This exam has 1 question, microshell: k
if you can make this code shorter, but readable, let me know!
- microshell.c
Allowed functions:
malloc, free, write, close, fork, waitpid, signal, kill, exit, chdir, execve, dup, dup2, pipe, strcmp, strncmp
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 ";"
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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...)
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You don't need to manage environment variables ($BLA ...)
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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
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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.
for example this should work:
$>./microshell /bin/ls "|" /usr/bin/grep microshell ";" /bin/echo i love my microshell
microshell
i love my microshell
$>
>./microshell
- Don't forget to pass the environment variable to execve
- Do not leak file descriptors!
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>
// Function to write an error message to stderr
int err(char *str)
{
// Loop through each character in the string and write it to stderr
while (*str)
write(2, str++, 1);
return 1;
}
// Function to change the current working directory
int cd(char **argv, int i)
{
// If the number of arguments is not 2, return an error
if (i != 2)
return err("error: cd: bad arguments\n");
// If changing the directory fails, return an error
if (chdir(argv[1]) == -1)
return err("error: cd: cannot change directory to "), err(argv[1]), err("\n");
return 0;
}
// Function to execute a command
int exec(char **argv, int i, char **envp)
{
int fd[2];
int status;
// Check if the command includes a pipe
int has_pipe = argv[i] && !strcmp(argv[i], "|");
// If the command is 'cd', execute it
if (!has_pipe && !strcmp(*argv, "cd"))
return cd(argv, i);
// If the command includes a pipe and creating the pipe fails, return an error
if (has_pipe && pipe(fd) == -1)
return err("error: fatal\n");
// Fork the process
int pid = fork();
if (!pid)
{
argv[i] = 0;
// If the command includes a pipe and setting up the pipe fails, return an error
if (has_pipe && (dup2(fd[1], 1) == -1 || close(fd[0]) == -1 || close(fd[1]) == -1))
return err("error: fatal\n");
// If the command is 'cd', execute it
if (!strcmp(*argv, "cd"))
return cd(argv, i);
// Execute the command
execve(*argv, argv, envp);
// If executing the command fails, return an error
return err("error: cannot execute "), err(*argv), err("\n");
}
// Wait for the child process to finish
waitpid(pid, &status, 0);
// If the command includes a pipe and setting up the pipe fails, return an error
if (has_pipe && (dup2(fd[0], 0) == -1 || close(fd[0]) == -1 || close(fd[1]) == -1))
return err("error: fatal\n");
// Return the exit status of the child process
return WIFEXITED(status) && WEXITSTATUS(status);
}
int main(int argc, char **argv, char **envp)
{
int i = 0;
int status = 0;
if (argc > 1)
{
// Loop through each argument
while (argv[i] && argv[++i])
{
// Move the pointer to the next argument
argv += i;
i = 0;
// Loop through each argument until a pipe or semicolon is found
while (argv[i] && strcmp(argv[i], "|") && strcmp(argv[i], ";"))
i++;
// If there are arguments, execute them
if (i)
status = exec(argv, i, envp);
}
}
return status;
}
Practice the exam just like you would in the real exam - https://github.com/JCluzet/42_EXAM