Difference between grep, egrep, fgrep, pgrep.

Introduction#

grep, egrep, fgrep, rgrep, pgrep - are commands in Unix-like operating systems that print lines matching a pattern. The grep searches the named input FILEs for lines containing a match to the given PATTERN. By default, it prints the matching lines. In addition, the variant programs egrep, fgrep, and rgrep are the same as grep -E, grep -F, and grep -r, respectively. These variants are deprecated, but are provided for backward compatibility.

Syntax#

grep [OPTIONS] PATTERN [FILE...]
grep [OPTIONS] [-e PATTERN]... [-f FILE]... [FILE...]

Parameters#

Symbol	Details Basic Regular Expressions (BRE)
`^`	the circumflex is used to match the beginning of a line.
`$`	used to match the end of a line.
`.`	matches any character except a new line.
`[]`	matches single character inside the brackets. If there’s a `^` inside, it would match anything but the characters in the bracket.
`\`	before any of the non-alphanumeric characters quotes them.
`*`	symbol matches the preceding character or subexpression zero, one or more times.
`\1`	backreferences 1-9 match the exact text by the corresponding group.
`\{m,n\}`	matches the preceding elements at least m and no more than n times.
`\|`	`foo\|bar` matches foo or bar.
`\?`	short for {0,1}
`\+`	(short for {1,}) match the preceding character or subexpression at most 1 time, or at least 1 time respectively.
`\n`	matches a newline, `\t` matches a tab, etc.
`\w`	matches any word constituent and \W matches any character that isn’t a word constituent.
`\<\>`	match the empty string only at the beginning or end of a word
`\b`	matches either and \B matches where \b doesn’t.
Symbol	Details Extended Regular Expressions (ERE)
`^`	match only at the beginning
`$`	match only at the end of a line.
`.`	matches any character (or any character except a newline).
`[…]`	matches any one character listed inside the brackets (character set). Add an initial ^ and ranges work like in BRE (see above).
`(…)`	syntactic group, for use with * or \DIGIT replacements.
`\|`	for alternation: `foo`\|`bar` matches foo or bar.
`*`	matches the preceding character or subexpression a number of times: 0, 1 or more times
`+`	matches 1 or more times preceding character.
`?`	matches preceding characters 0 or 1 times.
`\`	Backslash quotes the next character if it is not alphanumeric.
`{m,n}`	matches the preceding character or subexpression between m and n times (missing from some implementations); n or m can be omitted, and `{m}` means exactly m

Remarks#

fgrep stands for “Fixed-string Global Regular Expressions Print”. fgrep is the same as grep -F. This commands is a faster 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.

pgrep is an acronym that stands for “Process-ID Global Regular Expressions Print”. pgrep looks through the currently running processes and lists the process IDs which matches the selection criteria to stdout. pgrep is handy when all you want to know is the process id integer of a process.

grep	egrep(grep -E)	fgrep(grep -F)	pgrep
Basic Regular Expressions (BRE)	Extended Regular Expressions (ERE)	Searches only strings	Searches process by name

For more information and reference use some of the following links:

What is the difference between grep, egrep and fgrep ? Unix&Linux StackExchange

Why does my regular expression work in X but not in Y? Unix&Linux StackExchange

What is the difference between grep, pgrep, egrep, fgrep? Superuser

grep with Basic Regular Expressions

$ grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin

egrep with Extended Regular Expressions

$ egrep '^(0|1)+ [a-zA-Z]+$' searchfile.txt
011 AaBBS

fgrep with no Regular expressions

$ fgrep "." .bashrc
# will match lines with a dot.

pgrep with name of process

$ pgrep python
1299