BASH & multithreading

If you need to perform two or more time consuming jobs, and these jobs can be run independently, you can still use BASH.

Just BASH

Bash allows running scripts, programs and functions as background tasks. Also, Bash provides joining mechanism. First running tasks in background.

function task_x {
    // time consuming task x
}

function task_y {
    // time consuming task y
}

task_x &
task_y &

Adding a lone & sign will run command as background task. Such command will end immediately after starting task in background, and allows running next command. Background task will be terminated if session is closed.

If there is need for synchronized work, use wait method.

function task_x {
    // time consuming task x
}

function task_y {
    // time consuming task y
}

task_x &
pid_x=$!

task_y &
pid_y=$!

wait $pid_x
wait $pid_y

$! returns pid of last background task, which was stared by current thread. Knowing pid opens wide range of options

wait command waits until end of task with given pid. If thread ended before wait command, current thread won’t wait, but will obtain data.

Important thing: wait returns exit code, returned by thread.

Command (or sometimes built-in function) – nohup

Presented multithreading has one downside (or not, depending on your needs). If terminal get closed or killed, then all subprocesses will die too. If screen-like behaviour is needed, nohup command enters stage.

nohup time_consuming_task

And, by default, result will be saved to nohup.out text file. Now you can close terminal.

nohup command saves output to nohup.out file. If you need other, you can change name using:

nohup time_consuming_task > output_file &

When closing bash session you may get warn, that background task is running. Don’t worry it will finish everything as expected and print output to given file.

Unfortunately, syntax with different output file may not work in some environments.

screen

For tasks, that should work even when terminal window is closed, please consider using screen.