0x16. C - Simple Shell

General

• 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”?
• All your files should end with a new line
• A README.md file, at the root of the folder of the project is mandatory
• Your code should use the Betty style. It will be checked using betty-style.pl and betty-doc.pl
• Your shell should not have any memory leaks
• No more than 5 functions per file
• All your header files should be include guarded
• Use system calls only when you need to (why?)
• Write a README with the description of your project
• You should have an AUTHORS file at the root of your repository, listing all individuals having contributed content to the repository. Format, see Docker

GitHub

*There should be one project repository per group. If you and your partner have a repository with the same name in both your accounts, you risk a 0% score. Add your partner as a collaborator. *

Output

Unless specified otherwise, your program must have the exact same output as sh (/bin/sh) as well as the exact same error output. The only difference is when you print an error, the name of the program must be equivalent to your argv[0]

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)

Compilation

Your shell will be compiled this way: gcc -Wall -Werror -Wextra -pedantic -std=gnu89 *.c -o hsh

Testing

Your shell should work like this in interactive mode:

$ ./hsh
($) /bin/ls
hsh main.c shell.c
($)
($) exit
$

But also in non-interactive mode:

$ echo "/bin/ls" | ./hsh
hsh main.c shell.c test_ls_2
$
$ cat test_ls_2
/bin/ls
/bin/ls
$
$ cat test_ls_2 | ./hsh
hsh main.c shell.c test_ls_2
hsh main.c shell.c test_ls_2
$

Checks

We strongly encourage the entire class to work together to create a suite of checks covering both regular tests and edge cases for each task. See task 8. Test suite.

GitHub repository: simple_shell

Tasks

Task 0. Betty would be proud

Write a beautiful code that passes the Betty checks

Task 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…

Task 2. Simple shell 0.2

Simple shell 0.1 + Handle command lines with arguments

Task 3. Simple shell 0.3

Simple shell 0.2 + Handle the PATH

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

Task 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

Task 5. Simple shell 1.0

Simple shell 0.4 + Implement the env built-in, that prints the current environment

Task 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

Task 7. Simple shell 0.2.1

Simple shell 0.2 + You are not allowed to use strtok

Task 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

Task 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

Task 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

Task 11. ;

Simple shell 1.0 + Handle the commands separator ;

Task 12. && and ||

Simple shell 1.0 + Handle the && and || shell logical operators

Task 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

Task 14. Variables

Simple shell 1.0 +

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

Task 15. Comments

Simple shell 1.0 + Handle comments (#)

Task 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

Authors: Ernest Bhekizwe Shongwe, Garrick Larson Potts. (Credit to my Printf patner, Zikona Mnyaka, for bringing this project ack to life)