The files are displayed with their name, the row and column the phrase was found and the phrase surrounded by words before and after. One of the issues that I found was that it was not possible to scroll horizontally. The only option to display all text was to increase the window size significantly.

Filters are available to narrow down the search results. It is for instance possible to search only specific file types or folders, or to exclude files and folders from the search.

Other filters include making the search case sensitive or to limit the results to full word matches. GGrep supports regular expressions just like the original grep application. This opens up several interesting options for more powerful search and replace operations. You could for instance search for euro|dollar to find documents containing either the phrase euro or dollar.

Another interesting feature of GGrep is the option to replace text. While it is possible to use simple text to text replacements, one could use regular expressions for better text matching.

Please note that the program is not displaying a list of possible matches first. When you click Replace the replacing of text happens instantly and without option to undo the changes (other than reversing search and replace strings).

The results of a search or replace can be exported into a csv file which can then be imported into programs like Excel. Windows users who are looking for a powerful search and replace tool should take a closer look at GGrep. The software is compatible with all recent 32-bit and 64-bit editions of the operating system.

In this article I am going to introduce you to the grep command and how it is used. Once you know this command, your life with the Linux command line will be made much easier.

Limitations

Although grep is a very versatile command to know, it does have its limitations. One of the biggest limitations it has is that it can not search within binary files. That’s all fine, because your typical binary file would be searchable with the application that created said binary file.

Grep also has a line limit of 2048 lines. This means if your file is beyond that, grep will stop searching at the line limit.

Installation

You are in luck. Grep is a tool that comes pre-installed with all Linux distributions. So no installation is required.

Usage

The basic usage of grep is:

grep [OPTIONS] [PATTERN] file

Their are numerous options to use with the Grep command. The more helpful of these options are:

-E Interpret the PATTERN section as an extended regular expression.
-P Interpret the PATTERN section as a Perl regular expression.
-e Use PATTERN as the pattern. If searching a single PATTERN you do not need the -e option. But this option allows you to search for multiple search patterns.
-f  Obtain patterns from a file, one per line.
-i Ignore case.
-c Suppress normal output and only output count of matching lines.
--color Display the matched strings in color.
-n Display the line number associated with the matching entry.

Examples

Let’s take a look at a very basic example first. You want to search the file /etc/test.conf for the string input. The grep command for this would be:

grep input /etc/test.conf

Now, let’s say that test.conf is a larger file and you need to know the line number the string input is on. For this the command would look like:

grep -n input /etc/test.conf

The above output would then include the line number associated with each matching entry. Now, let’s say you wanted to search the same file for both input and output and you want to know the line numbers associated with each. For this the command would look like:

grep -n -e input -e output /etc/test.conf

But what if you only want to know how many times the string input is found in the file /etc/test.conf. For this you could use the command like so:

grep -c input /etc/test.conf

Final thoughts

Grep is one of those commands that you will use time and again, in many, various ways. I always tell new users that, once you reach the point where you begin using the command line, grep is one of the first commands you should master.

But because grep is so handy, it’s hard to know where to start. Of course you can use grep in the standard way, but when you can do so much with a command, why not learn some cool tricks. And that’s what we’ll do here – learn how to use grep to really make it useful.

Basic usage

Before we get into some fun stuff, we should look at the basic command structure. To use grep in its most basic form you follow this structure:

grep STRING FILE

Where STRING is the string of characters you want to search for and FILE is the file you want to search.

Say, for instance, I want to search the file test.txt for the string “Linux” (no quotes). I can do this with the command:

grep Linux test.txt

What grep will do is print out (at the prompt) every line in the file that contains the word Linux. But what if you are searching a large configuration file and you need to find the location of a specific option. To do this you can add the “n” switch which will print out the line numbers associated with each line. Let’s look at a different example. Say you need to find out what port Dansguardian listens to. Instead of scrolling through every line of the /etc/dansguardian/dansguardian.conf file for the string “port” (no quotes), you could issue the command:

grep -n port /etc/dansguardian/dansguardian.conf

which would report back something like:

6:# Web Access Denied Reporting (does not affect logging)
10:#  1 = report why but not what denied phrase
11:#  2 = report fully
14:reportinglevel = 3
17:# The HTML template within this dir is only used when reportinglevel
87:# the port that DansGuardian listens to.
88:filterport = 8080
93:# the port DansGuardian connects to proxy on
94:proxyport = 3128
97:# dansguardian reporting script was copied. Only used in reporting levels 1 and 2.
153:# the naughtyness limit will be log

So you can see that line 88 is the filter port and 94 is the proxy port. Easy. Now let’s see what else grep can do.

Lines before and after

What if you not only want to see the single line associated with a string, but also a certain amount of lines above and below that line. You can do this using the -A and -B switches like so (we’ll stick with our dansguardian example):

grep -B1 -A2 -n port dansguardian.conf

The above command would print out not only each line containing the string “port” (no quotes) but also the 1 line above it and the next two lines below it.

Pipe other commands

You remember the dmesg command. This command prints out the kernel buffer – it’s where you learn a lot about your machine. What if you want to check out CPU information in the dmesg output. You can issue the command dmesg | less and scroll around until you find it, or you can issue the command:

dmesg | grep -n CPU

which will print out all the dmesg output that contains the string CPU and the lines numbers associated with each line.

Search directories

You can also have grep help you in your search of directories. To do this you would use it in conjunction with, say, the find command. Say you were looking for the configuration file for the nano text editor but you had no idea what it was called or where it was located. You could pipe the output of the find command to grep (as the root or sudo user) like so:

find / | grep nano | less

and you would see among the output:

/etc/nanorc
Bingo! There’s your configuration file.

Final thoughts

As you can see there are a number of ways that the grep command can be of assistance. And this is only scratching the surface. Do you have a nifty way to use grep? If so, share it with your fellow ghacks members.