Created SC2257 (markdown)

SC2257.md

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+## Arithmetic modifications in command redirections may be discarded. Do them separately.
+
+### Problematic code:
+
+```sh
+curl "$URL" > "image$((i++)).jpg"
+```
+
+### Correct code:
+
+```sh
+i=$((i+1))
+curl "$URL" > "image$i.jpg"
+```
+### Rationale:
+
+You are using an arithmetic expression that modifies a variable, e.g. `$((x+=1))` or `$((x++))`, in the name of a file to redirect from/to, in a here document, or in a here string.
+
+The scope of these modifications depends on whether the command itself will fork:
+
+```sh
+echo foo > $((var++)).txt  # Updates in BusyBox and Bash
+cat  foo > $((var++)).txt  # Updates in Busybox, not in Bash
+gcc  foo > $((var++)).txt  # Does not update in either
+
+gcc() { /opt/usr/bin/gcc "$@"; }
+gcc  foo > $((var++)).txt  # Now suddenly updates in both
+```
+
+Rather than rely on knowing which commands do and don't fork, or are and aren't overridden, simply do the updates in a separate command as in the correct code.
+
+### Exceptions:
+
+If you know your variable is scoped the way you want it, you can ignore this warning.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!