get text between keywords in log file
One line of sed can do this for you:
sed -n 's/.*1234 \(.*\)9876.*/\1/p' textfile.txt > log
normally you can do this with grep and the -o param.
-o, --only-matching
Print only the matched (non-empty) parts of a matching line, with each such part on a separate output line.
so it would be something like:
grep -Po '1234.*9876' >> log
Not 100% sure about the regex btw, I did not test it