What does rm -rf *(1)* do?

Question

I was trying to remove duplicate downloads and made the error of running

rm -rf *(1)*

This deleted everything in ~/Downloads without an error message.

What exactly does this regex do?

Answer 1

In a POSIX shell, the parenthesis are reserved for spawning the inner commands (cmd1; cmd2; cmd3) in a subshell. Since in *(1)* the parenthesis are not separated from the asterisks by newlines or semicolons, it is a syntax error. Of course, even *;(1);* would not make sense, although not a syntax error per se.

In Bash, if extglob is enabled, as can be verified with shopt -p |& grep extglob, "*(pattern-list) matches zero or more ocurrences of the given patterns". In this case, *(1)* always expands to all the files in the directory, since *(1) can match zero occurrence of the pattern 1 and * will match all other names, whatever they are. The result is that all files (and directories, which are files too) are removed.

Also notice that shells use shell patterns (as known as globs), which are not the same as regular expressions. See Gilles' answer for a detailed more account of that. In your case, to remove every file whose name contains (1), you would need to escape or quote the parenthesis to preserve their literal meaning, i.e.,

rm -rf *'(1)'*
rm -rf *\(1\)*
rm -rf *"(1)"*