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authorEgor Tensin <Egor.Tensin@gmail.com>2020-05-07 01:48:11 +0000
committerEgor Tensin <Egor.Tensin@gmail.com>2020-05-07 01:48:11 +0000
commitadb9b3fe169a92ea2da7b8c804c8b91f966c21a4 (patch)
treefe340f4de0afa6763e220194d2e6bea3366b4c71 /_notes/bash.md
parentbind-mounts: rename the file (diff)
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----
-title: GNU bash
----
-(Associative) arrays
---------------------
-
-### Declaration
-
-`"${#xs[@]}"` doesn't work with `nounset` if `xs` wasn't defined, i.e. was
-declared with either of
-
- local -a xs
- declare -a xs
- local -A xs
- declare -A xs
-
-Therefore, if you want to extract the length of an array, append `=()` to the
-statements above.
-
- local -a xs=()
- declare -a xs=()
- ...
-
-And now `"${#xs[@]}"` works with `nounset`.
-It doesn't affect expansion (see below) though.
-
-### Expansion
-
-#### Do
-
- func ${arr[@]+"${arr[@]}"}
-
-#### Don't
-
- func "${arr[@]}" # Doesn't work with `nounset`.
- func "${arr[@]+"${arr[@]}"}" # Doesn't work properly with `declare -a arr=('')`.
-
-### `unset`
-
-#### Do
-
- unset -v 'arr[x]'
- unset -v 'arr[$i]'
-
-#### Don't
-
- unset -v arr[x] # May break due to globbing.
- unset -v arr[$i] # The same as above + a possible problem with quotation.
- unset -v 'arr["x"]' # Doesn't work for some reason.
- unset -v 'arr["]"]' # The same as above; just highlighting the problem with funny characters in array indices.
- unset -v 'arr["$i"]' # Also rejected.
-
- # An insightful discussion on the topic: https://lists.gnu.org/archive/html/help-bash/2016-09/msg00020.html.
-
-`errexit`
----------
-
-### Do
-
- bar_output="$( bar )"
- foo "$bar_output"
-
-### Don't
-
- foo "$( bar )" # With `errexit`, foo will still get executed.
- # I don't know why.