shell case pattern [...] disallowed chars

Question

Bracket expressions within case patterns seem to disallow [() &;]. However I can't seem to find any such restrictions (or escaping workarounds) in the POSIX shell spec, or in the bash manual for that matter.

case '&' in
# *[&]*)  echo y ;;  # won't parse
  *[\&]*) echo y ;;  # will parse & work
esac
# similar for ';', ' ', '(', ')'
# not a problem for ${var#[&; ()]}

This is in a sh shell script function that can't afford to call external utilities (but I'm curious about bash too). So... is there any spec that describes backslash-ing these characters within a bracket expression pattern?

Answer 1

No, I don't think it is explicitly documented anywhere.

But it can be deduced that Token Recognition Rule 6 is applied while the pattern list is being parsed. That is, unless quoted, control operators, redirection operators, and end of input are recognized as operators, and delimit a pattern. The shell expects | (indicates that another pattern follows) or ) (marks the end of the pattern list) to do that; and anything else causes a parse error.

As square brackets have no special meaning to the parser during tokenization, whether an operator occurs between them is irrelevant. And ${var#[&; ()]} is a different case; covered in Token Recognition Rule 5 and Parameter Expansion.