Bash job control

Oftentimes you may wish to start a process on the Bash shell without having to wait for it to actually complete, but still be notified when it does. Similarly, it may be helpful to temporarily stop a task while it’s running without actually quitting it, so that you can do other things with the terminal. For these kinds of tasks, Bash’s built-in job control is very useful.

Backgrounding processes

If you have a process that you expect to take a long time, such as a long cp or scp operation, you can start it in the background of your current shell by adding an ampersand to it as a suffix:

$ cp -r /mnt/bigdir /home &
[1] 2305

This will start the copy operation as a child process of your bash instance, but will return you to the prompt to enter any other commands you might want to run while that’s going.

The output from this command shown above gives both the job number of 1, and the process ID of the new task, 2305. You can view the list of jobs for the current shell with the builtin jobs:

$ jobs
[1]+  Running  cp -r /mnt/bigdir /home &

If the job finishes or otherwise terminates while it’s backgrounded, you should see a message in the terminal the next time you update it with a newline:

[1]+  Done  cp -r /mnt/bigdir /home &

Foregrounding processes

If you want to return a job in the background to the foreground, you can type fg:

$ fg
cp -r /mnt/bigdir /home &

If you have more than one job backgrounded, you should specify the particular job to bring to the foreground with a parameter to fg:

$ fg %1

In this case, for shorthand, you can optionally omit fg and it will work just the same:

$ %1

Suspending processes

To temporarily suspend a process, you can press Ctrl+Z:

$ cp -r /mnt/bigdir /home
[1]+  Stopped  cp -r /mnt/bigdir /home

You can then continue it in the foreground or background with fg %1 or bg %1 respectively, as above.

This is particularly useful while in a text editor; instead of quitting the editor to get back to a shell, or dropping into a subshell from it, you can suspend it temporarily and return to it with fg once you’re ready.

Dealing with output

While a job is running in the background, it may still print its standard output and standard error streams to your terminal. You can head this off by redirecting both streams to /dev/null for verbose commands:

$ cp -rv /mnt/bigdir /home &>/dev/null

However, if the output of the task is actually of interest to you, this may be a case where you should fire up another terminal emulator, perhaps in GNU Screen or tmux, rather than using simple job control.

Suspending SSH sessions

As a special case, you can suspend an SSH session using an SSH escape sequence. Type a newline followed by a ~ character, and finally press Ctrl+Z to background your SSH session and return to the terminal from which you invoked it.

tom@conan:~$ ssh crom
tom@crom:~$ ~^Z [suspend ssh]
[1]+  Stopped  ssh crom

You can then resume it as you would any job by typing fg:

tom@conan:~$ fg %1
ssh crom