OR
$ grep "1\|2" 1.txt 1 2
$ grep -E "1|2" 1.txt 1 2
grep -E "^1$|2" 1.txt 1 2
grep -e 1 -e 2 1.txt 1 2
egrep "1|2" 1.txt 1 2
AND
grep -E '1.*2' 2.txt 1 2
grep '1' 2.txt | grep '2' 2.txt 1 2 2
NOT
grep -v 1 2.txt
Tools: Grep
OR
$ grep "1\|2" 1.txt 1 2
$ grep -E "1|2" 1.txt 1 2
grep -E "^1$|2" 1.txt 1 2
grep -e 1 -e 2 1.txt 1 2
egrep "1|2" 1.txt 1 2
AND
grep -E '1.*2' 2.txt 1 2
grep '1' 2.txt | grep '2' 2.txt 1 2 2
NOT
grep -v 1 2.txt