From a0fa71c92c4f6f42f3f0b212158c22dedf1d52b9 Mon Sep 17 00:00:00 2001
From: Egor Tensin <Egor.Tensin@gmail.com>
Date: Mon, 30 Sep 2019 05:26:59 +0300
Subject: import bash.md from egor-tensin/notes.git

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 _notes/bash.md | 67 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 _notes/bash.md

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+---
+layout: note
+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.
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