Redirect Commands to Other Terminals

I had a friend ask me today if it would be possible to send commands to another terminal in Linux. Now I am not talking about how to send stdio to other terminals (echo "Test" > /dev/ttyX), but to actually run the command in that terminal.

I came across a small tool to do this for us.

Create a file called ttysend.c and put the following inside it:

#include <stdio.h> 
#include <stdlib.h> 
#include <fcntl.h> 
#include <sys/stat.h> 
#include <sys/ioctl.h> 
#include <string.h> 
#include <unistd.h> 

void print_help(char *prog_name) {
        printf("Usage: %s [-n] DEVNAME COMMAND\n", prog_name);
        printf("Usage: '-n' is an optional argument if you want to push a new line at the end of the text\n");
        printf("Usage: Will require 'sudo' to run if the executable is not setuid root\n");
        exit(1);
}

int main (int argc, char *argv[]) {
    char *cmd, *nl = "\n";
    int i, fd;
    int devno, commandno, newline;
    int mem_len;
    devno = 1; commandno = 2; newline = 0;
    if (argc < 3) {
        print_help(argv[0]);
    }
    if (argc > 3 && argv[1][0] == '-' && argv[1][1] == 'n') {
        devno = 2; commandno = 3; newline=1;
    } else if (argc > 3 && argv[1][0] == '-' && argv[1][1] != 'n') {
        printf("Invalid Option\n");
        print_help(argv[0]);
    }
    fd = open(argv[devno],O_RDWR);
    if(fd == -1) {
        perror("open DEVICE");
        exit(1);
    }
    mem_len = 0;
    for ( i = commandno; i < argc; i++ ) {
        mem_len += strlen(argv[i]) + 2;
        if ( i > commandno ) {
            cmd = (char *)realloc((void *)cmd, mem_len);
        } else { //i == commandno
            cmd = (char *)malloc(mem_len);
        }

        strcat(cmd, argv[i]);
        strcat(cmd, " ");
    }
  if (newline == 0)
        usleep(225000);
    for (i = 0; cmd[i]; i++)
        ioctl (fd, TIOCSTI, cmd+i);
    if (newline == 1)
        ioctl (fd, TIOCSTI, nl);
    close(fd);
    free((void *)cmd);
    exit (0);
} 

Now compile it by running "make ttysend".

To run the tool, first find out what terminal you are currently using that you want to send commands to by typing "tty". Then in another terminal run the following:

sudo ./ttysend -n [tty_to_redirect_to] [command_plus_args]

This ttysend must be run as user root or setuid root (chmod u+s).