Grep Tutorial


Contents

Why grep ?

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grep is not only one of the most useful commands, but also, mastery of grep opens the gates to mastery of other tools such as awk , sed and perl .

So what does it do ?

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grep basically searches. More precisely,

grep foo file returns all the lines that contain a string matching the expression "foo" in the file "file".
For now, we will just think of an expression as a string. So grep returns all matching lines that contain foo as a substring.

Another way of using grep is to have it accept data through STDIN. instead of having it search a file. For example,

ls |grep blah lists all files in the current directory whose names contain the string "blah"

Compatibility Notes

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This tutorial is based on the GNU version of grep. It is recommended that you use this version. To use it, firstly, it needs to be installed on your system. Secondly, your PATH needs to be set so that GNU grep is used in preference to the standard version.

Wildcards For Grep

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The Basics: Wildcards for grep

So the first question that probably comes to mind is something like "does this grep thing support wildcards ? And the answer is better than yes. In fact saying that grep supports wildcards is a big understatement. grep uses regular expressions which go a few steps beyond wildcards. But we will start with wildcards. The canonical wildcard character is the dot "." Here is an example : 
>cat file

big
bad bug 
bag
bigger
boogy

>grep b.g file

big
bad bug 
bag
bigger
notice that boogy didn't match, since the "." matches exactly one character. To match arbitrary strings, we use the star, which works in the following way:
the expression consisting of a character followed by a star matches any number (possibly zero) of repetitions of that character. In particular, .* matches any string, and hence acts as a "wildcard".
To illustrate, we show some examples:

Examples: Wildcards

The File for These Examples

>cat file
big
bad bug 
bag
bigger
boogy

Wildcards #1

>grep "b.*g" file
big
bad bug 
bag
bigger
boogy

Wildcards #2

>grep "b.*g." file
bigger
boogy

Wildcards #3

>grep "ggg*" file
bigger

Taking it Further - Regular Expressions

Back to top The wildcards are a start, but the idea could be taken further. For example, suppose we want an expression that matches Frederic Smith or Fred Smith. In other words, the letters eric are "optional".

First, we introduce the concept of an "escaped" character.

An escaped character is a character preceded by a backslash. The preceding backslash does one of the following:
(a) removes an implied special meaning from a character (b) adds special meaning to a "non-special" character

Examples

To search for a line containing text hello.gif, the correct command is
grep 'hello\.gif' file
since grep 'hello.gif' file will match lines containing hello-gif , hello1gif , helloagif , etc.

Now we move on to grouping expressions, in order to find a way of making an expression to match Fred or Frederic

First, we start with the ? operator.
an expression consisting of a character followed by an escaped question mark matches one or zero instances of that character.

Example

bugg?y matches all of the following: bugy , buggy but not bugggy We move on to "grouping" expressions. In our example, we want to make the string "ederic" following "Fred" optional, we don't just want one optional character.
An expression surrounded by "escaped" parentheses is treated by a single character.

Examples

Fred\(eric\)\? Smith matches Fred Smith or Frederic Smith
\(abc\)* matches abc , abcabcabc etc. (i.e. , any number of repetitions of the string abc , including the empty string.) Note that we have to be careful when our expressions contain white spaces or stars. When this happens, we need to enclose them in quotes so that the shell does not mis-interpret the command, because the shell will parse whitespace-separated strings as multiple arguments, and will expand an unquoted * to a glob pattern. So to use our example above, we would need to type
grep "Fred\(eric\)\? Smith" file

We now mention several other useful operators.

More on Regular Expressions

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Matching a list of characters

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To match a selection of characters, use [].

Example

[Hh]ello matches lines containing hello or Hello
Ranges of characters are also permitted.

Example

[0-3] is the same as [0123]
[a-k] is the same as [abcdefghijk]
[A-C] is the same as [ABC]
[A-Ca-k] is the same as
[ABCabcdefghijk]
There are also some alternate forms :
[[:alpha:]] is the same as [a-zA-Z]
[[:upper:]] is the same as [A-Z]
[[:lower:]] is the same as [a-z]
[[:digit:]] is the same as [0-9]
[[:alnum:]] is the same as [0-9a-zA-Z]
[[:space:]] matches any white space including tabs

These alternate forms such as [[:digit:]] are preferable to the direct method [0-9]

The [] may be used to search for non-matches. This is done by putting a carat ^ as the first character inside the square brackets.

