More fun with regexps: Adding "[no output]" in shell logs (Shallow Thoughts)

Akkana's Musings on Open Source Computing, Science, and Nature.

Sun, 12 Oct 2008

More fun with regexps: Adding "[no output]" in shell logs

Someone on LinuxChix' techtalk list asked whether she could get tcsh to print "[no output]" after any command that doesn't produce output, so that when she makes logs to help her co-workers, they will seem clearer.

I don't know of a way to do that in any shell (the shell would have to capture the output of every command; emacs' shell-mode does that but I don't think any real shells do) but it seemed like it ought to be straightforward enough to do as a regular expression substitute in vi. You're looking for lines where a line beginning with a prompt is followed immediately by another line beginning with a prompt; the goal is to insert a new line consisting only of "[no output]" between the two lines.

It turned out to be pretty easy in vim. Here it is:

:%s/\(^% .*$\n\)\(% \)/\1[no results]\r\2/

Explanation:

:
starts a command
%
do the following command on every line of this file
s/
start a global substitute command
\(
start a "capture group" -- you'll see what it does soon
^
match only patterns starting at the beginning of a line
%
look for a % followed by a space (your prompt)
.*
after the prompt, match any other characters until...
$
the end of the line, after which...
\n
there should be a newline character
\)
end the capture group after the newline character
\(
start a second capture group
%
look for another prompt. In other words, this whole
expression will only match when a line starting with a prompt
is followed immediately by another line starting with a prompt.
\)
end the second capture group
/
We're finally done with the mattern to match!
Now we'll start the replacement pattern.
\1
Insert the full content of the first capture group
(this is also called a "backreference" if you want
to google for a more detailed explanation).
So insert the whole first command up to the newline
after it.
[no results]
After the newline, insert your desired string.
\r
insert a carriage return here (I thought this should be
\n for a newline, but that made vim insert a null instead)
\2
insert the second capture group (that's just the second prompt)
/
end of the substitute pattern

Of course, if you have a different prompt, substitute it for "% ". If you have a complicated prompt that includes time of day or something, you'll have to use a slightly more complicated match pattern to match it.

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[ 13:34 Oct 12, 2008    More linux/editors | permalink to this entry | comments ]
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