sed & awk, 2nd Edition

Chapter 4. Writing sed Scripts

To use sed, you write a script that contains a series of editing actions and then you run the script on an input file. Sed allows you to take what would be a hands-on procedure in an editor such as vi and transform it into a look-no-hands procedure that is executed from a script.

When performing edits manually, you come to trust the cause-and-effect relationship of entering an editing command and seeing the immediate result. There is usually an “undo” command that allows you to reverse the effect of a command and return the text file to its previous state. Once you learn an interactive text editor, you experience the feeling of making changes in a safe and controlled manner, one step at a time.

Most people new to sed will feel there is greater risk in writing a script to perform a series of edits than in making those changes manually. The fear is that by automating the task, something will happen that cannot be reversed. The object of learning sed is to understand it well enough to see that your results are predictable. In other words, you come to understand the cause-and-effect relationship between your editing script and the output that you get.

This requires using sed in a controlled, methodical way. In writing a script, you should follow these steps:

20. Think through what you want to do before you do it.

21. Describe, unambiguously, a procedure to do it.

22. Test the procedure repeatedly before committing to any final changes.

These steps are simply a restatement of the same process we described for writing regular expressions in Chapter 3. They describe a methodology for writing programs of any kind. The best way to see if your script works is to run tests on different input samples and observe the results.

With practice, you can come to rely upon your sed scripts working just as you want them to. (There is something analogous in the management of one’s own time, learning to trust that certain tasks can be delegated to others. You begin testing people on small tasks, and if they succeed, you give them larger tasks.)

This chapter, then, is about making you comfortable writing scripts that do your editing work for you. This involves understanding three basic principles of how sed works:

• All editing commands in a script are applied in order to each line of input.

• Commands are applied to all lines (globally) unless line addressing restricts the lines affected by editing commands.

• The original input file is unchanged; the editing commands modify a copy of original input line and the copy is sent to standard output.

After covering these basic principles, we’ll look at four types of scripts that demonstrate different sed applications. These scripts provide the basic models for the scripts that you will write. Although there are a number of commands available for use in sed, the scripts in this chapter purposely use only a few commands. Nonetheless, you may be surprised at how much you can do with so few. (Chapter 5, and Chapter 6, present the basic and advanced sed commands, respectively.) The idea is to concentrate from the outset on understanding how a script works and how to use a script before exploring all the commands that can be used in scripts.