Advanced Flow Control Commands

You have already seen several examples of changes in sed’s normal flow control. In this section, we’ll look at two commands that allow you to direct which portions of the script get executed and when. The branch (b) and test (t) commands transfer control in a script to a line containing a specified label. If no label is specified, control passes to the end of the script. The branch command transfers control unconditionally while the test command is a conditional transfer, occurring only if a substitute command has changed the current line.

A label is any sequence of up to seven characters.[1] A label is put on a line by itself that begins with a colon:

:mylabel

There are no spaces permitted between the colon and the label. Spaces at the end of the line will be considered part of the label. When you specify the label in a branch or test command, a space is permitted between the command and the label itself:

b mylabel

Be sure you don’t put a space after the label.

Branching

The branch command allows you to transfer control to another line in the script.

[address]b[label]

The label is optional, and if not supplied, control is transferred to the end of the script. If a label is supplied, execution resumes at the line following the label.

In Chapter 4, we looked at a typesetting script that transformed quotation marks and hyphens into their typesetting counterparts. If we wanted to avoid making these changes on certain lines, then we could use the branch command ...

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