Grep

grep searches the named input FILEs (or standard input if no files are named, or if a single hyphen-minus (-) is given as file name) for lines containing a match to the given PATTERN. By default, grep prints the matching lines. grep is a powerful tool.

Using grep

Recursively search DIR for "STRING" (also ignores caps)

    grep -ri "STRING" /DIR/

Piping

grep can also be piped from or to other outputs using the pipe "|"

   grep "STRING" /LOCATION/logfile.log |grep ANOTHERSTRING

Example usage:

   grep "13:00:" /var/log/messages |grep "September"

   tail -f /var/log/somefile.log |grep specificstring

   sudo cat /usr/local/apache/logs/error_log | grep -i "modsec" | awk '{print $10}' | sort | uniq -c | sort -n

Remove everything but grep'd

This will show only lines with "firstline" within file.txt.

   cat file.txt | grep '^firstline'