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
.
-
,