Grep a Log file for the last occurrence of a string between two strings

Question

I have a log file trace.log. In it I need to grep for the content contained within the strings <tag> and </tag>. There are multiple sets of this pair of strings, and I just need to return the content between last set (in other words, from the tail of the log file).

Extra Credit: Any way I can return the content contained within the two strings only if the content contains "testString"?

Thanks for looking.

EDIT: The search parameters and are contained on different lines with about 100 lines of content separating them. The content is what I'm after...

Answer 1

Use tac to print the file the other way round and then grep -m1 to just print one result. The look behind and look ahead checks text in between <tag> and </tag>.

tac a | grep -m1 -oP '(?<=tag>).*(?=</tag>)' 

Test

Given this file

$ cat a <tag> and </tag> aaa <tag> and <b> other things </tag> adsaad <tag>and  last one</tag>  $ tac a | grep -m1 -oP '(?<=tag>).*(?=</tag>)' and  last one 

---

Update

EDIT: The search parameters and are contained on different lines with about 100 lines of content separating them. The content is what I'm after...

Then it is a bit more tricky:

tac file | awk '/<\/tag>/ {p=1; split($0, a, "</tag>"); $0=a[1]};                 /<tag>/   {p=0; split($0, a, "<tag>");  $0=a[2]; print; exit};                 p' | tac 

The idea is to reverse the file and use a flag p to check if the <tag> has appeared yet or not. It will start printing when </tag> appears and finished when <tag> comes (because we are reading the other way round).

• split($0, a, "</tag>"); $0=a[1]; gets the data before </tag>
• split($0, a, "<tag>" ); $0=a[2]; gets the data after <tag>

Test

Given a file a like this:

<tag> and </tag> aaa <tag> and <b> other thing come here and here </tag>  some text<tag>tag is starting here blabla and ends here</tag> 

The output will be:

$ tac a | awk '/<\/tag>/ {p=1; split($0, a, "</tag>"); $0=a[1]}; /<tag>/ {p=0; split($0, a, "<tag>"); $0=a[2]; print; exit}; p' | tac tag is starting here blabla and ends here 

Answer 2

If like me, you don't have access to tac because your sysadmin won't play ball you can try:

grep pattern file | tail -1