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+## In POSIX sh, arithmetic base conversion is undefined
+
+
+### Problematic code:
+
+```sh
+#!/bin/sh
+echo "Hex $1 is decimal $((16#$1))"
+```
+
+### Correct code:
+
+```sh
+#!/bin/sh
+echo "Hex $1 is decimal $(echo "ibase=16; $1" | bc)"
+```
+
+### Rationale:
+
+Your script uses a shell feature not supported by the shebang. Either rewrite the script to be portable, or change the shebang to explicitly require a shell like Bash.
+
+In this case, using radix specifiers in arithmetic contexts, such as `$((16#DEADBEEF))` is not portable. `bc` ibase/obase can be used instead.
+
+### Exceptions:
+
+None
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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