Example

grep "([^()]*)a" file returns any line containing a pair of parentheses that are innermost and are followed by the letter "a". So it matches these lines 
(hello)a
(aksjdhaksj d ka)a
But not this 
x=(y+2(x+1))a

Matching a Specific Number Of Repetitions of a Pattern

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Suppose you want to match a specific number of repititions of a pattern. A good example is phone numbers. You could search for a 7 digit phone number like this:
grep "[:digit:]\{3\}[ -]\?[:digit:]\{4\}" file
This matches phone numbers, possibly containing a dash or whitespace in the middle.

Nailing it Down to Start of the Line and End of the Line

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Here's the deal. Suppose you want to search for lines containing a line consisting of white space, then the word hello, then the end of the line. Let us start with an example. 
>cat file

	hello
hello world
	hhello

>grep hello file

	hello
hello world
	hhello
This is not what we wanted. So what went wrong ? The problem is that grep searches for lines containing the string "hello" , and all the lines specified contain this. To get around this problem, we introduce the end and beginning of line characters
The $ character matches the end of the line. The ^ character matches the beginning of the line.

Examples

returning to our previous example,
grep "^[[:space:]]*hello[[:space:]]*$" file
does what we want (only returns one line) Another example:
grep "^From.*mscharmi" /var/spool/mail/elflord searches my inbox for headers from a particular person. This kind of regular expression is extremely useful, and mail filters such as procmail use it all the tims.

This or That: matching one of two strings

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The expression consisting of two expressions seperated by the or operator \| matches lines containing either of those two expressions.
Note that you MUST enclose this inside single or double quotes.

Example

grep "cat\|dog" file matches lines containing the word "cat" or the word "dog"
grep "I am a \(cat\|dog\)" matches lines containing the string "I am a cat" or the string "I am a dog".

Backpedalling and Backreferences

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Suppose you want to search for a string which contains a certain substring in more than one place. An example is the heading tag in HTML. Suppose I wanted to search for <H1>some string</H1> . This is easy enough to do. But suppose I wanted to do the same but allow H2 H3 H4 H5 H6 in place of H1. The expression <H[1-6]>.*</H[1-6]> is not good enough since it matches <H1>Hello world</H3> but we want the opening tag to match the closing one. To do this, we use a backreference
The expression \n where n is a number, matches the contents of the n'th set of parentheses in the expression
Woah, this really needs an example!

Examples

>H\([1-6]\).*</H\1> matches what we were trying to match before.
"Mr \(dog\|cat\) came home to Mrs \1 and they went to visit Mr \(dog\|cat\) and Mrs \2 to discuss the meaning of life matches ... well I'm sure you can work it out. the idea is that the cats and dogs should match up in such a way that it makes sense.

Some Crucial Details: Special Characters and Quotes

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Special Characters

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Here, we outline the special characters for grep. Note that in egrep (which uses extended regular expressions), which actually are no more functional than standard regular expressions if you use GNU grep ) , the list of special characters increases ( | in grep is the same as \| egrep and vice versa, there are also other differences. Check the man page for details ) The following characters are considered special and need to be "escaped": 
?  \  .  [  ]  ^  $
Note that a $ sign loses its meaning if characters follow it (I think) and the carat ^ loses its meaning if other characters precede it.
Square brackets behave a little differently. The rules for square brackets go as follows:

Quotes

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Firstly, single quotes are the safest to use, because they protect your regular expression from the shell. For example, grep "!" file will often produce an error (since the shell thinks that "!" is referring to the shell command history) while grep '!' file will not.

When should you use single quotes ? the answer is this: if you want to use shell variables, you need double quotes. For example,

grep "$HOME" file
searches file for the name of your home directory, while
grep '$HOME' file
searches for the string $HOME

Extended Regular Expression Syntax

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We now discuss egrep syntax as opposed to grep syntax. Ironically, despite the origin of the name (extended), egrep actually has less functionality as it is designed for compatibility with the traditional egrep. A better way to do an extended "grep" is to use grep -E which uses extended regular expression syntax without loss of functionality.

grep grep -E Available for egrep?
a\+ a+ yes
a\? a? yes
expression1\|expression2 expression1|expression2? yes
\(expression\) (expression1) yes
\{m,n\} {m,n} no
\{,n\} {,n} no
\{m,} {m,} no
\{m} {m} no