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