12.2 Filename Globbing

Before the shell passes arguments to an external command or interprets a built-in command, it scans the command line for certain special characters and performs an operation known as filename globbing. Filename globbing resembles the processing of wildcards used in MS-DOS commands, but it's much more sophisticated. Table 12-1 describes the special characters, known as filename metacharacters, used in filename globbing.

Table 12-1. Filename metacharacters

Metacharacter	Function
*	Matches a string of zero or more characters
?	Matches exactly one character
[abc...]	Matches any of the characters specified
[a-z]	Matches any character in the specified range
[!abc...]	Matches any character other than those specified
[!a-z]	Matches any character not in the specified range
~	The home directory of the current user
~userid	The home directory of the specified user
~+	The current working directory
~-	The previous working directory

In filename globbing, just as in MS-DOS wildcarding, the shell attempts to replace metacharacters appearing in arguments in such a way that arguments specify filenames. Filename globbing makes it easier to specify names of files and sets of files.

For example, suppose the current working directory contains the files file1, file2, file3, and file04. Suppose you want to know the size of each file. The following command reports that information:

ls -l file1 file2 file3 file04

However, the following command reports the same information and is much easier to type:

ls -l file*

As Table 12-1 shows, the * filename metacharacter can match any string of characters. Suppose you issued the following command:

ls -l file?

The ? filename metacharacter can match only a single character. Therefore, file04 would not appear in the output of the command.

Similarly, the command:

ls -l file[2-3]

would report only file2 and file3, because only these files have names that match the specified pattern, which requires that the last character of the filename be in the range 2-3.

You can use more than one metacharacter in a single argument. For example, consider the following command:

ls -l file??

This command will list file04, because each metacharacter matches exactly one filename character.

Most commands let you specify multiple arguments. If no files match a given argument, the command ignores the argument. Here's another command that reports all four files:

ls -l file0* file[1-3]

Suppose that a command has one or more arguments that include one or more metacharacters. If none of the arguments matches any filenames, the shell passes the arguments to the program with the metacharacters intact. When the program expects a valid filename, an unexpected error may result.

The tilde (~) metacharacter lets you easily refer to your home directory. For example, the following command:

ls ~

would list the files in your home directory.

Filename metacharacters don't merely save you typing. They let you write scripts that selectively process files by name. You'll see how that works later in this chapter.