34.20 The sed Test Command

The test command, t , branches to a label (or the end of the script) if a successful substitution has been made on the currently addressed line. It implies a conditional branch. Its syntax is as follows:

[ address ] t [ label ]

If no label is supplied, control falls through to the end of the script. If label is supplied, then execution resumes at the line following the label.

Let's look at a spelling corrector written by Greg Ubben. The script fixes common (in this example, silly) spelling goofs; the t command tells about corrections that were made:

h s/seperate/separate/g s/compooter/computer/g s/said editor/sed editor/g s/lable/label/g t changed b : changed p g s/.*/[WAS: &]/ t

First, h ( 34.13 ) holds a copy of the current input line. Then, if any of the four substitutions succeed, the command t changed branches to the corresponding label ( : changed ) at the end of the script. Otherwise, if no s succeeded, the b command restarts the script on the next line (as always in sed , the input line is printed before the script re-starts).

After the label, the script prints the current input line (the line with a spelling error - which, by now, has been corrected). Then g ( 34.13 ) gets the original uncorrected line. An s command brackets that line [WAS:  xxx ] . Here's some sample output:

$ 

sed -f sedscr afile

 This is a separate test. [WAS: This is a seperate test.] I put a label on my computer! [WAS: I put a lable on my compooter!] That's all for now.

The final t in the script is a work-around for a bug in some versions of sed . Greg says "The t flag is supposed to be reset after either the t command is executed or a new line of input is read, but some versions of sed don't reset it on a new line of input. So I added a do-nothing t to make sure it's reset after the previous always-true s/// ." Try the script without the extra t ; if adding it makes the script work right, your sed has the bug and you might try a new version, like GNU gsed .