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users/29882: Explain $RANDOM predictability

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Bart Schaefer 2024-05-24 19:27:14 -07:00
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@ -1,5 +1,7 @@
2024-05-24 Bart Schaefer <schaefer@zsh.org>
* users/29882: Etc/FAQ.yo: Explain $RANDOM predictability
* 52910: Functions/Misc/zmv: Improve handling of **/ patterns
* 52904: Completion/Unix/Command/_git: Improve quoting of paths

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Etc/FAQ.yo
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@ -137,6 +137,7 @@ Chapter 3: How to get various things to work
3.28. How do I edit the input buffer in $EDITOR?
3.29. Why does `which' output for missing commands go to stdout?
3.30. Why doesn't the expansion mytt(*.{tex,aux,pdf}) do what I expect?
3.31. Why does mytt($RANDOM) return the same number more than once?
Chapter 4: The mysteries of completion
4.1. What is completion?
@ -2219,6 +2220,37 @@ sect(Why doesn't the expansion mytt(*.{tex,aux,pdf}) do what I expect?)
This is harder for the user to remember but easier for the shell to
parse!
sect(Why does mytt($RANDOM) return the same number more than once?)
As tt(zshparam(1)) says:
verb(
The values of RANDOM form an intentionally-repeatable
pseudo-random sequence; subshells that reference RANDOM
will result in identical pseudo-random values unless the
value of RANDOM is referenced or seeded in the parent shell
in between subshell invocations.
)
You can use a function, including an anonymous function, to always
evaluate mytt($RANDOM) in the parent shell. This example illustrates
the difference:
verb(
for i in {1..10}; do
echo subshell: $(echo $RANDOM) $RANDOM
() { echo parent: $(echo $1) $2 } $RANDOM $RANDOM;
done
)
Remember that for a pipe like mytt(A | B), zsh runs A in a subshell
and B in the current shell. This means that, for example:
verb(
for i in {1..10}; do
echo $RANDOM | tee
done
)
also repeats the same value, because mytt($RANDOM) is evaluated in
the subshell and the parent sequence is left unchanged.
chapter(The mysteries of completion)
sect(What is completion?)