Trjtdtk

There is nothing wrong with the previous two answers, but I thought I would make you aware that finding the third line after a pattern can be done in a single sed call:

sed -n "/four/ n; n; p " SourceData.txt

Because a single program does the work, this is more efficient than running multiple filters. The above command outputs the third line after every instance of "four", except where this occurs again in one of the two lines following a match (the other solutions don't handle this case in the expected manner either); also, no output is generated if the pattern is in the last or second-last line of the file, which may or may not be what you want.

To match the first instance only:

sed -n "/four/ n; n; p; q " SourceData.txt

(Note that this answer is as efficient as possible, by ending the scan as soon as the match is found.)

I add this solution because it is worth getting to know sed and, despite its rather off-putting syntax (regular expressions are bad enough!), it can often be extremely useful. This tutorial is a good introduction.

^{Note: this answer was originally written before the OP clearly stated the pattern appears just once. It is designed not to miss any occurrence (unless near the end, so there's no "n-th line after") and I'm going to leave it this way. If you're sure there's only one occurrence or if you want only the first one to be found, you may consider some other solution that stops immediately and doesn't parse the whole input stream/file in vain.}

This solution prints the current line iff there was a match two lines ago. It is slightly different from few other answers because it won't miss another match even if it occurs soon after the previous match.

awk -v delay=2 'for (i=delay; i>=0; i--) t[i]=t[i-1] /four/ t[0]="m" if (t[delay]) print'

Whenever there's a match, the information is stored in t[0]. With each line the t array is shifted (including shifting t[-1] to t[0] to reset the value of t[0]). The line is printed iff the array indicates there was a match two lines ago.

You can easily set a different delay (e.g. delay=7) or use another pattern (e.g. /sda[[:digit:]]/)

You can use this expression (input.txt):

grep "four" -A 2 input.txt | tail -n 1

Output is:

six 6

The grep option "-A 2" states that two lines after the matched line are outputted.

And the tail option "-n 1" states that only the last 1 lines of this result are returned.

For multiple occurences, and assumming that no lines start with --:

( grep -A 2 pattern data.txt; echo '--' ) | grep -E -B1 '^--' | grep -Ev '^--'

In slo-mo:

• 
  ( grep -A 2 pattern data.txt; echo '--' ) prints the pattern and the next two lines, and inserts a -- line between the groups. echo '--' makes sure that the last group is also followed by --.


• 
  grep -E -B1 '^--' print the separators and the lines just before (whoch are the ones we are looking for)


• 
  grep -Ev '^--' drops the separators, leaving only the lines we are looking for.

Looks like a good use case for ex, the POSIX-specified scriptable file editor.

Unlike sed and awk, ex is actually designed for file editing, not stream editing, and is capable of going backwards and forwards in a file. It's actually the non-visual form of the vi editor.

But the important aspect here is that ex is capable of chaining addresses. So referring to the line that's two lines after a particular text pattern is trivial.

Here is a command which prints all the lines that come two lines after lines containing four:

printf '%sn' 'g/four/+2p' | ex file.txt

I've written a lot of answers using ex on the Unix & Linux Stack Exchange; this one in particular has some additional explanations that may help.

You already got very good answers, which are easily the true go-to solutions for one-liners, but just in case you'd like or need to do it pure-bash only you might start from the following:

 while read letters _ && [ "$letters" != four ] ; do :; done ; read && read _ number _ && echo $number ; < data.txt

It only yields the first occurrence of "four".

Here I used _ as the name for a placeholder variable, i.e. a variable that receives values to be discarded, and as an "added bonus" you have the | cut -f2 result already built into the script.

Also please note that above script is just a proof-of-concept: it only works with the exampled input data.

It could be enhanced by:

• reading the entire (unsplit) line to match it against a regex via the [[ command (in place of [) along with its =~ operator, like in: while read line && ! [[ "$line" =~ four ]] but beware the then additional complexity given by bash’s own escaping rules if the regex gets more complex than a simple "four"; also note the ! preceding the [[ to negate the test


• joining the [ (or [[) command with the two read and the && echo ... together inside the while loop, to not stop after first match, like in:

 while read letters _ ; do [ "$letters" = four ] && read && read _ number _ && echo $number ; done ; < data.txt

Note also the change of the comparison operator. However beware of missing possibly adjacent matching lines. In order to address this possibility you’d need some kind of look-ahead or back-caching, for which you’d need some more advanced bash constructs.

Lastly, of course you can rather output the entire unsplit line via read line && echo $line in place of read _ number _ && echo $number