I was surprised with this comment in other question:
Sending dd the USR1 signal too soon after it has started (i.e. in a bash script, the line after you started it) will in fact terminate it
Can anybody explain why?
asked May 13, 2012 at 22:08
1 Answer
Each signal has a "default disposition" -- what a process does by default when it receives that signal. There's a table in the signal(7) man page listing them:
Signal Value Action Comment
──────────────────────────────────────────────────────────────────────
...
SIGUSR1 30,10,16 Term User-defined signal 1
SIGUSR2 31,12,17 Term User-defined signal 2
SIGUSR1 and SIGUSR2 both have the default action Term -- the process is terminated. dd registers a handler to intercept the signal and do something useful with it, but if you signal too quickly it hasn't had time to register that handler yet, so the default action happens instead
answered May 13, 2012 at 22:14
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2I wish I could upvote twice for being made aware of this obscurity. Seeing processes die randomly after removing an explicit signal handler was disconcerting. Commented Mar 9, 2016 at 19:39
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3Is there any practical way to control this race condition instead of just sleeping for a reasonable amount of time (~0.5-1 sec)? (I mean, beside something ludicrous like capturing and parsing
straceoutput in a shell script…) Commented Apr 20, 2018 at 17:16 -
I had a shell script working fine. But suddenly stop working because of likely!: I had hyperthreding on now off. The subprocess sending kill -s SIGUSR1 $PARENT_PID gets too fast now?. The grand parent thinks the parent is terminated normally, but the parent is still executing the loop. This is a good posting. I have been spending most of the day trying to figure this out. Commented Nov 26, 2018 at 23:02
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2@AdrianGünter "Is there any practical way to control this race condition …?" – Run
trap '' USR1in the (sub)shell that is going to rundd. Comparetrap - USR1; dd if=/dev/zero of=/dev/null & kill -s USR1 "$!"; sleep 1; kill -s USR1 "$!"; sleep 1; kill "$!"totrap '' USR1; dd if=/dev/zero of=/dev/null & kill -s USR1 "$!"; sleep 1; kill -s USR1 "$!"; sleep 1; kill "$!". Commented Jun 29, 2023 at 10:38
{ dd if=/dev/zero of=/dev/null & }; kill -USR1 $!; jobs; sleep 1; jobsto reproduce the effect you're describing.