Exam Question

This exam has 1 question, microshell:

if you can make this code shorter, but readable, let me know!

Excepted Files

microshell.c
microshell.h

Subject Text

Allowed functions:

malloc, free, write, close, fork, waitpid, signal, kill, exit, chdir, execve, dup, dup2, pipe, strcmp, strncmp

The Program

Write a program that will behave like executing a shell command

The command line to execute will be the arguments of this program
Executable's path will be absolute or relative but your program must not build a path (from the PATH variable for example)
You must implement "|" and ";" like in bash
- we will never try a "|" immediately followed or preceded by nothing or "|" or ";"
Your program must implement the built-in command cd only with a path as argument (no '-' or without parameters)
- if cd has the wrong number of argument your program should print in STDERR "error: cd: bad arguments" followed by a '\n'
- if cd failed your program should print in STDERR "error: cd: cannot change directory to path_to_change" followed by a '\n' with path_to_change replaced by the argument to cd
- a cd command will never be immediately followed or preceded by a "|"
You don't need to manage any type of wildcards (*, ~ etc...)
You don't need to manage environment variables ($BLA ...)
If a system call, except execve and chdir, returns an error your program should immediatly print "error: fatal" in STDERR followed by a '\n' and the program should exit
If execve failed you should print "error: cannot execute executable_that_failed" in STDERR followed by a '\n' with executable_that_failed replaced with the path of the failed executable (It should be the first argument of execve)
Your program should be able to manage more than hundreds of "|" even if we limit the number of "open files" to less than 30.

Example

for example this should work:

$>./microshell /bin/ls "|" /usr/bin/grep microshell ";" /bin/echo i love my microshell
microshell
i love my microshell
$>

>./microshell

Hints

Don't forget to pass the environment variable to execve
Do not leak file descriptors!

Exam Practice Tool

Practice the exam just like you would in the real exam - https://github.com/JCluzet/42_EXAM

Code Commented

#include <unistd.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <string.h>

// Function to write an error message to standard error (file descriptor 2)
int err(char *str) 
{
    while (*str)
        write(2, str++, 1); // Writing each character of the string to stderr
    return 1;
}

// Function to change the current directory
int cd(char **argv, int i) 
{
    if (i != 2)
        return err("error: cd: bad arguments\n"); // Return an error if the argument count is not 2
    else if (chdir(argv[1]) == -1)
        return err("error: cd: cannot change directory to "), err(argv[1]), err("\n"); // Return an error if directory change fails
    return 0; // Return 0 on success
}

// Function to execute a command
int exec(char **argv, char **envp, int i) 
{
    int fd[2];
    int status;
    int has_pipe = argv[i] && !strcmp(argv[i], "|"); // Check if a pipe is present in the command

    if (has_pipe && pipe(fd) == -1)
        return err("error: fatal\n"); // Return an error if pipe creation fails

    int pid = fork(); // Create a child process
    if (!pid) 
    {
        argv[i] = 0; // Set the element after the pipe or semicolon to null
        if (has_pipe && (dup2(fd[1], 1) == -1 || close(fd[0]) == -1 || close(fd[1]) == -1))
            return err("error: fatal\n"); // Return an error if pipe redirection or closing fails
        execve(*argv, argv, envp); // Execute the command
        return err("error: cannot execute "), err(*argv), err("\n"); // Return an error if execution fails
    }

    waitpid(pid, &status, 0); // Wait for the child process to finish
    if (has_pipe && (dup2(fd[0], 0) == -1 || close(fd[0]) == -1 || close(fd[1]) == -1))
        return err("error: fatal\n"); // Return an error if pipe redirection or closing fails
    return WIFEXITED(status) && WEXITSTATUS(status);
}

int main(int argc, char **argv, char **envp) 
{
    int    i = 0;
    int    status = 0;

    if (argc > 1) 
    {
        while (argv[i] && argv[++i]) 
        {
            argv += i; // Move the argv pointer forward by i
            i = 0; // Reset i to 0
            while (argv[i] && strcmp(argv[i], "|") && strcmp(argv[i], ";"))
                i++; // Find the next pipe or semicolon or end of arguments
            if (!strcmp(*argv, "cd"))
                status = cd(argv, i); // Execute cd command
            else if (i)
                status = exec(argv, envp, i); // Execute other commands
        }
    }
    return status; // Return the status of the last executed command
}

brmoretti/42-School-Exam-Rank-04