Job Control

Job control is what lets one terminal session run several commands at once without opening several terminals - starting things in the background, pausing a foreground command, and bringing things back to the foreground as needed.

Foreground vs. background

A command runs in the foreground by default, occupying the shell until it finishes. Append & to start it in the background instead, returning control of the shell immediately:

long-task.sh &

[1] 4821

The [1] is a job number (specific to this shell session); 4821 is the process's actual PID (see Process Lifecycle).

Suspending and resuming

Ctrl+Z suspends whatever's running in the foreground, handing control back to the shell without killing it - the process is paused, not terminated:

some-command
# press Ctrl+Z

[1]+  Stopped                 some-command

From there:

jobs        # list jobs in this shell, with their state
fg          # resume the most recent job in the foreground
fg %1       # resume job number 1 specifically, in the foreground
bg %1       # resume job number 1, but in the background
kill %1     # send SIGTERM to job number 1

See Signals for what's actually happening underneath Ctrl+Z (SIGTSTP), fg/bg (SIGCONT), and kill.

Jobs die when the shell does - unless you say otherwise

By default, a background job is tied to the shell session that started it: closing the terminal (or losing an SSH connection) sends SIGHUP to every job still attached to it, which terminates them. Two ways around this:

nohup - makes a command ignore SIGHUP specifically, so it survives the shell exiting:

nohup long-task.sh &

disown - detaches an already-running background job from the shell's job table, after the fact:

long-task.sh &
disown

Neither of these keeps a process usable interactively after you log out - for that, you want a session multiplexer.

For anything you need to reattach to: tmux or screen

nohup and disown solve "don't kill my job when I disconnect," but they don't let you reconnect to see its output later. tmux (or the older screen) runs an entire persistent terminal session on the server, independent of any one SSH connection - you can detach from it, log out, log back in later, and reattach to find everything exactly as you left it. This is the standard tool for long-running interactive work (a build, a database migration, a training job) on a remote machine, and is generally preferable to nohup for anything you might need to check back in on.

tmux new -s mysession    # start a named session
# ... do work, then detach with Ctrl+B, D ...
tmux attach -t mysession # reattach later, from any SSH connection