redirecting output to a file in C

Question

i coded a basic shell in C for executing basic commands it\'ll execute commands ls, ls -al , ls -al | more etc.

i want to exec

Answer 1

Before you call execve(2) in the newly created process to execute the command, you can redirect its standard input or output via the dup2(2) system call:

/* redirect stdout to a file */
dup2(1, some_open_file_descriptor);

Of course, you need to have some error handling.

Answer 2

Don't use pipe when the output is to go to a file.

When you fork the child to run the ls command, you note the redirection, and open the file; you then use dup2() (or close() and dup()) so that the file descriptor is now standard output for the child; you close the duplicated file descriptor - the one returned by open(); then you execute ls as usual; its standard output is now sent to the file.

Note that you do the non-pipe I/O redirection after forking, not before. Pipes have to be set up before forking, but other I/O redirection does not.

Answer 3

This is the result of my testing things out with dup2

The more subtle point is remembering fflush at the right times :) Otherwise, you'll get very surprising results.

Also, prefer fileno instead of hardcoding 1 (stdout) 2 (stderr).

Redirecting stdin was left as an exercise for the reader

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>

int main(int argc, const char *argv[])
{
    int out = open("cout.log", O_RDWR|O_CREAT|O_APPEND, 0600);
    if (-1 == out) { perror("opening cout.log"); return 255; }

    int err = open("cerr.log", O_RDWR|O_CREAT|O_APPEND, 0600);
    if (-1 == err) { perror("opening cerr.log"); return 255; }

    int save_out = dup(fileno(stdout));
    int save_err = dup(fileno(stderr));

    if (-1 == dup2(out, fileno(stdout))) { perror("cannot redirect stdout"); return 255; }
    if (-1 == dup2(err, fileno(stderr))) { perror("cannot redirect stderr"); return 255; }

    puts("doing an ls or something now");

    fflush(stdout); close(out);
    fflush(stderr); close(err);

    dup2(save_out, fileno(stdout));
    dup2(save_err, fileno(stderr));

    close(save_out);
    close(save_err);

    puts("back to normal output");

    return 0;
}