Sed command to print matching lines and 2 lines above..
 

Hello everyone,

I'm looking to print matching lines including the 3 lines above the matched line. I have come up with the following command but it's only printing 1 line above. In the example below the output is missing the Host Name and the following blank line. Anyone known what I'm missing here?


sed -n /192.168.12/{x;p;x;p};h <input.txt >output.txt

input.txt

Host Name: SERVER1

NetBIOS over TCP/IP: Enabled
IP Address: 192.168.12.111

-----------------------------
output.txt


NetBIOS over TCP/IP: Enabled
IP Address: 192.168.12.111

If you're not trying to replace any of the text, then the grep command has an option to do this:You may play around with it to get what you want, but the man pages of grep have a section called 'Context Line Control' that discusses what you are trying to do.

Thanks crc123, but is there anyway SED can do this?

The rational answer is: Why?
This said, I will confess that I often do such things---if only to see if it can be done.

The problem with SED is that it operates one line at a time. To do what you want, you have to append lines to the hold register and then pull out the combination to test it. If the test failed, you would then have to delete one of the lines and add a new one.

I visualize some messy-looking code....

Really good SED tutorial here:
http://www.grymoire.com/Unix/Sed.html

Look what I found:
http://www.grymoire.com/Unix/Sed.html#uh-54
He calls it "context grep"

Maybe this could work:

I think OP wanted the 3 previous lines without knowing the content...ie the only thing known was the desired pattern in the last line.

sed is not the correct tool for such task. there's actually an algorithm for this. It uses variables to store every 3 lines.
output:
the order is jumbled, but you get the picture.

If the file is not too large:

That's correct, I don't know the value of the previous lines.

I finally figured out the original syntax:

What this does is to write (copy) each line to the hold buffer until the pattern is found. Then it exchanges the pattern buffer and hold buffer, prints, and then repeats. Thus you get the last thing copied to the hold buffer, and then the line where the pattern matched.

What you need is "H" (append), instead of "h" (copy)

This is crude, but works:

In the reference I gave you, read carefully about h, H, g, G, and x

The following is adapted from a sed emulation of the tail command at
http://www.gnu.org/software/sed/manual/sed.html#tail
It creates a moving window of four lines.
The '$' end of line marker is essential as the pattern space will usually contain four file lines and '192.168.12.111' could be in any of them, but we only want to print when its in the last one, i.e. at the end of the pattern space.
This is a more pragmatic method which uses tac (to reverse the order of the lines) and GNU sed:-
EDIT:
These solutions give the matched line and the three lines above.
If only two lines above are required change 4 to 3, and 3 to 2 in the code.
(Both versions of the problem appear in the thread)

Thank You! I get it now...you guys are amazing!