I implement this program since flock(1) in Linux doesn't support timeout of pending.

I've never used BSD. After reading the link provided by you, I think my program becomes no value, if there is a Linux version of lockf(1). Sad.

Oh, after carefully check the man page of flock(1) in Linux, it supports timeout. So the last value of this program disappears. Please ignore my ignorance and disturbance. I wish I could delete the post but there is no way to do it.

You still got to learn about POSIX!

1. I want to run commands in background, but sequentially 2. Used in a server application for queued long-time task

(so; why; not; run; in; background;)& ?

You are right in this situation.

The scene for this program is that I don't know all the tasks.

Namely, tasks are generated in real time.

1. I posted some task to the queue and continue my work, then the work make me post another work. e. g. in a shell script

2. For server application to queue tasks

Thanks for the explanation (and patience lol)