linux-grep commond
http://www.panix.com/~elflord/unix/grep.html
Why grep ?
Back to topgrep 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 ?
Back to topgrep 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
Back to topThis 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
Back to topThe Basics: Wildcards for grep
The Wildcard Character
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
The repetition character
To match repetitions of a character, 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, the expression ".*" matches any string, and hence acts as a "wildcard".To illustrate, we show some examples:
Examples: Wildcards
The File for These Examples |
Wildcards #1 |
Wildcards #2 |
repetition |
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 matchesFrederic 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 texthello.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
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
Back to topMatching a list of characters
Back to topTo match a selection of characters, use [].
Example
Ranges of characters are also permitted.[Hh]ellomatches lines containinghelloorHello
Example
There are also some alternate forms :[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]
[[: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]
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)aBut not this
x=(y+2(x+1))a
Matching a Specific Number Of Repetitions of a Pattern
Back to topSuppose you want to match a specific number of repetitions 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
Back to topHere'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.
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>cat file hello hello world hhello >grep hello file hello hello world hhello
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
Back to top
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
Back to topSuppose 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 expressionWoah, 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
Back to topSpecial Characters
Back to topHere, 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:
- A closing square bracket loses its special meaning if placed first in a list. for example
[]12]matches ] , 1, or 2. - A dash - loses it's usual meaning inside lists if it is placed last.
- A carat ^ loses it's special meaning if it is not placed first
- Most special characters lose their meaning inside square brackets
Quotes
Back to topFirstly, 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
Back to topWe 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 |