[SOLVED] Sed -n '/pattern/{n;p;}' is missing matches in file

Euler2 · 06-16-2011, 05:45 PM

I have an interesting problem, sed '/pattern/{n;p;}' file, doesn't seem to be catching all of the matches in my file. As an example of this see below.

test:

Turn
Turn
Turn
H310
Turn
Turn
Alpha
Coil
Turn
Turn

sed -n '/Turn/{n;p}' test
output:
Turn
H310
Turn
Turn

But what I expect is:

Turn
Turn
H310
Turn
Alpha
Turn

Any ideas what's going wrong and how to fix it?

(my thought is that the pattern is being matched, sed moves to the next line, prints, but then doesn't bother to check if there is a match)

jschiwal · 06-16-2011, 07:59 PM

Try sample data which adds line numbers. Then it will be easier to debug. Usually the `-n' option is used with pattern matches and the `p' command.

Euler2 · 06-16-2011, 08:26 PM

Following your suggestion, I went back and added line numbers, now the output is:

2 Turn
4 H310
6 Turn
10 Turn

while I expect:

2 Turn
3 Turn
4 H310
6 Turn
7 Alpha
10 Turn

Which matches what I thought was happening, that sed is identifying matches, advancing to the next line and printing, but after advancing is not checking to see if the line it advanced to matches the pattern. Any ideas on how to get my desired result?

grail · 06-16-2011, 09:57 PM

Not sure on getting the desired result but you are aware that the n;p says get the next line and print it.
Sed cannot then say I need to check the next line as well (not as written).

Now I am guessing one of the gurus will have this a better way, but the following seems to work:

Code:

sed -n 'x;/Turn/{x;p;x}' file

Reuti · 06-17-2011, 05:57 AM

What about a loop in case of a match:

Code:

$ sed -n ": start;/Turn/{n;p;b start}" test

Euler2 · 06-17-2011, 07:28 AM

The loop worked like a charm, thanks! (Solved!)