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From: Grisha Levit <grishalevit@gmail.com>
Newsgroups: gnu.bash.bug
Subject: nounset and $! behavior
Date: Mon, 12 Nov 2018 18:55:37 -0500
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The following crashes bash:

bash-5.0$ set -u
bash-5.0$ echo ${!,}
bash: $!: unbound variable
bash: !: unbound variable
bash-5.0$ echo ${!,}
bash: $!: unbound variable

Testing if $! is set also doesn't seem to be possible:

bash-5.0$ echo ${!-}
bash: $!: unbound variable