Order of Commands in a Script (Unix Power Tools, 3rd Edition)

34.6. Order of Commands in a Script

Combining a series of edits in a script can have unexpected results. You might not think of the consequences one edit can have on another. New users typically think that sed applies an individual editing command to all lines of input before applying the next editing command. But the opposite is true. sed applies every editing command to the first input line before reading the second input line and applying the editing script to it. Because sed is always working with the latest version of the original line, any edit that is made changes the line for subsequent commands. sed doesn't retain the original. This means that a pattern that might have matched the original input line may no longer match the line after an edit has been made.

Let's look at an example that uses the substitute command. Suppose someone quickly wrote the following script to change pig to cow and cow to horse:

s/pig/cow/
s/cow/horse/

The first command would change pig to cow as expected. However, when the second command changed cow to horse on the same line, it also changed the cow that had been a pig. So, where the input file contained pigs and cows, the output file has only horses!

This mistake is simply a problem of the order of the commands in the script. Reversing the order of the commands -- changing cow into horse before changing pig into cow -- does the trick.

Another way to deal with this effect is to use a pattern you know won't be in the document except when you put it there, as a temporary placeholder. Either way, you know what the "document" looks like after each step in the program.

s/pig/cXXXoXXXw/
s/cow/horse/
s/cXXXoXXXw/cow/

Some sed commands change the flow through the script. For example, the N command (Section 34.16) reads another line into the pattern space without removing the current line, so you can test for patterns across multiple lines. Other commands tell sed to exit before reaching the bottom of the script or to go to a labeled command. sed also maintains a second temporary buffer called the hold space. You can copy the contents of the pattern space to the hold space and retrieve it later. The commands that make use of the hold space are discussed in Section 34.14 and other articles after it.

-- DD