Grep

I love grep, and I use it all the time. A closely related program is find.

This is my favorite way to use it. If I am looking for any file with a particular line in it:

grep -rI 'the string' .

Explanation:

• -r: Recurse through directories.
• -I: Ignore binary files.
• 'the string': The string to find. I always use single quotes.
• .: The directory to search through.

Here are the most common patterns I use in grep: (See Regex)

• \|: Alternatives.
• *: The Kleene Star-0 or more of the preceeding.
• \+: One or more.
• \?: Zero or one. In other words, optional.
• \<, \>: Word boundaries. For instance, if I was looking for the word 'python' and not 'pythons' or 'unpythonic', I would type '\<python\>'.
• \(, \): Grouping parens.

Sometimes I want a case-insensitive search (rarely though). Just use the -i option to grep.

I never, never use egrep. I am too used to grep's regex syntax.

-A NUM	Show NUM lines after the matching line.
-B NUM	Show NUM lines before the matching line.
-C NUM	Show NUM lines around the matching line.
--colour	Color the matching expressions. Some people do "alias cgrep grep --color". I don't.
-c	Just show how many lines matched.
-I	Ignore binary files.
-i	Ignore case.
-l	Show only the filenames that match.
-L	Show only the filenames that don't match.
-n	Show the line numbers.
-o	Show only the match.
-r	Recurse through directories.
-v	Show lines that don't match the expression. This is great for using with tail when you don't want to see certain lines.
-P	Use perl's regex engine.

Patterns

See regex.

Most of grep's regex syntax needs to be escaped. Don't forget that bash does some magic on double-quotes, so use single-quotes if possible.

Characters

[chars]	Any char. You can also use POSIX groups inside.
.	Any char.
\w	Any word char.
\W	Any non-word char.
\char	char if char is a special character.

POSIX groups can be used inside the square brackets. These are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:]. Don't forget that these can be combined, as in [[:lower:][:digit:]%].

Grouping

\(pattern\)	Basic grouping.
\|	Alternation.

Positions

^	Start of line.
$	End of line.
\< \>	Beginning, end of word.
\b	Word boundary
\B	Non-word boundary

Multipliers

*	Zero or more
\+	One or more
\?	Zero or one
\{n\}	n times
\{n,m\}	n to m times, inclusive
\{n,\}	n or more times

These are all greedy. Non-greedy has a '\?' after the multiplier.