Sunday, June 1, 2008

Using grep

New Linux users unfamiliar with this standard Unix tool may not
realize how useful it is. In this tutorial for the novice user, Eric
demonstrates grep techniques.

When I first started working in systems integration, I was primarily a
PC support person. I spent a lot of time installing and supporting
Windows applications in various PC LAN configurations, running various
versions (and vendors) of TCP/IP transports. Since then, I have
successfully ditched DOS and moved on. Now, after working on various
versions of Unix for a few years, I porting some of our networking and
data manipulation libraries to other platforms and environments, such
as the AS/400 minicomputer and the Macintosh. This ongoing experience
has given me a chance to appreciate just how powerful the tools we
take for granted with Linux really are.

Searching for a word (or any other value) in a group of files is a
very common task. Whether it's searching for a function in a group of
source code modules, trying to find a parameter in a set of
configuration files, or simply looking for a misplaced e-mail message,
text searching and matching operations are common in all environments.

Unfortunately, this common task doesn't have an easy solution on all
platforms. On most, the best solution available is to use the search
function in an editor. But when it comes to Linux (and other Unix
descendants), you have many solutions. One of them is grep.

grep is an acronym for "global regular expression print," a reference
to the command in the old ed line editor that prints all of the lines
in a file containing a specified sequence of characters. grep does
exactly that: it prints out lines in a file that contain a match for a
regular expression. We'll gradually delve into what a regular
expression is as we go on.
Starting Out

First, let's look at a quick example. We will search for a word in the
Configure script provided with Linux for setting up the Linux kernel
source, which is usually installed in the /usr/src/linux directory.
Change to that directory and type (the $ character is the prompt,
don't type it):

$ grep glob Configure

You should see:

# Disable filename globbing once and for all.

glob is in bold to illustrate what grep matched. grep does not
actually print matches in bold.

grep looked for the sequence of characters glob and printed the line
of the Configure file with that sequence. It did not look for the word
glob. It looked for g followed by l followed by o followed by b. This
points out one important aspect of regular expressions: they match
sequences of characters, not words.

Before we dig any deeper into the specifics of pattern matching, let's
look at grep's "user interface" with a few examples. Try the following
two commands:

$ grep glob < Configure
$ cat Configure | grep glob

both of these two commands should print

# Disable filename globbing once and for all.

which probably looks familiar.

In all of these commands, we have specified the regular expression as
the first argument to grep. With the exception of any command line
switches, grep always expects the regular expression as the first
argument.

However, we presented grep with three different situations and
received the same response. In the first exercise, we provided grep
with the name of a file, and it opened that file and searched it. grep
can also take a list of filenames to search.

In the other two exercises we illustrated a feature that grep shares
with many other utilities. If no files are specified on the command
line, grep reads standard input. To further illustrate standard input
let's try one more example:

$ grep foo

When you run that, grep appears to "hang" waiting for something. It
is. It's waiting for input. Type:

tttt

and press return. Nothing happens. Now type:

foobar

and press enter. This time, grep sees the string foo in foobar and
echos the line foobar back at you, which is why foobar appears twice.
Now type ctrl-d, the "end-of-file" character, to tell grep that it has
reached the end of the file, whereupon it exits.

You just gave grep an input file that consisted of tttt, a newline
character, foobar, a newline character, and the end-of-file character.

Piping input into grep from standard input also has another frequent
use: filtering the output of other commands. Sometimes cutting out the
unnecessary lines with grep is more convenient than reading output
page by page with more or less:

$ ps ax | grep cron

efficiently gives you the process information for crond.
Special Characters

Many Unix utilities use regular expressions to specify patterns.
Before we go into actual examples of regular expressions, let's define
a few terms and explain a few conventions that I will use in the
exercises.

