general regular expression print // file pattern searcher
-o Print only the matching string, not the entire line
-l Only print filename of matching files
-h Hide filename of matching files
-c Show the number of matches found
-m Limit to <num> matches. Surprisingly useful to avoid firehoses.
--line-buffered Stops grep buffering data, and output it as soon as it arrives
-f Pass in a file of newline-separated patterns. Note: empty lines match every input line.
-w Match whole words only
-P Set PCRE, useful when you're writing a complex pattern
--color This is enabled by default for grep, but not for flavours like fgrep/egrep.
-B/ -A Show <num> lines before and after matches
-h /-H Do/don't print filename. -H defaults on when searching multiple files
-v Display all entries which don't match
-r Search recursively in a directory
-n Show line numbers