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Updated SC1102 (markdown)

koalaman
2016-06-26 22:11:12 -07:00
parent be1ba26da3
commit 6c299008ea
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SC1102.md

@@ -1,21 +1,21 @@
## Shells differ in parsing ambiguous $(((. Use spaces: $( (( . ## Shells disambiguate $(( differently or not at all. If the first $( should start command substitution, add a space after it.
### Problematic code: ### Problematic code:
```sh ```sh
echo "$((( n > 0)) && mycommand --flags)" echo "$((cmd "$@") 2>&1)"
``` ```
### Correct code: ### Correct code:
```sh ```sh
echo "$( (( n > 0)) && mycommand --flags)" echo "$( (cmd "$@") 2>&1)"
``` ```
### Rationale: ### Rationale:
You are using `$(((` (or `(((`) to mean `$( ((`: command expansion with an arithmetic command. The more common interpretation is `$(( (`: arithmetic expansion with parentheses. You appear to be using `$((` with two (or more) parentheses in a row, where the first `$(` should open a subshell.
This is an ill-defined structure that is parsed differently between different shells and shell versions. Prefer adding a space to make it unambiguous, both to shells and humans. This is an ill-defined structure that is parsed differently between different shells and shell versions. Prefer adding spaces to make it unambiguous, both to shells and humans.
Consider the `$(((` in `$(((1)) )`: Consider the `$(((` in `$(((1)) )`:
@@ -23,4 +23,6 @@ Ash, dash and Bash 1 parses it as `$(( (` and subsequently fail to find the matc
### Exceptions: ### Exceptions:
None. **Alternatively**, you may indeed have correctly spaced your parentheses, but ShellCheck failed to parse `$((` as an arithmetic expression while accidentally succeeding in parsing it as `$(` + `(`.
In these cases, double check the syntax to ensure ShellCheck can parse the `$((`, or ignore this error and hope that it won't affect analysis too severely.