0x16. C - Simple Shell

Learning Objectives

• Who designed and implemented the original Unix operating system
• Who wrote the first version of the UNIX shell
• Who invented the B programming language (the direct predecessor to the C programming language)
• Who is Ken Thompson
• How does a shell work
• What is a pid and a ppid
• How to manipulate the environment of the current process
• What is the difference between a function and a system call
• How to create processes
• What are the three prototypes of main
• How does the shell use the PATH to find the programs
• How to execute another program with the execve system call
• How to suspend the execution of a process until one of its children terminates
• What is EOF / “end-of-file”?

List of allowed functions and system calls

• access (man 2 access)
• chdir (man 2 chdir)
• close (man 2 close)
• closedir (man 3 closedir)
• execve (man 2 execve)
• exit (man 3 exit)
• _exit (man 2 _exit)
• fflush (man 3 fflush)
• fork (man 2 fork)
• free (man 3 free)
• getcwd (man 3 getcwd)
• getline (man 3 getline)
• getpid (man 2 getpid)
• isatty (man 3 isatty)
• kill (man 2 kill)
• malloc (man 3 malloc)
• open (man 2 open)
• opendir (man 3 opendir)
• perror (man 3 perror)
• read (man 2 read)
• readdir (man 3 readdir)
• signal (man 2 signal)
• stat (__xstat) (man 2 stat)
• lstat (__lxstat) (man 2 lstat)
• fstat (__fxstat) (man 2 fstat)
• strtok (man 3 strtok)
• wait (man 2 wait)
• waitpid (man 2 waitpid)
• wait3 (man 2 wait3)
• wait4 (man 2 wait4)
• write (man 2 write)

Tasks

0. Write a beautiful code that passes the Betty checks

1. Write a UNIX command line interpreter.

   • Usage: simple_shell
     Your Shell should:
     
   • Display a prompt and wait for the user to type a command. A command line always ends with a new line.
   • The prompt is displayed again each time a command has been executed.
   • The command lines are simple, no semicolons, no pipes, no redirections or any other advanced features.
   • The command lines are made only of one word. No arguments will be passed to programs.
   • If an executable cannot be found, print an error message and display the prompt again.
   • Handle errors.
   • You have to handle the “end of file” condition (Ctrl+D)
     You don’t have to:
     
   • use the PATH
   • implement built-ins
   • handle special characters : ", ', , `, *, &, #
   • be able to move the cursor
   • handle commands with arguments execve will be the core part of your Shell, don’t forget to pass the environ to it…

2. Simple shell 0.1 +

   • Handle command lines with arguments

3. Simple shell 0.2 +

   • Handle the PATH
   • fork must not be called if the command doesn’t exist

4. Simple shell 0.3 +

   • Implement the exit built-in, that exits the shell
   • Usage: exit
   • You don’t have to handle any argument to the built-in exit

5. Simple shell 0.4 +

   • Implement the env built-in, that prints the current environment