Asynchronous commands

There is a special notation that when used, allows you to run a command, a pipeline or an entire list asyncronously in the background. When you run a command in the background. The program will continue executing without waiting the command to exit.

Asynchronous commands run in a sub-shell. And read input from the /dev/null unless explicitly redirected.

The format is simple:

command &

You simply put an ampersand & at the end of the statement.

You can run an entire pipeline in the background:

command1 | command2 | command3 &

You can take this further, and run an entire list in the background:

command1 | command2 && command3 || command4 &

Waiting for background commands

When you invoke a command in background. the program will continue without waiting for the command to finish. that's why they're called background commands.

For example:

sleep 20 &
echo Hello

The command sleep will be invoked and put in background. Then the program will continue to run echo. In this particular case. the program will exit immediatly after writing Hello to stdout because the command sleep will sleep for 20 seconds. And because the program will not wait for it to finish. The program will move on to echo and exit.

If you want to wait for background commands to finish, you can use the wait keyword:

sleep 20 &
echo Hello

wait

The wait keyword waits for all commands in background to finish. This means. the above example will indeed wait for the sleep command to finish before it exits.

Exit Code

When you run a command in background. the exit code is always set to zero 0.