As well as using the shell to run commands you can use its built-in programming language to write your own commands or programs. You can put commands into a file - known as a shell script - and then execute that file as you would a command or program.
The use of the Bourne shell (sh) is illustrated as this is available on all UNIX systems. Bourne shell scripts will also work under the Bourne Again SHell. The C and TC shells use a programming language which is similar to the C programming language.