Section 6.1. Filenames Versus Patterns

6.1. 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.