Chapter 1. Power Tools for Editing

Initially, using sed and awk will seem like the long way to accomplish a task. After several attempts you may conclude that the task would have been easier to do manually. Be patient. You not only have to learn how to use sed and awk but you also need to learn to recognize situations where using them pays off. As you become more proficient, you will solve problems more quickly and solve a broader range of problems.

You will also begin to see opportunities to find general solutions to specific problems. There is a way of looking at a problem so you see it related to a class of problems. Then you can devise a solution that can be reused in other situations.

\*[CHerrorhand]

"CHerrorhand" is the name giving the reference and "\*[" and "]" are calling sequences that distinguish the reference from other text. In a central file, the names used for cross references in the document are defined as sqtroff strings. For instance, "CHerrorhand" is defined to be "Chapter 16, Error Handling." (The advantage of using a symbolic cross-referencing scheme like this, instead of explicit referencing, is that if chapters are added or deleted or reordered, only the central file needs to be edited to reflect the new organization.) When the formatting software processes the document, the references are properly resolved and expanded.

The problem we faced was that we had to use the same files to create an online version of the book. Because our sqtroff formatting software would not be used, we needed some way to expand the cross references in the files. In other words, we did not want files containing "\*[CHerrorhand]"; instead we wanted what "CHerrorhand" referred to.

There were three possible ways to solve this problem:

1. Use a text editor to search for all references and replace each of them with the appropriate literal string.

2. Use sed to make the edits. This is similar to making the edits manually, only faster.

3. Use awk to write a program that (a) reads the central file to make a list of reference names and their definitions, (b) reads the document searching for the reference calling sequence, and (c) looks up the name of the reference on the list and replaces it with its definition.

The first method is obviously time-consuming (and not very interesting!). The second method, using sed, has an advantage in that it creates a tool to do the job. It is pretty simple to write a sed script that looks for "\*[CHerrorhand]" and replaces it with "Chapter 16, Error Handling" for instance. The same script can be used to modify each of the files for the document. The disadvantage is that the substitutions are hard-coded; that is, for each cross reference, you need to write a command that makes the replacement. The third method, using awk, builds a tool that works for any cross reference that follows this syntax. This script could be used to expand cross references in other books as well. It spares you from having to compile a list of specific substitutions. It is the most general solution of the three and designed for the greatest possible reuse as a tool.

Part of solving a problem is knowing which tool to build. There are times when a sed script is a better choice because the problem does not lend itself to, or demand, a more complex awk script. You have to keep in mind what kinds of applications are best suited for sed and awk.