From 2660d91200c49ddd3ace7e215f9a7304de8aa3db Mon Sep 17 00:00:00 2001
From: Vidar Holen <vidar@vidarholen.net>
Date: Tue, 3 Aug 2021 13:19:07 -0700
Subject: [PATCH] Created SC2295 (markdown)

---
 SC2295.md | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)
 create mode 100644 SC2295.md

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+## Expansions inside ${..} need to be quoted separately, otherwise they will match as a pattern.
+
+### Problematic code:
+
+```sh
+relative_path() {
+  printf '%s\n' "${2#$1}"
+}
+
+# Results in "/tmp/King_Kong_[1933]/extras/trailer.mkv" because the prefix fails to match
+relative_path "/tmp/King_Kong_[1933]/" "/tmp/King_Kong_[1933]/extras/trailer.mkv"
+
+# Results in "cover.jpg" even though the prefix is different
+relative_path "/tmp/King_Kong_[1933]/" "/tmp/King_Kong_3/cover.jpg"
+
+```
+
+### Correct code:
+
+```sh
+relative_path() {
+  printf '%s\n' "${2#"$1"}"
+}
+# Results in "extras/trailer.mkv" as expected
+relative_path "/tmp/King_Kong_[1933]/" "/tmp/King_Kong_[1933]/extras/trailer.mkv"
+
+# Results in "/tmp/King_Kong_3/cover.jpg" as expected
+relative_path "/tmp/King_Kong_[1933]/" "/tmp/King_Kong_3/cover.jpg"
+```
+
+### Rationale:
+
+When using expansions in a parameter expansion prefix/suffix expression, the expansion needs to be quoted separately or it will match as a pattern. The quotes around the outer parameter expansion does not protect against this.
+
+This means that any variable that contains e.g. brackets, asterisks or question marks may not match as expected. In the example, `[1933]` was interpreted as a pattern character range and would therefore match `/tmp/King_Kong_3/` but not `/tmp/King_Kong_[1933]/` as was the intention.
+
+### Exceptions:
+
+If you wanted to treat the string as a pattern, such as `suffix=".*"; file="${var%$suffix}";` then you can [[ignore]] this suggestion. 
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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