bash Cookbook

Confusing Shell Wildcards and Regular Expressions

Problem

Sometimes you see .* sometimes just *, and sometimes you see [a-z]* but it means something other than what you thought. You use regular expressions for grep and sed but not in some places in bash. You can’t keep it all straight.

Solution

Relax; take a deep breath. You’re probably confused because you’re learning so much (or just using it too infrequently to remember it). Practice makes perfect, so keep trying.

The rules aren’t that hard to remember for bash itself. After all, regular expression syntax is only used with the =~ comparison operator in bash. All of the other expressions in bash use shell pattern matching.

Discussion

The pattern matching used by bash uses some of the same symbols as regular expressions, but with different meanings. But it is also the case that you often have calls in your shell scripts to commands that use regular expressions—commands like grep and sed.

We asked Chet Ramey, the current keeper of the bash source and all-around bash guru, if it was really the case that the =~ was the only use of regular expressions in bash. He concurred. He also was kind enough to supply a list of the various parts of bash syntax that use shell pattern matching. We’ve covered most, but not all of these topics in various recipes in this book. We offer the list here for completeness.

Shell pattern matching is performed by:

• Filename globbing (pathname expansion)

• == and != operators for [[

• case statements

• $GLOBIGNORE handling