Wildcards

Like UNIX (and DOS, for that matter), Linux supports wildcards in command lines and shells scripts. A wildcard is merely shorthand for a character or a string of characters. Wildcards can come in handy if you’re looking for a file and you’ve forgotten the specific filename (geez, I know the file ends in 1996), or if you want to see a list of files that fall within specific parameters (such as ending with .c, useful if you plan on using Linux for software development).

There are three types of Linux wildcards: *, ?, and [...]. Each will be explained.

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NOTE:  Technically, wildcards are the province of the shell, and in theory a discussion of wildcards should take place with a discussion of shells. For our purposes, however, we’re going to discuss wildcards at this point in the Linux discussion, because what we’re saying here applies to all shells.

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In the previous section covering the ls command, we covered the command’s use when it’s applied to single files. However, there may be times when you want to list a set of files that share a common characteristic, such as ending with .c. In this instance, you can tell ls to look for every file that ends with .c, using the following command line:

     gilbert:~$ ls *.c

     aardvark.c     stuff.c     titles.c     xylophone.c

In this instance, ls is told to substitute * for any portion of a filename preceding an ending of .c. And, as you can see from the list of files, the command was successful. The ls is used to match any number of characters in a string, including zero characters:

     gilbert:~$ ls titles*

     titles     titles.c

In the case of titles, the wildcard matched zero characters.

The asterisk (*) can be used at the beginning or end of a wildcard expression. You can also use more than one asterisk in an expression:

     gilbert:~$ ls t*.*

     titles.c

If you wanted to list the files with the string titles anywhere in the filename, you could use the following command line:

     gilbert:~$ ls *titles*

     subtitles     titles     titles.c

The asterisk wildcard is the most expansive wildcard available. On the other end of the spectrum is the question-mark wildcard, which is used to match a single character:

     gilbert:~$ ls title?

     titles

In this instance, ls did not match titles.c, which contains two characters after the search string of title. Titles, meanwhile, contained only one character after the search string of title, which matched the parameters of the ls command.

The final wildcard is used to return specific characters, as defined by brackets ([]). For example, you’re looking through a directory filled with memos from the last 12 months. Since you’ve been a good Linux user, you’ve been placing a number at the end of every file, signifying the month it was written. (Yes, we know you’re not likely to have too many files if you’ve just installed Linux. Think of this advice as something you’ll need in the future.) You want to track down a memo you wrote sometime in the summer, but you can’t remember the name of the file, and a reading through the directory listings don’t spark a memory. In this instance, you’ll want to narrow down the directory listings to files ending in 6, 7, or 8 (corresponding to June, July, and August). To do this with the ls command, you’d enter 6–8 in brackets:

     gilbert:~/memos$ ls *[6-8]

     golf.8        golfanne.8     golfpat.6     golfjim.6

     golftod.6     golftom.7

This narrows down the list of files returned by ls. It also means you probably play too much golf.

In the preceding example, we asked ls to return files that ended with a range of characters, i.e., in 6, 7, or 8. You can also use this wildcard to return a single character:

     gilbert:~/memos$ ls *[6]

     golfpat.6     golfjim.6     golftod.6

If you’re searching for a character (remembering, of course, that Linux distinguishes between uppercase and lowercase letters at all times) or range of characters, you can list them in the brackets:

     gilbert:~/memos$ ls report.[Ee]rc

     report.Erc     report.erc

Wildcards can be used with any Linux command.

Creating Directories with Mkdir

The mkdir command is used to create directories. If you plan on using Linux for most of your day-to-day stuff, we advise creating directories to help organize the many files Linux (and any other version of UNIX, for that matter) creates. Using mkdir is simple:

     gilbert:~$ mkdir directory

where directory is the name of the directory you want to create. To create a directory named letters in your home directory, you’d use the following command:

     gilbert:~$ mkdir letters

To see if the directory was really created, you can use the ls command:

     gilbert:~$ ls

     letters/     text

You can also use it to create a new directory elsewhere in the directory hierarchy:

     gilbert:~$ mkdir /users/kevin/letters

     gilbert:~$ ls /users/kevin

     letters/

Mkdir can create more than one directory on a command line:

     gilbert:~$ mkdir letters data

     gilbert:~$ ls

     data/          letters/     text

Mkdir can also create a directory and a subdirectory in a single command line:

     gilbert:~$ mkdir -p /letters/eric

Other options to mkdir are listed in Table 4.6.