Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
cathy ~> ls -ld [[:digit:]]*<br />
drwxrwxr-x 2 cathy cathy 4096 Apr 20 13:45 2/<br />
cathy ~> ls -ld [[:upper:]]*<br />
drwxrwxr-- 3 cathy cathy 4096 Sep 30 2001 Nautilus/<br />
drwxrwxr-x 4 cathy cathy 4096 Jul 11 2002 OpenOffice.org1.0/<br />
-rw-rw-r-- 1 cathy cathy 997376 Apr 18 15:39 Schedule.sdc<br />
When the extglob shell option is enabled (using the shopt built-in), several extended pattern matching<br />
operators are recognized. Read more in the <strong>Bash</strong> info pages, section Basic shell features->Shell<br />
Expansions->Filename Expansion->Pattern Matching.<br />
4.4. Summary<br />
Regular expressions are powerful tools for selecting particular lines from files or output. A lot of UNIX<br />
commands use regular expressions: vim, perl, the PostgreSQL database and so on. They can be made<br />
available in any language or application using external libraries, and they even found their way to non-UNIX<br />
systems. For instance, regular expressions are used in the Excell spreadsheet that comes with the MicroSoft<br />
Windows Office suite. In this chapter we got the feel of the grep command, which is indispensable in any<br />
UNIX environment.<br />
The grep command can do much more than the few tasks we discussed here; we only used it as an<br />
example for regular expressions. The GNU grep version comes with plenty of documentation, which<br />
you are strongly advised to read!<br />
<strong>Bash</strong> has built-in features for matching patterns and can recognize character classes and ranges.<br />
4.5. Exercises<br />
<strong>Bash</strong> <strong>Guide</strong> for <strong>Beginners</strong><br />
These exercises will help you master regular expressions.<br />
1. Display a list of all the users on your system who log in with the <strong>Bash</strong> shell as a default.<br />
2. From the /etc/group directory, display all lines starting with the string "daemon".<br />
3. Print all the lines from the same file that don't contain the string.<br />
4. Display localhost information from the /etc/hosts file, display the line number(s) matching the<br />
search string and count the number of occurrences of the string.<br />
5. Display a list of /usr/share/doc subdirectories containing information about shells.<br />
6. How many README files do these subdirectories contain? Don't count anything in the form of<br />
"README.a_string".<br />
7. Make a list of files in your home directory that were changed less that 10 hours ago, using grep, but<br />
leave out directories.<br />
8. Put these commands in a shell script that will generate comprehensible output.<br />
9. Can you find an alternative for wc -l, using grep?<br />
10. Using the file system table (/etc/fstab for instance), list local disk devices.<br />
11. Make a script that checks whether a user exists in /etc/passwd. For now, you can specify the user<br />
name in the script, you don't have to work with arguments and conditionals at this stage.<br />
12. Display configuration files in /etc that contain numbers in their names.<br />
Chapter 4. Regular expressions 61
Change language