Grep:

• Grep (Global Regular Expression Print) is a command-line utility for searching plain-text data sets for lines matching a regular expression. … Grep was originally developed for the Unix operating system, but is available today for all Unix-like systems.

egrep

• egrep is an acronym for “Extended Global Regular Expressions Print”. It is a program which scans a specified file line by line, returning lines that contain a pattern matching a given regular expression. The standard egrep command looks like: egrep <flags> ‘<regular expression>’ <filename>

fgrep

• The fgrep command differs from the grep and egrep commands because it searches for a string instead of searching for a pattern that matches an expression.
• The fgrep command uses a fast and compact algorithm. The $, *, [, … , (, ), and \ characters are interpreted literally by the fgrep command.

difference between `grep`, `egrep`, and `fgrep`:

grep:

• grep is an acronym that stands for “Global Regular Expressions Print”. grep is a program which scans a specified file or files line by line, returning lines that contain a pattern.
• A pattern is an expression that specifies a set of strings by interpreting characters as meta-characters.
• For example the asterisk meta character (*) is interpreted as meaning “zero or more of the preceding element”.
• This enables users to type a short series of characters and meta characters into a grep command to have the computer show us what lines in which files match.

The standard grep command looks like:

grep <flags> '<regular expression>' <filename>

grep prints the search results to the screen (stdout) and returns the following exit values:

0 A match was found.
1 No match was found.
>1 A syntax error was found or a file was inaccessible
(even if matches were found).

Some common flags are: -c for counting the number of successful matches and not printing the actual matches, -i to make the search case insensitive, -n to print the line number before each match printout, -v to take the complement of the regular expression (i.e. return the lines which don’t match), and -l to print the file names of files with lines which match the expression.

egrep

• egrep is an acronym that stands for “Extended Global Regular Expressions Print”.
• The ‘E’ in egrep means treat the pattern as a regular expression. “Extended Regular Expressions” abbreviated ‘ERE’ is enabled in egrep. egrep (which is the same as grep -E) treats +, ?, |, (, and ) as meta-characters.
• In basic regular expressions (with grep), the meta-characters ?, +, {, |, (, and ) lose their special meaning. If you want grep to treat these characters as meta-characters, escape them \?, \+, \{, \|, \(, and \).
• For example, here grep uses basic regular expressions where the plus is treated literally, any line with a plus in it is returned.

grep "+" myfile.txt

egrep on the other hand treats the “+” as a meta character and returns every line because plus is interpreted as “one or more times”.

egrep "+" myfile.txt

Here every line is returned because the + was treated by egrep as a meta character. normal grep would have searched only for lines with a literal +.

fgrep

• fgrep is an acronym that stands for “Fixed-string Global Regular Expressions Print”.
• fgrep (which is the same as grep -F) is fixed or fast grep and behaves as grep but does NOT recognize any regular expression meta-characters as being special. The search will complete faster because it only processes a simple string rather than a complex pattern.

For example, if you wanted to search you .bash_profile for a literal dot (.) then using grep would be difficult because you would have to escape the dot because dot is a meta character that means ‘wild-card, any single character’:

grep "." myfile.txt

The above command returns every line of myfile.txt. Do this instead:

fgrep "." myfile.txt

Then only the lines that have a literal ‘.’ in them are returned. fgrep helps us not bother escaping our meta characters.