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Variable USE_TLS is to be unset, null, or empty when TLS is not to be used, but set to 1 when TLS must be enforced. I require it to be enforced by default (i.e., set), but not if I specify another variable do_not_use_tls be set to 1.

# this assigns null or empty
USE_TLS=${do_not_use_tls:-""}

# this assigns empty or null
USE_TLS=${do_not_use_tls:+""}

Is there a substitution which will set USE_TLS when do_not_use_tls is null or unset, but not assign it a value otherwise? I was trying the following, but these do not work:

USE_TLS=${do_not_use_tls:+${do_not_use_tls:-""}}

USE_TLS=${do_not_use_tls:-${do_not_use_tls:+null}}
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  • are you intentionally trying to avoid an if construct? Commented Oct 27, 2023 at 14:03
  • Also I have a hard time following the logic you've laid out. Can you show examples of if do_not_use_tls=1 then USE_TLS should be set to foo? Commented Oct 27, 2023 at 14:05
  • What's wrong with [ "${do_not_use_tls:-0}" -eq 1 ] && unset USE_TLS? Commented Oct 27, 2023 at 14:09
  • @jesse_b, yes I was trying inline, and @Chris Davies, thank you for unset. Commented Oct 27, 2023 at 14:42

1 Answer 1

Reset to default
2

What you're looking for seems to be

unset -v USE_TLS
[ "${do_not_use_tls-}" = 1 ] || USE_TLS=1

This first unsets the USE_TLS variable (this is not strictly needed if you are confident that it won't have a previous value), and then sets it to 1 unless $do_not_use_tls is 1.

Or even more unambiguously,

if [ "${do_not_use_tls-}" = 1 ]; then
    unset -v USE_TLS
else
    USE_TLS=1
fi

I use "${do_not_use_tls-}" rather than just "$do_not_use_tls" to not generate an error in case set -u aka set -o nounset is in effect.

You can't really use the examples that you show (all on the form of an assignment to USE_TLS with a value that is some expansion of the do_not_use_tls variable) because you need to test the value of $do_not_use_tls to see whether it's 1 or not.

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  • or, to explicitly start with the stated default: USE_TLS=1; [ "${do_not_use_tls-}" = 1 ] && unset USE_TLS Commented Oct 27, 2023 at 15:33
  • @ilkkachu Yes, that's certainly another possibility. Commented Oct 27, 2023 at 15:43

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