-e 옵션 빼도 동일한 결과네요.

옵션 설명을 보면, 정규표현식 옵션 같은데...
-e PATTERN, --regexp=PATTERN
Use PATTERN as the pattern; useful to protect patterns beginning with -.


[root@oracle test]# grep -e search_word ./test -R
./test/test.txt:search_word
[root@oracle test]# grep search_word ./test -R
./test/test.txt:search_word


[root@oracle test]# grep -e [a-z] ./test -R
./test/test.txt:search_word
./test/test.txt:search-word