in commands you will examine are eval, :, and type.
The eval Command
The eval command is used when you want the shell to execute a command after performing substitution. The basic syntax is
Here cmd is any valid shell command. The eval command is normally used ...