Chapter 33. Wildcards

33.1. File-Naming Wildcards

Wildcards (Section 1.13) are the shell's way of abbreviating filenames. Just as in poker, where a wildcard is a special card that can match any card in the deck, filename wildcards are capable of matching letters or groups of letters in the alphabet. Rather than typing a long filename or a long chain of filenames, a wildcard lets you provide parts of names and then use some "wildcard characters" for the rest. For example, if you want to delete all files whose names end in .o, you can give the following command:

% rm *.o

You don't have to list every filename.

I'm sure you already know that wildcards are useful in many situations. If not, they are summarized in Section 33.2. Here are a few of my favorite wildcard applications:

• If you remember part of a filename, but not the whole name, you can use wildcards to help you find it. If I have a file on genetics saved in a directory with several hundred other files, a command like:

  % ls *gene*

  will often find what I want. It's quicker and easier than find (Section 9.1).

• Wildcards are a natural when you want to work with groups of files. If I have a general purpose directory that's full of filenames ending in .c and .h, I can make new subdirectories and use wildcards to move the files easily:

  % mkdir c h
  % mv *.c c
  

• Wildcards often help you to work with files with inconvenient characters in their names. Let's say you have a file named abcxe, where x is some unknown control character. You can delete or rename that file by using the wildcarded name abc?e. (When you do this, be careful that your wildcard doesn't match more than you intend, perhaps by running an ls using the pattern first.)

• Wildcards can appear in any component of a pathname. This can often be used to your advantage. For example, let's say that you have a directory named /work, split into subdirectories for a dozen different projects. For each project, you have a schedule, in a file called (obviously enough) schedule.txt. You can print all the schedules with a command like:

  % lpr /work/*/schedule.txt

  (However, you can occasionally run into problems (Section 33.5).)

It's a common misconception, particularly among new users, that application programs and utilities have something to do with wildcards. Given a command like grep ident *.c, many users think that grep handles the * and looks to see which files have names that end in .c. If you're at all familiar with Unix's workings, you'll realize that this is the wrong picture. The shell interprets wildcards. That is, the shell figures out which files have names ending in .c, puts them in a list, puts that list on the command line, and then hands that command line to grep. As it processes the command line, the shell turns grep ident *.c into grep ident file1.c file2.c....

Since there are several shells, one might think (or fear!) that there should be several different sets of wildcards. Fortunately, there aren't. The basic wildcards work the same for all shells.

-- ML