grep command is used to print lines matching patterns. It input from file or pipe and print lines matching.
grep [OPTIONS] PATTERN [FILENAME...]
grep [OPTIONS] -f FILE_PATTERN ... [FILENAME...]
[ -i ] :Case-insensitive strings.
[ -v ] :Non-matching lines
[ -w ] :It prints who matches whole word.
[ -x ] :It prints lines who matches whole line.
1. Finding stings in text.
2. Find strings by case-insensitive type, it helps in finding possible all strings.
3. Finding strings in file by pattern or regexp.
4. Finding pattern from a file by taking input of regexp from another file.
5. Piping data to grep
6. finding non matching lines in file.
This tool is very useful and big. explore more on man pages. `man grep`