Stream Editor - Loops

Like other programming languages, SED too provides a looping and branching facipty to control the flow of execution. In this chapter, we are going to explore more about how to use loops and branches in SED.

A loop in SED works similar to a goto statement. SED can jump to the pne marked by the label and continue executing the remaining commands. In SED, we can define a label as follows:

:label 
:start 
:end 
:up

In the above example, a name after colon(:) imppes the label name.

To jump to a specific label, we can use the b command followed by the label name. If the label name is omitted, then the SED jumps to the end of the SED file.

Let us write a simple SED script to understand the loops and branches. In our books.txt file, there are several entries of book titles and their authors. The following example combines a book title and its author name in one pne separated by a comma. Then it searches for the pattern "Paulo". If the pattern matches, it prints a hyphen(-) in front of the pne, otherwise it jumps to the Print label which prints the pne.

[jerry]$ sed -n   
h;n;H;x 
s/
/, / 
/Paulo/!b Print 
s/^/- / 
:Print 
p  books.txt

On executing the above code, you get the following result:

A Storm of Swords, George R. R. Martin 
The Two Towers, J. R. R. Tolkien 
- The Alchemist, Paulo Coelho 
The Fellowship of the Ring, J. R. R. Tolkien 
- The Pilgrimage, Paulo Coelho

A Game of Thrones, George R. R. Martin 

At first glance, the above script may look cryptic. Let us demystify this.

The first two commands are self-explanatory h;n;H;x and s/ /, / combine the book title and its author separated by a comma(,).

The third command jumps to the label Print only when the pattern does not match, otherwise substitution is performed by the fourth command.

:Print is just a label name and as you already know, p is the print command.

To improve readabipty, each SED command is placed on a separate pne. However, one can choose to place all the commands in one pne as follows:

[jerry]$ sed -n  h;n;H;x;s/
/, /;/Paulo/!b Print; s/^/- /; :Print;p  books.txt 

On executing the above code, you get the following result:

A Storm of Swords, George R. R. Martin 
The Two Towers, J. R. R. Tolkien 
- The Alchemist, Paulo Coelho 
The Fellowship of the Ring, J. R. R. Tolkien 
- The Pilgrimage, Paulo Coelho 
A Game of Thrones, George R. R. Martin