How to suspend and bring a background process to foreground

Question

I have a process originally running in the foreground. I suspended by Ctrl+Z, and then resume its running in the background by bg <jobid>.

I wonder how to suspend a process running in the background?

How can I bring a background process to foreground?

Edit:

The process outputs to stderr, so how shall I issue the command fg <jobid> while the process is outputting to the terminal?

Answer 1

As Tim said, type fg to bring the last process back to foreground.

If you have more than one process running in the background, do this:

$ jobs
[1]   Stopped                 vim
[2]-  Stopped                 bash
[3]+  Stopped                 vim 23

fg %3 to bring the vim 23 process back to foreground.

To suspend the process running in the background, use:

kill -STOP %job_id

The SIGSTOP signal stops (pauses) a process in essentially the same way Ctrl+Z does.

example: kill -STOP %3.

sources:

• How to send signals to processes in Linux and Unix
• How to manage background and foreground jobs.

Answer 2

This should be true of any shell with job control, which (for the most part) you can take for granted unless dealing with a truly ancient shell. It's in the POSIX standard, so even dash supports job control (when run interactively or with -m).

Interactive

• Ctrl+z will suspend the currently foregrounded program
• bg will background the most recently suspended program
  (use bg %2 with the job number, which you can check with jobs)
• fg will foreground the most recently suspended program

In zsh, you can write a key binding to implicitly run fg from the prompt via another Ctrl+z:

_zsh_cli_fg() { fg; }
zle -N _zsh_cli_fg
bindkey '^Z' _zsh_cli_fg

There's probably also a clever way to implicitly run bg upon suspending, but it seems unwise; at least for me, the majority of my Ctrl+z usage is because Ctrl+c is failing to break out; I want to follow that with e.g. kill %1 rather than bg, and I certainly don't want to default to killing! (This logic also extends into why I no longer use this key binding: If I'm hammering on Ctrl+z to stop a process, the last thing I want it to do is resume!)

Non-interactive

If you're in a different shell instance (or a different user, sometimes including sudo commands), you likely won't be able to use job numbers.

You can still act on another process once you know its process ID (PID). You can get the PID with pgrep …, or ps aux |grep … (or from the same shell, jobs -l, or $!) and you can then run:

kill -STOP $PID  # suspend
kill -CONT $PID  # continue (resume)

If you do not know the process ID and aren't worried about suspending other instances of the process by name, you can pass signals to one of these:

killall -STOP program_name
pkill -STOP program_name
pkill -f -STOP program_name_or_args

A CONT signal to a program stopped with Ctrl+z (and not bg'd) will resume its progress (in the foreground), the same as if you were to fg it.

Re: Standard Error

The edit to this question asks about standard error:

The process outputs to stderr, so how shall I issue the command fg <jobid> while the process is outputting to the terminal?

Unless the job in question has components that are backgrounded (or the whole job is backgrounded, perhaps via kill -CONT), you shouldn't actually see output while it is suspended.

If it is still outputting data (be it to standard output or standard error), it will certainly make your terminal visually cluttered, but all of that output will be ignored as it is not part of your input. This might make it harder to know you haven't entered any typos, but (blindly) typing fgEnter should suffice (unless you have multiple jobs and the one in question isn't the most recent, in which case you will indeed need the job descriptor).

If you do need to find the job descriptor, use another terminal to send it the STOP signal via the non-interactive methods above. This should free up your display (perhaps hit Enter a few times or run clear or Ctrl+L) so you can then run jobs to find the job descriptor and then run fg %N where N is that number.

Answer 3

Type fg to bring it to the foreground.

Answer 4

1. list jobs with jobs command
2. Bring your target job to the foreground with fg; e.g.: fg %4
3. Hit CTRL Z to suspend
4. To start it running in the background, use bg; e.g.: bg %4

Answer 5

Ending a process can be done in several different ways. Often, from a console-based command, sending a Ctrlc keystroke (the default interrupt character) will exit the command. This works when process is running in foreground mode.

If a process is running in background mode then first you would need to get its Job ID using the ps command and after that you can use kill command to kill the process as follows:

$ps -f
UID      PID  PPID C STIME    TTY   TIME CMD
amrood   6738 3662 0 10:23:03 pts/6 0:00 first_one
amrood   6739 3662 0 10:22:54 pts/6 0:00 second_one
amrood   3662 3657 0 08:10:53 pts/6 0:00 -ksh
amrood   6892 3662 4 10:51:50 pts/6 0:00 ps -f
$kill 6738
Terminated

Here the kill command would terminate the first_one process. If a process ignores a regular kill command, you can use kill -9 followed by the process ID as follows:

$kill -9 6738
Terminated