0x16. C - Simple Shell

• By Julien Barbier
• Project to be done in teams of 2 people (Team: Aminat Oyeniyi, Jamila Moseka)

Concepts

For this project, students are expected to look at these concepts:

• Everything you need to know to start coding your own shell
• Approaching a Project

Background Context

Write a simple UNIX command interpreter.

^ "The Gates of Shell", by Spencer Cheng, featuring Julien Barbier

Resources

Read or watch:

• Unix shell
• Thompson shell
• Ken Thompson
• Everything you need to know to start coding your own shell concept page

man or help:

• sh (Run sh as well)

Learning Objectives

At the end of this project, you are expected to be able to explain to anyone, without the help of Google:

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"?

Requirements

General

• Allowed editors: vi, vim, emacs
• All your files will be compiled on Ubuntu 20.04 LTS using gcc, using the options -Wall -Werror -Wextra -pedantic -std=gnu89
• 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?)

GitHub

There should be one project repository per group. If you clone/fork/whatever a project repository with the same name before the second deadline, you risk a 0% score.

More Info

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] (See below)

Example of error with sh:

$ echo "qwerty" | /bin/sh
/bin/sh: 1: qwerty: not found
$ echo "qwerty" | /bin/../bin/sh
/bin/../bin/sh: 1: qwerty: not found
$

Same error with your program hsh:

$ echo "qwerty" | ./hsh
./hsh: 1: qwerty: not found
$ echo "qwerty" | ./././hsh
./././hsh: 1: qwerty: not found
$

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

The Checker will be released at the end of the project (1-2 days before the deadline). 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.

Tasks

0. README, man, AUTHORS

mandatory

• Write a README
• Write a man for your shell.
• You should have an AUTHORS file at the root of your repository, listing all individuals having contributed content to the repository. Format, see Docker

Repo:

• GitHub repository: simple_shell
• File: README.md, man_1_simple_shell, AUTHORS

1. Betty would be proud

mandatory

Write a beautiful code that passes the Betty checks

Repo:

• GitHub repository: simple_shell

2. Simple shell 0.1

mandatory

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

julien@ubuntu:~/shell$ ./shell
#cisfun$ ls
./shell: No such file or directory
#cisfun$ /bin/ls
barbie_j       env-main.c  exec.c  fork.c  pid.c  ppid.c    prompt   prompt.c  shell.c  stat.c         wait
env-environ.c  exec    fork    mypid   ppid   printenv  promptc  shell     stat test_scripting.sh  wait.c
#cisfun$ /bin/ls -l
./shell: No such file or directory
#cisfun$ ^[[D^[[D^[[D
./shell: No such file or directory
#cisfun$ ^[[C^[[C^[[C^[[C
./shell: No such file or directory
#cisfun$ exit
./shell: No such file or directory
#cisfun$ ^C
julien@ubuntu:~/shell$ echo "/bin/ls" | ./shell
#cisfun$ barbie_j       env-main.c  exec.c  fork.c  pid.c  ppid.c    prompt   prompt.c  shell.c stat.c         wait
env-environ.c  exec    fork    mypid   ppid   printenv  promptc  shell     stat test_scripting.sh  wait.c
#cisfun$ julien@ubuntu:~/shell$

Repo:

• GitHub repository: simple_shell

3. Simple shell 0.2

mandatory

Simple shell 0.1 +

• Handle command lines with arguments

Repo:

• GitHub repository: simple_shell

4. Simple shell 0.3

mandatory

Simple shell 0.2 +

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

julien@ubuntu:~/shell$ ./shell_0.3
:) /bin/ls
barbie_j       env-main.c  exec.c  fork.c  pid.c  ppid.c    prompt   prompt.c  shell_0.3  stat    test_scripting.sh  wait.c
env-environ.c  exec    fork    mypid   ppid   printenv  promptc  shell     shell.c    stat.c  wait
:) ls
barbie_j       env-main.c  exec.c  fork.c  pid.c  ppid.c    prompt   prompt.c  shell_0.3  stat    test_scripting.sh  wait.c
env-environ.c  exec    fork    mypid   ppid   printenv  promptc  shell     shell.c    stat.c  wait
:) ls -l /tmp
total 20
-rw------- 1 julien julien    0 Dec  5 12:09 config-err-aAMZrR
drwx------ 3 root   root   4096 Dec  5 12:09 systemd-private-062a0eca7f2a44349733e78cb4abdff4-colord.service-V7DUzr
drwx------ 3 root   root   4096 Dec  5 12:09 systemd-private-062a0eca7f2a44349733e78cb4abdff4-rtkit-daemon.service-ANGvoV
drwx------ 3 root   root   4096 Dec  5 12:07 systemd-private-062a0eca7f2a44349733e78cb4abdff4-systemd-timesyncd.service-CdXUtH
-rw-rw-r-- 1 julien julien    0 Dec  5 12:09 unity_support_test.0
:) ^C
julien@ubuntu:~/shell$

Repo:

• GitHub repository: simple_shell

5. Simple shell 0.4

mandatory

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

Repo:

• GitHub repository: simple_shell

6. Simple shell 1.0

mandatory

Simple shell 0.4 +

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

julien@ubuntu:~/shell$ ./simple_shell
$ env
USER=julien
LANGUAGE=en_US
SESSION=ubuntu
COMPIZ_CONFIG_PROFILE=ubuntu
SHLVL=1
HOME=/home/julien
C_IS=Fun_:)
DESKTOP_SESSION=ubuntu
LOGNAME=julien
TERM=xterm-256color
PATH=/home/julien/bin:/home/julien/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin
DISPLAY=:0
$ exit
julien@ubuntu:~/shell$

Repo:

• GitHub repository: simple_shell

7. What happens when you type ls -l *.c in the shell

mandatory

Write a blog post describing step by step what happens when you type ls -l *.c and hit Enter in a shell. Try to explain every step you know of, going in as much details as you can, give examples and draw diagrams when needed. You should merge your previous knowledge of the shell with the specifics of how it works under the hoods (including syscalls).

• Have at least one picture, at the top of the blog post
• Publish your blog post on Medium or LinkedIn
• Share your blog post at least on LinkedIn
• Only one blog post by team
• The blog post must be done and published before the first deadline (it will be part of the manual review)
• Please, remember that these blogs must be written in English to further your technical ability in a variety of settings