Executing string sent from one terminal in another in Linux pseudo-terminal

Question

Lets say I have one terminal where the output of \"tty\" is \"/dev/pts/2\" From another terminal, I want to send a command to the first terminal and execute it. Using: echo \"ls

Answer 1

Usually getty, login, and shell programs are needed for executing commands from tty.

But you can also put a shell directly executing commands from a pseudo terminal. This is simplified example (all error checkings removed):

int main( int argc, char** argv )
{
    int master_fd = create_my_own_psudo_terminal() ;

    // Wait until someone open the tty
    fd_set fd_rset;
    FD_ZERO( &fd_rset );
    FD_SET( master_fd, &fd_rset );
    select( master_fd + 1, &fd_rset, NULL, NULL, NULL );

    dup2( master_fd, STDIN_FILENO );
    execl("/bin/sh", "sh", 0 );

    return 0;
}

Now you can do the following:

Start this simple program in the first terminal.

And send your command from the second terminal:

echo "ls" > /dev/pts/5

And you will get listing in the first terminal.

Note: This is quite unsecure, because login is not done.