Mac OS X Tiger in a Nutshell

Filenames Versus Patterns

When you issue a command on the command line, special characters are seen first by the shell, and then by the program; therefore, unquoted metacharacters are interpreted by the shell for filename expansion. The command:

$ grep [A-Z]* chap[12]

can, for example, be transformed by the shell into:

$ grep Array.c Bug.c Comp.c chap1 chap2

and can then try to find the pattern Array.c in files Bug.c, Comp.c, chap1, and chap2. To bypass the shell and pass the special characters to grep, use quotes:

$ grep "[A-Z]*" chap[12]

Double quotes suffice in most cases, but single quotes are the safest bet.

Note also that in pattern matching, ? matches zero or one instance of a regular expression; in filename expansion, ? matches a single character.