5.9 Shell programming - Scripts5 Class notes5.7 Quiz 15.8 Regular expressions and grep

5.8 Regular expressions and grep

grep is a good way to introduce regular expressions. Regular Expressions, or re, are used by many programs. It is the use of special characters to represent general characters. For example, the use of an asterisk "*" to represent any possible string of characters as part of a file name when "talking" to a shell. With regular expressions you will also have a way of escaping from the special meaning of a special character such as the asterisk "*". This way you will still be able to search for file names that contain asterisks. (This is NOT recommended.) The escape sequence is to place a backslash in front of the special character. This also works for the backslash.

grep is a fast way to search the content of a file. Please look at the manual pages for grep. The manual pages will give you more information on switches and how to use grep.

I have used grep many, many times. Two examples come to mind that have been very useful. I will outline these uses.

The first was to find files on floppy disks, after I had a hard disk. At this time I had several hundred floppy disks. I wrote a program that would do a recursive directory search of a disk creating a file of the disks content and printing a disk label. I also ran this program on the hard disks of systems that I was using at the time. The name of the file was the number on the printed label (for floppy disks). The content of the file was for each file on the disk one line. This line included the disk number, the full path name of the file, and the creation/modification time of the file. If I modified a disk I could re-run the program recreating a new directory file, overwriting the old entry. I placed all of the resulting files in a single directory on my hard disk. I could then use grep to search all of the files for a file name. greps output would tell me what disk had the file, when it was last modified, and what directory the file was in.

The second example relates to maintenance programming. I was involved in a writing and updating some product test code for some multi-bus products here in Tucson. It was a summer job. The code was all new to me. I needed to be able to change sections of the "C" code that was broken up into about 60 different files, both program and header files.

I used grep to search all of the modules for references to a function names, variable names, constants - both symbolic and numeric, file names and other constructs. It allowed me to quickly get a grasp of where "things" where both declared and used. It also let me verify that I had made changes in all of the needed locations.


Instructor: ltaber@pima.edu ** My new Home at GeoApps in Tucson ** The Pima College Site

5.9 Shell programming - Scripts5 Class notes5.7 Quiz 15.8 Regular expressions and grep