Task 4-2

Task 4-2

You are writing a graphics file conversion utility for use in creating a web page. You want to be able to take a PCX file and convert it to a JPEG file for use on the web page.^[7]

^[7] PCX is a popular graphics file format under Microsoft Windows. JPEG (Joint Photographic Expert Group) is a common graphics format on the Internet and is used to a great extent on web pages.

Graphics file conversion utilities are quite common because of the plethora of different graphics formats and file types. They allow you to specify an input file, usually from a range of different formats, and convert it to an output file of a different format. In this case, we want to take a PCX file, which can't be displayed with a web browser, and convert it to a JPEG which can be displayed by nearly all browsers. Part of this process is taking the filename of the PCX file, which ends in .pcx, and changing it to one ending in .jpg for the output file. In essence, you want to take the original filename and strip off the .pcx, then append .jpg. A single shell statement will do this:

outfile=${filename%.pcx}.jpg

The shell takes the filename and looks for .pcx on the end of the string. If it is found, .pcx is stripped off and the rest of the string is returned. For example, if filename had the value alice.pcx, the expression ${filename%.pcx} would return alice. The .jpg is appended to form the desired alice.jpg, which is then stored in the variable outfile.

If filename had an inappropriate value (without the .pcx) such as alice.xpm, the above expression would evaluate to alice.xpm.jpg: since there was no match, nothing is deleted from the value of filename, and .jpg is appended anyway. Note, however, that if filename contained more than one dot (e.g., if it were alice.1.pcx—the expression would still produce the desired value alice.1.jpg).