Parallel commands in Bash

Bash natively supports a wait command.

Run parallel processes

Using default wait behavior

By default, wait will wait for all child processes to complete before continuing an execution. For most cases, this is sufficient.

1sleep 2 && echo "hi" &
2sleep 2 && echo "hi2" &
3
4# Wait for child processes to finish
5wait

Using the & character at the ending of a line sends the process to the background to complete. Without wait, the the script would end.

The script will complete in 2 seconds.

Output variation 1:

1hi
2hi2

Output variation 2:

1hi2
2hi

Using wait with process ids

wait allows for a process id argument value. This may be useful for establishing chained waits.

In the example below, the “hi3” task depends on “hi”; however “hi2” may run in parallel with “hi”.

 1sleep 2 && echo "hi" &
 2hi_pid=$!
 3
 4# "hi2" does not require "hi"
 5sleep 2 && echo "hi2" &
 6
 7wait $hi_pid
 8
 9# "hi3" requires "hi"
10sleep 2 && echo "hi3" &
11
12wait

Output:

1hi2
2hi
3hi3

The process id of the last execution is returned using $!.