A Shell is simply a program that conveniently allows you to run other programs.

Features

1. The internal shell command "exit" which terminates the shell

• System calls: exit()

2. A command with no arguments

• Example: ls, cp, rm ...etc
• Details: the shell must block until the command completes and, if the return code is abnormal, print out a message to that effect.
• System calls: fork(), execvp(), exit(), wait()

3. A command with arguments

• Example: ls –l
• Details: Argument 0 is the name of the command.

4. A command, with or without arguments, executed in the background using &.

• Example: firefox &
• Details: In this case, the shell must execute the command and return immediately, not blocking until the command finishes.