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cexecvpfreopen

freopen and execvp in c

I am doing a minor shell implementation in c, and I am stuck.

I want to be able to redirect stdin and stdout, but I am confused. In, my shell, when I want to start a program, I use the execvp function. Now I would like to be able to redirect stdout, so If I run another process, the output should be redirected to a file instead of the screen.

Here is sample code:

pid_t pid;

// Child process
pid = vfork();

if((pid == 0)){
    freopen("myfile.txt", "w", stdout);
    char* arr[3];
    arr[0] = "cat";
    arr[1] = "someFileToCat.txt";
    arr[2] = NULL;
   execvp("cat", arr);
   fclose(stdout);
}

It does however print in the terminal, and not in the file.

Solution

  • File streams are a C abstraction. What you are looking for are lower level system calls such as open, close, and dup2.

    See Redirecting exec output to a buffer or file for a full example.