Well, depending on what you are doing grep can be one of the most usfull tools. I used to leave nmap on for hours doing random port scans of stuff I was interested in and logging it to a file and then I hate a perl script that would extract ip addresses that had that port open and run further test to see if the system was vulnerable to a certain bug. Grep was helpful in every step of the way. Anytime you have big logfiles grep is a lifesaver.
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