The Unix Grep Command Under Linux

I love the grep command.
It is so useful!

grep can be used to find
useful information inside of
a file.

For example, if I'm looking for
a name in my address book, I might
use grep. That's one way
to do it.

Here. Let me look for my own name
as an example:

grep -i Abbott AddressBook

The above grep command looks
for every occurance of the word
Abbott in my AddressBook
file.

I've asked grep to ignore
case sensitivity. That is to say,
I want to find Abbott, which
is capitalized, or abbott, which
is uncapitalized.

The following grep option ensures
that case sensitivity is ignored:

-i

What does grep do? It prints
all lines that have Abbott in
them on my screen.

Here's the command again:

grep -i Abbott AddressBook

Here's the output:

@Abbott, Ed

That one line of output.

If I want more, I can ask
the grep command to
print more.

For example, let's say I
want 3 lines of context.

Here's how I would specifiy
3 lines of context:

grep -i -c 3 Abbott AddressBook

The above command should give
me 7 lines.

• Three lines before
• The line with Abbott in it
• Three lines after

This could be handy as my name in my
address book could immediately be
followed by my phone number.
Sometimes, context is helpful.

My examples here are somewhat
contrived. In reality, I use
the grep command for other things.

For example, I keep a journal
on my laptop computer. If I want
to look up a specific topic, such
as basketball, which may span
more than one journal, I can do it
this way:

grep -i -C 3 basketball *

This simple command will look at
every journal in the current directory
and pick out 7 lines of context for
each mention of basketball.

This is very helpful if you are trying
to find information fast.

The -C option of grep
is actually part of a family of options.

This family gives context. Here's the
family:

-A	after context
-B	before context
-C	before and after context

Note that in this family of context options,
each member of the family is mnemonic for
something.

Here's the same table expressed as a memory
aid:

-A	after
-B	before
-C	context

The context options, as I call them,
always print context lines in addition
to the line specified.

I'll give an example.

Let's say I have a file called Numbers
that consists of the following ten lines
of text:

ONE
TWO
THREE
FOUR
FIVE
SIX
SEVEN
EIGHT
NINE
TEN

OK. Now let's say I
type the following command:

grep SIX Numbers

The grep command will
go looking for the pattern
SIX in the file. When
it finds it, we get the following
input/output:

$ grep SIX Numbers
SIX
$

The final $ is just my
command line prompt being returned
to me.

Notice that grep defaults
to printing just one line. That's
important! This helps you to understand
the behavior of the context options.

All context options print lines that
are in addition to the one line that
grep defaults to.

Here's a chart of how many lines each
context option will print:

-A 3	4 lines total
-B 3	4 lines total
-C 3	7 lines total

Here's a chart that describes what each
option does:

-A 3	print the line plus 3 lines after the line
-B 3	print 3 lines before the line plus the line
-C 3	print 3 lines before the line, the line, and 3 lines after the line

Ed Abbott