Appreciate the madness of Minihell.
This project may seem simple at first glace, however, it took a good portion of a 4 month
period to perfect and complete, and indroduced my teammate and I to many new concepts
that we will continue to practice throughout our coding careers.
For example:
Multi-processing using the fork() function in C, and piping the output of one child
process to the next using pipe() and dup() / dup2().
One of the most prevelent and important subjects that this project taught me was how
shell script is parsed, and expansions are applied, seeing as in the 42 cursus we
incorporate the use of shells every day it is critical that we as students understand
how shells are used as primary means of communication between the user and the machine.
It is important to note that not all shells are created equal, and neither are the
Minishells produced by 42 students. No shell will ever perfect.
-
Must display a prompt.
SEA SHELL ➜ -
The history must be working.
-
Must be able to search and launch the correct executables according to the PATH variable included in the envp.
-
More than one global variable is forbidden.
-
No need to interpret special characters like '\', ';' or handle unclosed quotes.
-
Must handle single quotes as a shell does ( prevents expansions ), and prevents interpreting of metacharacters.
-
Must handle double quotes which prevents interpreting of metacharacters, however, expansions ('$') are not prevented.
E.g: echo "hello$USER:" should output helloUserName: whereas echo 'hello$USER:' should output hello$USER: without expanding. -
Must implement redirections ( > , < , >> , <<)
Types of redirection:
> , >> : stdout redirections - creates files, and redirects outputs to those files. '>' overwrites pre-existing content, whereas '>>' appends new content to existing content. < , << : stdin redirections - < redirects input from a file to a child process. << (Heredoc) redirects input from the stdin on the terminal to a child process, it may also display a prompt whilst waiting for input. << (Heredoc) will stop once it receives a signal, or the deliminator is entered. The syntax for Heredoc is always 'Command << deliminator.'When you encounter a sequence of the same redirection type only the last redirection in the sequence is used. The same applies to the Heredoc ( << ) redirection. E.g: cat < test1 < test2 < test3 - only test3 will be piped to cat, similarly cat > test1 < test2 > test3 < test4 - test4 will be piped into cat and the output will be piped to test3. Mixed E.g: cat < test1 > test2 << bob >> test3 - Only the << (Heredoc) with bob as the deliminator will be taken as input, and the output will be appended into test3. Hint: Even though only the last redirection of a particular redir. type is used the shell will still ensure that each file exists, or in the case of stdout redir. it will go and create each file, but only output to the last one. -
Must implement pipes ( | ):
The output of each child process in the pipeline should be piped to the next child process in the pipeline. E.g: echo hello world | cat | grep -o hello | cat -e The output of echo is piped to cat, then cat's output is piped to the grep command, and so forth until the end. The last command will then display the output on the regular STDOUT or file descriptor 1. Pipes and redirections can be mixed as well inorder to redirect stdin or stdout within a pipeline. E.g: cat < test.txt | cat -e > test2.txt | echo hi | cat -e -
Handle '$' which will cause it and the consecutive characters to expand.
If a '$' is found within a string of characters the shell takes the characters to the right up until the first non apha / numerical character as the name of an evironmental variable. It will expand the variable to it's value in the envp. E.g: echo $PWD will output the current directory: /Users/username/Desktop/minishell similarly echo $USER will expand to the username in the envp. Sometimes even mixed: E.g: echo Hello$PWD: this will output Hello/Users/username/Desktop/minishell: All of this was easily achieved using my ft_strncmp(), modified ft_strjoin(), and ft_substr() included in libft. In order to see the list of environmental variables, take in envp as 3rd argument for your main() function. This little program will display the envp from the parent shell: #include <stdio.h> int main(int argc, char **argv, char **envp) { int i; i = -1; while (envp[++i]) printf("%s\n", envp[i]); (void) argc; (void) argv; return (0); } Alternatively if you're lazy like me, just type env in the terminal ;) -
One non-env expansion: '$?,' which expands to the exit status of the last child-process to be executed.
-
Crtl-c (SIGINT), crtl-d (eof), and crtl-\ (SIGQUIT).
Type man signal into the terminal to findout more about SIGINT and SIGQUIT. Crtl-d is simply an EndOfFile input. The behaviour of these signals should be like in bash. They work normally on the child-processes, however, SIGINT simply displays a new prompt and SIGQUIT does nothing. It is worth noting that SIGINT changes the last exit status. -
It is required by the subject that we implement several built-ins (thank goodness it's not all of bash's builtins).
1. echo - with the option to enter -n as a flag. 2. cd - with only absolute or relative path. 3. pwd - print working directory. 4. export - exports new environmental variables. 5. unset - unsets environmental variables. 6. env - without any arguments or options. 7. and - exit without options.
Throughout the process of developing we encountered a number of problems
to overcome.
For example: it took me 3 weeks to create the perfect parser through trial
and error, the expansions process we redesigned multiple times in order
to accomodate for quotes within the expansions, and the fact that neither
I nor my partner had any prior experience in multi-processing to benefit
from. Non-the-less we persevered and became better coders for it.
One lesson that formed a big part of the project was knowing when you've
done enough. The many features you add to your minishell must work
flewlessly, thus the more you steer away from the minimum the project
requires, the more effort you should put into testing every aspect.
As 42 students time is a rare and valuable commodity when you've got a
blackhole threatening to swallow you.
Building a shell is not an easy task, and the more features you want to implement,
the longer it takes to complete. For any 42 student busy with this project,
there are plenty of resources online, but the most important resource will always
be your peers.
And for those who are just curious about this project, you are free to clone and
enjoy my personal mini - version of bash :) just type 'make' and execute './minishell'
inside a unix / linux environment. For windows users, you might have to install a
virtual machine / docker, or use WSL.
Thank you for the read and I hope this was informative.