linux - 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 execute following command in my shell. like ;

ls -al > a.txt

this will give me a.txt file which contain output of ls -al process. i found a one solution, it's changing the command in my shell like [command1] | tee [filename]. in this instance it'll change ls -al > a.txt to ls -al | tee a.txt. but this process gives the output to file and terminal as well. how to stop printing output in terminal.

or are there are any better solution rather than using tee command. thanks in advance...

Answer 1

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;
}