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I’ve been working with UNIX operating systems for 20+ years and I’m still amazed at how much I still don’t know about it. Recently at work I came across a script (Bourne) that made use of the wait command. Now if someone had asked me if a command like this existed I would probably have guessed yes, so I wasn’t completely shocked by it, but I was kinda surprised that I had never seen it actually used anywhere in the wild.

The wait command gives you the ability to block a script on the termination of another process or list of processes. This can come in handy when you want to make sure a program finished before you’re script moves on. Specifically for a program where it runs as a backgrounded process.

I like examples so here are a couple that demonstrate how wait works.

Example 1

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# wait with a process id
 
# run the sleep command for 15 seconds backgrounded
% sleep 15 &
[1] 8546
 
# wait for sleep's process id ($!) to finish, and then echo "hi"
% wait $! && echo "hi"
[1]+  Done                    sleep 15
hi

Example 2

Additionally you can give wait the job id (%1, %2, etc.) instead.

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# wait with a job id
 
% sleep 15 &
[1] 8551
% wait %1 && echo "hi"
[1]+  Done                    sleep 15
hi

Example 3

You can also drop the process id entirely if the wait command occurs right after the backgrounded command.

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# wait without any argument
 
% sleep 10 &
[1] 8552
% wait && echo "hi"
[1]+  Done                    sleep 10
hi

Example 4

Finally you can tell wait to wait on more than one process.

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# wait on 2 processes
 
% sleep 10 &
[1] 8570
% sleep 10 &
[2] 8571
% wait %1 %2 && echo "hi"
[1]-  Done                    sleep 10
[2]+  Done                    sleep 10
hi

For more information about the wait command check out the wait section of the Bash FAQ. Check out more about special variables in Bash.

NOTE: For further details regarding my one-liner blog posts, check out my one-liner style guide primer.