Grabbing output from exec

Question

I'm trying to write a C program that grabs command output and then i'll be passing that to another program.

I'm having an issue, I cant work out how to get the command output and store it. Below is a sample of what I have

if(fork() == 0){    execl("/bin/ls", "ls", "-1", (char *)0);    /* do something with the output here */ } else{     //*other stuff goes here* } 

so basically im wondering if there is any way i can get the output from the "execl" and pass it to some thing else (e.g. via storing it in some kind of buffer).

Suggestions would be great.

Answer 1

You have to create a pipe from the parent process to the child, using pipe(). Then you must redirect standard ouput (STDOUT_FILENO) and error output (STDERR_FILENO) using dup or dup2 to the pipe, and in the parent process, read from the pipe. It should work.

#include <stdio.h> #include <stdlib.h> #include <unistd.h>  #define die(e) do { fprintf(stderr, "%s\n", e); exit(EXIT_FAILURE); } while (0);  int main() {   int link[2];   pid_t pid;   char foo[4096];    if (pipe(link)==-1)     die("pipe");    if ((pid = fork()) == -1)     die("fork");    if(pid == 0) {      dup2 (link[1], STDOUT_FILENO);     close(link[0]);     close(link[1]);     execl("/bin/ls", "ls", "-1", (char *)0);     die("execl");    } else {      close(link[1]);     int nbytes = read(link[0], foo, sizeof(foo));     printf("Output: (%.*s)\n", nbytes, foo);     wait(NULL);    }   return 0; } 

Answer 2

Open a pipe, and change stdout to match that pipe.

 #include <sys/types.h>  #include <unistd.h>  #include <stdio.h>  #include <stdlib.h>   int pipes[2];   pipe(pipes); // Create the pipes   dup2(pipes[1],1); // Set the pipe up to standard output 

After that, anything which goes to stdout,(such as through printf), comes out pipe[0].

FILE *input = fdopen(pipes[0],"r"); 

Now you can read the output like a normal file descriptor. For more details, look at this