Linux in a Nutshell, 6th Edition

Filenames Versus Patterns

Metacharacters used in pattern matching are different from metacharacters used for filename expansion (see Chapter 6). However, several metacharacters have meaning for both regular expressions and for filename expansion. This can lead to a problem: the shell sees the command line first, and can potentially interpret an unquoted regular expression metacharacter as a filename expansion. For example, the command:

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

could be transformed by the shell into:

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

and grep would 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 as follows:

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

Double quotes suffice in most cases, but single quotes are the safest bet, since the shell does absolutely no expansions on single-quoted text.

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