*

Character any printable symbol, such as a letter, number, or
punctuation mark.
*

String a sequence of characters, such as cat or segment
(sometimes referred to as a literal).
*

Expression also a sequence of characters. The difference between
a string and an expression is that while strings are to be taken
literally, expressions must be evaluated before their actual value can
be determined. (The manual page for GNU grep compares regular
expressions to mathematical expressions.) An expression usually can
stand for more than one thing, for example the regular expression
th[ae]n can stand for then or than. Also, the shell has its own type
of expression, called globbing, which is usually used to specify file
names. For example, *.c matches any file ending in the characters .c.
*

Metacharacters the characters whose presence turns a string into
an expression. Metacharacters can be thought of as the operators that
determine how expressions are evaluated. This will become more clear
as we work through the examples below.

Interference

You have probably entered a shell command like

$ ls -l *.c

at some time. The shell "knows" that it is supposed to replace *.c
with a list of all the files in the current directory whose names end
in the characters .c.

This gets in the way if we want to pass a literal * (or ?, |, $, etc.)
character to grep. Enclosing the regular expression in `single quotes'
will prevent the shell from evaluating any of the shell's
metacharacters. When in doubt, enclose your regular expression in
single quotes.
Basic Searches

The most basic regular expression is simply a string. Therefore a
string such as foo is a regular expression that has only one match:
foo.

We'll continue our examples with another file in the same directory,
so make sure you are still in the /usr/src/linux directory:

$ grep Linus CREDITS

Linus
N: Linus Torvalds
E: Linus.Torvalds@Helsinki.FI
D: Personal information about Linus

This quite naturally gives the four lines that have Linus Torvalds'
name in them.

As I said earlier, the Unix shells have different metacharacters, and
use different kinds of expressions. The metacharacters . and * cause
the most confusion for people learning regular expression syntax after
they have been using shells (and DOS, for that matter).

In regular expressions, the character . acts very much like the ? at
the shell prompt: it matches any single character. The *, by contrast,
has quite a different meaning: it matches zero or more instances of
the previous character.

If we type

$ grep tha. CREDITS

we get this (partial listing only):

S: Northampton
E: Hein@Informatik.TU-Clausthal.de

As you can see, grep printed every instance of tha followed by any
character. Now try

$ grep 'tha*' CREDITS
S: Northampton
D: Author of serial driver
D: Author of the new e2fsck
D: Author of loopback device driver

We received a much larger response with "*". Since "*" matches zero or
more instances of the previous character (in this case the letter
"a"), we greatly increase our possibility of a match because we made
th a legal match!
Character Classes

One of the most powerful constructs available in regular expression
syntax is the character class. A character class specifies a range or
set of characters to be matched. The characters in a class are
delineated by the [ and ] symbols. The class [a-z] matches the
lowercase letters a through z, the class [a-zA-Z] matches all letters,
uppercase or lowercase, and [Lh] would match upper case L or lower
case h.

$ grep 'sm[ai]' CREDITS
E: csmith@convex.com
D: Author of several small utilities

since our expression matches sma or smi. The command

$ grep '[a-z]' CREDITS

gives us most of the file. If you look at the file closely, you'll see
that a few lines have no lowercase letters; these are the only lines
that grep does not print.

Now since we can match a set of characters, why not exclude them
instead? The circumflex, ^, when included as the first member of a
character class, matches any character except the characters specified
in the class.

$ grep Sm CREDITS

gives us three lines:

D: Small patches for kernel, libc
D: Smail binary packages for Slackware and Debian
N: Chris Smith
$ grep 'Sm[^i]' CREDITS

gives us two

D: Small patches for kernel, libc
D: Smail binary packages for Slackware and Debian

because we excluded i as a possible letter to follow Sm.

To search for a class of characters including a literal ^ character,
don't place it first in the class. To search for a class including a
literal -, place it the very last character of the class. To search
for a class including the literal character ], place it the first
character of the class.

