Created SC2324 (markdown)

SC2324.md

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+## var+=1 will append, not increment. Use (( var += 1 )), declare -i var, or quote number to silence.
+
+### Problematic code:
+
+```sh
+var=2 n=3
+var+=$n
+```
+
+### Correct code:
+
+In bash/ksh, use an `(( arithmetic context ))` 
+
+```sh
+(( var += n ))
+```
+
+or declare the variable as an integer type:
+
+```sh
+declare -i var=2
+n=4
+var+=$n
+```
+
+For POSIX sh, use an `$((arithmetic expansion))`:
+
+```sh
+var=$((var+n))
+```
+
+
+### Rationale:
+
+The problematic code attempts to add 2 and 3 to get 5.
+
+Instead, `+=` on a string variable will concatenate, so the result is 23.
+
+### Exceptions:
+
+If you *do* want to concatenate a number, for example to append trailing zeroes, you can silence the warning by quoting the number:
+
+```sh
+var+="000"
+```
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!