0x16. C - Simple Shell

Tasks

Pseudocode

WHILE TRUE
	show_prompt()
	cmd = read_user_input()
	IF cmd == "exit"
		exit() //system call
	ELSE
		tokens = tokenize_user_input(cmd)
	IF check_builtin_cmd(tokens)
		path = check_executable_file(cmd[0])
		IF path == null
			perror("Command not found")
		ELSE
			p_id = fork() //system call
			IF p_id == 0
				run_cmd(path,tokens)
			ELSE
				pp_id = p_id
				wait(pp_id) //system call

Flowchart

0. Betty would be proud

• Write a beautiful code that passes the Betty checks.

Solution: simple_shell

1. Simple shell 0.1

• 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…

Solution: simple_shell

2. Simple shell 0.2

• Simple shell 0.1 +

  • Handle command lines with arguments

Solution: simple_shell

3. Simple shell 0.3

• Simple shell 0.2 +

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

Solution: simple_shell

4. Simple shell 0.4

• 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

Solution: simple_shell

5. Simple shell 1.0

• Simple shell 0.4 +

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

Solution: simple_shell

6. Simple shell 0.1.1

• Simple shell 0.1 +

  • Write your own getline function
  • Use a buffer to read many chars at once and call the least possible the read system call
  • You will need to use static variables
  • You are not allowed to use getline *You don’t have to:
  • be able to move the cursor

Solution: simple_shell

7. Simple shell 0.2.1

• Simple shell 0.2 +

  • You are not allowed to use strtok

Solution: simple_shell

8. Simple shell 0.4.1

• Simple shell 0.4 +

  • handle arguments for the built-in exit
  • Usage: exit status, where status is an integer used to exit the shell

Solution: simple shell

9. setenv, unsetenv

• Simple shell 1.0 +

• Implement the setenv and unsetenv builtin commands

  • setenv
    • Initialize a new environment variable, or modify an existing one
    • Command syntax: setenv VARIABLE VALUE
    • Should print something on stderr on failure
  • unsetenv
    • Remove an environment variable
    • Command syntax: unsetenv VARIABLE
    • Should print something on stderr on failure

Solution: simple_shell

10. cd

• Simple shell 1.0 +

• Implement the builtin command cd:

  • Changes the current directory of the process.
  • Command syntax: cd [DIRECTORY]
  • If no argument is given to cd the command must be interpreted like cd $HOME
  • You have to handle the command cd -
  • You have to update the environment variable PWD when you change directory

• man chdir, man getcwd

Solution: simple_shell

11. ;

• Simple shell 1.0 +

  • Handle the commands separator ;

Solution: simple_shell

12. && and ||

• Simple shell 1.0 +

  • Handle the && and || shell logical operators

Solution: simple_shell

13. alias

• Simple shell 1.0 +

• Implement the alias builtin command
• Usage: alias [name[='value'] ...]
  • alias: Prints a list of all aliases, one per line, in the form name='value'
  • alias name [name2 ...]: Prints the aliases name, name2, etc 1 per line, in the form name='value'
  • alias name='value' [...]: Defines an alias for each name whose value is given. If name is already an alias, replaces its value with value

Solution: simple_shell

14. Variables

• Simple shell 1.0 +

  • Handle variables replacement
  • Handle the $? variable
  • Handle the $$ variable

Solution: simple_shell

15. Comments

• Simple shell 1.0 +

  • Handle comments (#)

Solution: simple_shell

16. File as input

• Simple shell 1.0 +

  • Usage: simple_shell [filename]
  • Your shell can take a file as a command line argument
  • The file contains all the commands that your shell should run before exiting
  • The file should contain one command per line
  • In this mode, the shell should not print a prompt and should not read from stdin

Solution: simple_shell