How to put a job in background without output?

Question

When putting a job in background with the command bg, I have the output of the command of the job.

How to put the job in background (like the bg command) but without any output?

PS: the job is linked to a command with an output (there is no >/dev/null 2>1 in the original command)

Answer 1

You need to tell the now backgrounded application to stop writing on the tty device. There is no generic way to do that.

You can do:

stty tostop

to make so that background jobs be suspended (with a SIGTTOU signal) when they try to write to the tty.

You can attach a debugger to the process and make it re-open the file descriptors it has open on the tty device to /dev/null. Like (here for stdout only):

gdb --batch -ex 'call close(1)' -ex 'call open("/dev/null",2)' -p "$pid"

(assuming the application is dynamically linked or has debug symbols and note that some systems will have security restrictions preventing you from doing it).

For dynamically linked applications, you could run them with a LD_PRELOAD blob that installs a handler on SIGTTOU (and do the stty tostop) that reopens stdout and stderr on /dev/null if they were going to a terminal. That would handle the case of non-setuid/setgid applications writing on the terminal via their stdout/stderr and don't handle SIGTTOU themselves.

Run:

 gcc -Wall -shared -fPIC -nostartfiles -o ~/lib/handle_tostop.so handle_tostop.c

Where handle_tostop.c contains:

#include <signal.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

static void reopen_on_null(int req_fd)
{
  int fd;
  if (close(req_fd) == 0) {
    fd = open("/dev/null", O_RDWR);
    if (fd != req_fd && fd >= 0) {
      dup2(fd, req_fd);
      close(fd);
    }
  }
}

static void handle_sigttou(int sig)
{
  if (isatty(1)) reopen_on_null(1);
  if (isatty(2)) reopen_on_null(2);
}
void _init()
{
  signal(SIGTTOU, handle_sigttou);
}

And then:

LD_PRELOAD=~/lib/handle_tostop.so
export LD_PRELOAD
stty tostop

Instead of /dev/null, you may want to redirect the output to some log file (which you might then want to open with O_APPEND, and possibly include the pid of the process in the file name so you know whose process the output is from) so it's not discarded if it's of any use to you.

Answer 2

If you use nohup your jobs will run in background, but this create a automatic output (nohup.out).