Now grep is smart enough to search for strings quickly without having to be told explicitly via -F that the regular expressions are in fact strings. I suspect that the performance difference between fgrep and grep used to be larger, but the latter has gotten more efficient. There may be instances where fgrep is much faster than grep, but I haven’t seen one first hand. I ran an analogous search on my own computer with different data and got similar results. When I timed it the difference was on the order of 1%. It was a tiny bit faster, not enough to notice. This is the kind of task where fgrep (“fast grep”) is supposed to be much faster than grep. I was working on a project for a client where I had to search for a long list of words in a long list of files. These variants are deprecated, but are provided for backward compatibility. In addition, the variant programs egrep, fgrep and rgrep are the same as grep -E, grep -F, and grep -r, respectively. On Linux, if you ask for the man (manual) page for fgrep you’ll be taken to the man page for grep which says Strictly speaking I used grep -F rather than fgrep. However, if your regular expressions are in fact simply text strings, fgrep may be much faster than grep. The grep utility searches text files for regular expressions, but it can search for ordinary strings since these strings are a special case of regular expressions.

Bash grep for files that are gigabytes