Often it is convenient to base searches on the position of the
characters on a line. The ^ character matches the beginning of a line
(outside of a character class, of course) and the $ matches the end.
(Users of vi may recognize these metacharacters as commands.) Earlier,
searching for Linus gave us four lines. Let's change that to:

grep 'Linus$' CREDITS

which gives us

Linus
D: Personal information about Linus

two lines, since we specified that Linus must be the last five
characters of the line. Similarly,

grep - CREDITS

produces 99 lines, while

grep '^-' CREDITS

produces only one line:

----------

In some circumstances you may need to match a metacharacter. Inside a
character class set all characters are taken as literals (except ^, -,
and ], as shown above). However, outside of classes we need a way to
turn a metacharacter into a literal character to match.
Matching Metacharacters

For this purpose the special metacharacter \ is used to escape
metacharacters. Escaped metacharacters are interpreted literally, not
as a component of an expression. Therefore \[ would match any sequence
with a [ in it:

$ grep '[' CREDITS

produces an error message:

grep: Unmatched [ or [^

but

$ grep '\[' CREDITS

produces two lines:

E: hennus@sky.ow.nl [My uucp-fed Linux box at home]
D: The XFree86[tm] Project

If you need to search for a \ character, escape it just like any other
metacharacter: \\
Options

As you can see, with just its support of regular expression syntax,
grep provides us with some very powerful capabilities. Its command
line options add even more power.

Sometimes you are looking for a string, but don't know whether it is
upper, lower, or mixed case. For this situation grep offers the -i
switch. With this option, case is completely ignored:

$ grep -i lINuS CREDITS
Linus
N: Linus Torvalds
E: Linus.Torvalds@Helsinki.FI
D: Personal information about Linus

The -v option causes grep to print all lines that do not contain the
specified regular expression:

$ grep -v '^#" /etc/syslog.conf | grep -v '^$'

prints all the lines from /etc/syslog.com that are neither commented
(starting with #) nor empty (^$). This prints six lines on my system,
although my syslog.conf file really has 21 lines.

If you need to know how many lines match, pass grep the -c option.
This will output the number of matching lines (not the number of
matches; two matches in one line count as one) without printing the
lines that match:

$ grep -c Linux CREDITS
33

If you are searching for filenames that contain a given string,
instead of the actual lines that contain it, use grep's -l switch:

$ grep -l Linux *
CREDITS
README
README.modules

grep also notifies us, for each subdirectory, that it can't search
through a directory. This is normal and will happen whenever you use a
wildcard that happens to include directory names as well as file
names.

The opposite of -l is -L. This option will cause grep to return the
names of files that do not contain the specified pattern.

If you are searching for a word and want to suppress matches that are
partial words use the -w option. Without the -w option,

$ grep -c a README

tells us that it matched 146 lines, but

$ grep -wc a README

returns only 35 since we matched only the word a, not every line with
the character a.

Two more useful options:

$ grep -b Linus CREDITS
301: Linus
17446:N: Linus Torvalds
17464:E: Linus.Torvalds@Helsinki.FI
20561:D: Personal information about Linus
$ grep -n Linus CREDITS
7: Linus
793:N: Linus Torvalds
794:E: Linus.Torvalds@Helsinki.FI
924:D: Personal information about Linus

The -b option causes grep to print the byte offset (how many bytes the
match is from the beginning of the file) of each match before the
corresponding line of output. The -n switch gives the line number.
Another grep

GNU also provides egrep (enhanced grep). The regular expression syntax
supported by GNU egrep adds a few other metacharacters:

*

? Like *, except that it matches zero or one instances instead
of zero or more.
*

+ the preceding character is matched one or more times.
*

| separates regular expressions by ORing them together.

$ egrep -i 'linux|linus' CREDITS

outputs any line that contains linus or linux.

To allow for legibility, parentheses "(" and ")" can be used in
conjunction with "|" to separate and group expressions.
More Than Just grep

This covers many of the features provided by grep. If you look at the
manual page, which I strongly recommend, you will see that I did leave
out some command-line options, such as different ways to format grep's
output and a method for searching for strings without employing
regular expressions.

Learning how to use these powerful tools provides Linux users with two
very valuable advantages. The first (and most immediate) of these is a
time-saving way to process files and output from other commands.

The second is familiarity with regular expressions. Regular
expressions are used throughout the Unix world in tools such as find
and sed and languages such as awk, perl and Tcl. Learning this syntax
prepares you to use some of the most powerful computing tools
available.

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