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;
}

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.