0x16. C - Simple Shell C
Group project
Syscall
Writting a simple UNIX command interpreter.
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)
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”?
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)
shell was compiled this way:
gcc -Wall -Werror -Wextra -pedantic -std=gnu89 *.c -o hsh
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 0.1 +
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
6 Simple shell 0.4 +
Implement the env built-in, that prints the current environment
7 Simple shell 0.2 +
You are not allowed to use strtok
8 Simple shell 0.4 +
handle arguments for the built-in exit
Usage: exit status, where status is an integer used to exit the shell
9 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
10 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
11 Simple shell 1.0 +
Handle the commands separator ;
12 Simple shell 1.0 +
Handle the && and || shell logical operators
13 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
14 Simple shell 1.0 +
Handle variables replacement
Handle the $? variable
Handle the $$ variable
15 Simple shell 1.0 +
Handle comments (#)
16 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.