Write program that pretends to be a TTY

Question

I'm writing a program that reads input from stdin, manipulates the input, and writes output to stdout. However, many programs check whether stdin is a terminal or a pipe (by calling a function like isatty), and generate output differently. How do I have my program pretend to be a TTY?

The solution should work on both Linux and macOS. Any programming language that generates a standalone binary is acceptable, but Go is preferred.

Note that I'm asking a programming question, not asking for a tool. So, things like script or unbuffer is not something I'm looking for.

Answer 1

The following is fully working code for running a command in a pty and capturing its output. (Not as many lines as you may have thought.)

#include <signal.h>
#include <stdlib.h>
#include <sysexits.h>
#include <unistd.h>
#include <util.h>

pid_t child = 0;

void sighandler(int signum) {
  if (child > 0) {
    killpg(child, signum);
    exit(signum);
  }
}

// Run a command in a pty.
// Usage: /path/to/this/binary command to run
int main(int argc, char *argv[]) {
  if (argc < 2) {
    return EX_USAGE;
  }

  int master;
  child = forkpty(&master, NULL, NULL, NULL);

  if (child == -1) {
    perror("failed to fork pty");
    return EX_OSERR;
  }

  if (child == 0) {
    // we're in the child process, so replace it with the command
    execvp(argv[1], argv + 1);
    perror("failed to execute command");
    return EX_OSERR;
  }

  // trap kill signals and forward them to child process
  signal(SIGHUP, sighandler);
  signal(SIGINT, sighandler);
  signal(SIGTERM, sighandler);

  const int buf_size = 1024;
  char buf[buf_size];
  fd_set fds;
  ssize_t bytes_read;

  // forward the output continuously
  while (1) {
    FD_ZERO(&fds);
    FD_SET(master, &fds);

    if (select(master + 1, &fds, NULL, NULL, NULL) > 0 && FD_ISSET(master, &fds)) {
      bytes_read = read(master, buf, buf_size);
      if (bytes_read <= 0) {
        return EXIT_SUCCESS;
      }

      if (write(STDOUT_FILENO, buf, bytes_read) != bytes_read) {
        perror("failed to write to stdout");
        return EX_OSERR;
      }
    }
  }
}