I'm making a C program where it basically reads in a line from the user, interprets it and then tries to execute the command with execve. I'm also forking the execve to a child process if '&' is in the input.
Now I wish to hide any terminal output which comes from the execve command when it's running in the child process. Is there any relatively easy way to do this?
You can hide the output by redirecting stdout and stderr to /dev/null after forking but before execve(). The idea is to open /dev/null, then make stdout and stderr duplicates of the obtained file descriptor with dup2() (which will also close the originals first). It's almost the same as redirecting to a pipe.
An example (incomplete program, and skipping most error checking):
#include <unistd.h>
#include <fcntl.h>
...
int pid = fork();
if (pid == -1) {
/* fork error */
exit(1);
} else if (pid == 0) {
/* child process */
/* open /dev/null for writing */
int fd = open("/dev/null", O_WRONLY);
dup2(fd, 1); /* make stdout a copy of fd (> /dev/null) */
dup2(fd, 2); /* ...and same with stderr */
close(fd); /* close fd */
/* stdout and stderr now write to /dev/null */
/* ready to call exec */
execve(cmd, args, env);
exit(1);
} else {
/* parent process */
...
I've written simple example, maybe it will help you.
First, try to call it without | echo $1 > /dev/null - it should print files. When you add it, output is empty.
#include <stdio.h>
#include <unistd.h>
int main()
{
int ret;
char *cmd[] = { "ls", "-l", (char *)0 };
char *env[] = {(char *)0 };
ret = execve ("/bin/ls | echo $1 > /dev/null", cmd, env);
return 0;
}
