egrep and fgrep

1. The Definitions

• egrep (Extended GREP): This version supports Extended Regular Expressions (ERE). It recognizes special metacharacters like |, +, ?, and () without needing to escape them with backslashes.

• fgrep (Fixed/Fast GREP): This version treats your pattern as a fixed string, not a regular expression. It does not recognize any special characters; it looks for the exact literal characters you typed.

2. Key Technical Differences

Feature egrep (grep -E) fgrep (grep -F)

Pattern Type Extended Regular Expression Literal/Fixed String

Speed Fast, but processes complex logic Very fast for simple, large strings

Special Chars *, +, ?, `, ()` are active

Use Case Complex pattern matching Searching for exact phrases or IP addresses

3. Practical Examples

Imagine you are looking for a string that contains a period and a plus sign, like an IP range or a specific versioning tag.

Using egrep

If you search for 192.168.1.1+, egrep thinks:

• . means "any character."

• + means "one or more of the previous character."

• Result: It will match "192x168y1z11111", which is probably not what you want. You would have to escape them: 192\.168\.1\.1\+.

Using fgrep

If you search for 192.168.1.1+, fgrep thinks:

• "I am looking for exactly these characters in this order."

• Result: It only matches that exact string. No escaping required.

4. The Modern Way

In modern Linux systems, egrep and fgrep are actually considered deprecated. While they still work for backward compatibility, the "correct" way to use them now is by using flags with the standard grep command:

• Instead of egrep, use grep -E

• Instead of fgrep, use grep -F

Example: search two different IPs at the same time

If you don't want to mess with regex at all and just want to match two exact strings, you can use the -e flag multiple times.

grep -F -e "192.168.1.1" -e "10.0.0.5" access.log

• Why use this? It’s the easiest to read. Because we used -F, we don't even have to escape the dots!

Confused about globbing and regex? Here is some background.