How to implement env | grep LANG with fork execve and pipe - c

Question

I'm writing my minishell and I can't understand why execve doesn't work when calling pid_2 ?
My main task is to implement env | grep LANG
int main(void)
{
pid_t pid_1, pid_2;
int fd[2];
int status;
char *mass_1[] = {"env", NULL};
char *mass_2[] = {"grep", "LANG", NULL};
pid_1 = fork();
pipe(fd);
if (pid_1 == 0)
{
dup2(fd[1], 0);
close(fd[0]);
execve(mass_1[0], mass_1, NULL);
exit(1);
}
pid_2 = fork();
if (pid_2 == 0)
{
dup2(fd[0], 0);
close(fd[1]);
execve(mass_2[0], mass_2, NULL);
exit(1);
}
close(fd[0]);
close(fd[1]);
waitpid(pid_1, &status, WUNTRACED);
waitpid(pid_2, &status, WUNTRACED);
return (0);
}

Answer 1

There are three issues in your code:
execve() doesn't search for the command in $PATH so generally it won't find neither env nor grep. Use execvp() instead.
You should call pipe() before the first fork() to prevent another pipe to be created and used by the child process
as #KamilCuk has pointed out: in the first child process, you should dup2() stdout instead of stdin
Fixing these bugs lead to this working code
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
#include <errno.h>
int main(void)
{
pid_t pid_1, pid_2;
int fd[2];
int status;
char *mass_1[] = {"env", NULL};
char *mass_2[] = {"grep", "LANG", NULL};
pipe(fd);
pid_1 = fork();
if (pid_1 == 0)
{
dup2(fd[1], 1);
close(fd[0]);
execvp(mass_1[0], mass_1);
exit(1);
}
pid_2 = fork();
if (pid_2 == 0)
{
dup2(fd[0], 0);
close(fd[1]);
execvp(mass_2[0], mass_2);
exit(1);
}
close(fd[0]);
close(fd[1]);
waitpid(pid_1, &status, WUNTRACED);
waitpid(pid_2, &status, WUNTRACED);
return (0);
}
Btw. you should add some error handling e.g. pipe() or fork() could fail
If you must not call execvp() you have to provide pathnames to execve() like "/usr/bin/env" or "/bin/grep". In this case you also have to provide the parent's environment at least to env otherwise it won't print anything.
The easiest way to do so is:
change the signature of main() to int main(int argc, char *argv[], char *envp[])
call execve(mass_1[0], mass_1, envp);