What kind of (different) return values do you need for if, while and until to signal truthiness or falsiness?

Question

I am curious about the expected value of conditionals in if, while and until statements, because I can't obviously not seem to get my conditionals working properly.

Do they expect true and false or an exit code, i.e. 0 or {1-255} ?

Answer 1

The conditionals test for an exit code. Zero is true, and non-zero is false. The true command is true by virtue of it returning a zero (0) exit code. Similarly, false returns a non-zero exit code (as it happens, it's 1).

if true; then echo 'happy days'; fi
if false; then echo 'maths has just died a horrible death'; fi

true; echo "exit status is $?"     # "0"
false; echo "exit status is $?"    # "1"

[ 0 -eq 0 ]; echo "exit status is $?"    # "0"
[ 0 -eq 1 ]; echo "exit status is $?"    # "1"