How To Use grep Command In Linux / UNIX

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How do I use grep command on Linux or Apple macOS/OS X? How can I use grep command on Unix operating systems? Can you give me a simple examples of the grep command?Theis used to search text. It searches the given file for lines containing a match to the given strings or words. It is one of the most useful commands on Linux and Unix-like system. Let us see how to use grep on a Linux or Unix like system.

The name, “grep”, derives from the command used to perform a similar operation, using the Unix/Linux text editor ed:

g/re/p

The grep utilities are a family that includes grep, egrep, and fgrep for searching duties. For most uses, you need to use fgrep as it the fastest and only look into strings and words. However, typing grep is easy. Hence, it is a personal choice.

grep command examples in Linux and Unix

Below is some standard grep command explained with examples to get you started with grep on Linux, macOS, and Unix:

Search any line that contains the word in filename on Linux: grep 'word' filename Perform a case-insensitive search for the word ‘bar’ in Linux and Unix: grep -i 'bar' file1 Look for all files in the current directory and in all of its subdirectories in Linux for the word ‘httpd’ grep -R 'httpd' . Search and display the total number of times that the string ‘nixcraft’ appears in a file named frontpage.md: grep -c 'nixcraft' frontpage.md

Let us see all commands and options in details.

Syntax

The syntax is as follows:

grep 'word' filename fgrep 'word-to-search' file.txt grep 'word' file1 file2 file3 grep 'string1 string2' filename cat otherfile | grep 'something' command | grep 'something' command option1 | grep 'data' grep --color 'data' fileName grep [ -options ] pattern filename fgrep [ -options ] words file grep 'word' filename fgrep 'word-to-search' file.txt grep 'word' file1 file2 file3 grep 'string1 string2' filename cat otherfile | grep 'something' command | grep 'something' command option1 | grep 'data' grep --color 'data' fileName grep [-options] pattern filename fgrep [-options] words file

How do I use grep to search a file on Linux?

Search /etc/passwd file for boo user, enter:

grep boo /etc/passwd

Sample outputs:

foo:x:1000:1000:boo,,,:/home/boo:/bin/ksh

We can use fgrep/grep to find all the lines of a file that contain a particular word. For example, to list all the lines of a file named address.txt in the current directory that contain the word “California”, run:

fgrep California address.txt

Please note that the above command also returns lines where “California” is part of other words, such as “Californication” or “Californian”. Hence pass the -w option with the grep/fgrep command to get only lines where “California” is included as a whole word:

fgrep -w California address.txt

You can force grep to ignore word case i.e match boo, Boo, BOO and all other combination with the -i option. For instance, type the following command:

grep -i "boo" /etc/passwd



The last grep -i "boo" /etc/passwd can run as follows using the cat command too:

cat /etc/passwd | grep -i "boo"

How to use grep recursively

You can search recursively i.e. read all files under each directory for a string “192.168.1.5”

$ grep -r "192.168.1.5" /etc/

OR

$ grep -R "192.168.1.5" /etc/

Sample outputs:

/etc/ppp/options:# ms-wins 192.168.1.50 /etc/ppp/options:# ms-wins 192.168.1.51 /etc/NetworkManager/system-connections/Wired connection 1:addresses1=192.168.1.5;24;192.168.1.2;

You will see result for 192.168.1.5 on a separate line preceded by the name of the file (such as /etc/ppp/options) in which it was found. The inclusion of the file names in the output data can be suppressed by using the -h option as follows:

$ grep -h -R "192.168.1.5" /etc/

OR

$ grep -hR "192.168.1.5" /etc/

Sample outputs:

# ms-wins 192.168.1.50 # ms-wins 192.168.1.51 addresses1=192.168.1.5;24;192.168.1.2;

How to use grep to search words only

When you search for boo, grep will match fooboo, boo123, barfoo35 and more. You can force the grep command to select only those lines containing matches that form whole words i.e. match only boo word:

$ grep -w "boo" file

How to use grep to search 2 different words

Use the egrep command as follows:

$ egrep -w 'word1|word2' /path/to/file

Ignore case

We can force grep to ignore case distinctions in patterns and data. For example, when I search for ‘bar’, match ‘BAR’, ‘Bar’, ‘BaR’ and so on:

$ grep -i 'bar' /path/to/file

In this example, I am going to include all subdirectories in a search:

$ grep -r -i 'main' ~/projects/

How can I count line when words has been matched

The grep can report the number of times that the pattern has been matched for each file using -c (count) option:

$ grep -c 'word' /path/to/file

Pass the -n option to precede each line of output with the number of the line in the text file from which it was obtained:

$ grep -n 'root' /etc/passwd

1:root:x:0:0:root:/root:/bin/bash 1042:rootdoor:x:0:0:rootdoor:/home/rootdoor:/bin/csh 3319:initrootapp:x:0:0:initrootapp:/home/initroot:/bin/ksh

Force grep invert match

You can use -v option to print inverts the match; that is, it matches only those lines that do not contain the given word. For example print all line that do not contain the word bar:

$ grep -v bar /path/to/file

$ grep -v '^root' /etc/passwd

Display lines before and after the match

Want to see the lines before your matches? Try passing the -B to the grep:

grep -B NUM "word" file

grep -B 3 "foo" file1

Similarly, display the lines after your matches by passing the -A to the grep:

grep -A NUM "string" /pth/to/file

grep -A 4 "dropped" /var/log/ufw.log

We can combine those two options to get most meaningful outputs:

grep -C 4 -B 5 -A 6 --color 'error-code' /var/log/httpd/access_log

Here is a sample shell script that fetches the Linux kernel download urls:

....... ... _out = "/tmp/out.$$" curl -s https: // www.kernel.org / > " $_out " ####################### ## grep -A used here ## ####################### url = " $(grep -A 2 '<td id="latest_button">' ${_out} | grep -Eo '(http|https) ://[^/" ] +. * xz ')" gpgurl="${url/tar.xz/tar.sign}" notify-send "A new kernel version ($remote) has been released." echo "* Downloading the Linux kernel (new version) ..." wget -qc "$url" -O "${dldir}/${file}" wget -qc "$gpgurl" -O "${dldir}/${gpgurl##*/}" ..... .. ....... ... _out="/tmp/out.$$" curl -s https://www.kernel.org/ > "$_out" ####################### ## grep -A used here ## ####################### url="$(grep -A 2 '<td id="latest_button">' ${_out} | grep -Eo '(http|https)://[^/"]+.*xz')" gpgurl="${url/tar.xz/tar.sign}" notify-send "A new kernel version ($remote) has been released." echo "* Downloading the Linux kernel (new version) ..." wget -qc "$url" -O "${dldir}/${file}" wget -qc "$gpgurl" -O "${dldir}/${gpgurl##*/}" ..... ..

UNIX / Linux pipes

grep command often used with shell pipes. In this example, show the name of the hard disk devices:

# dmesg | egrep '(s|h)d[a-z]'

Display cpu model name:

# cat /proc/cpuinfo | grep -i 'Model'

However, above command can be also used as follows without shell pipe:

# grep -i 'Model' /proc/cpuinfo

model : 30 model name : Intel ( R ) Core ( TM ) i7 CPU Q 820 @ 1.73GHz model : 30 model name : Intel ( R ) Core ( TM ) i7 CPU Q 820 @ 1.73GHz model : 30 model name : Intel(R) Core(TM) i7 CPU Q 820 @ 1.73GHz model : 30 model name : Intel(R) Core(TM) i7 CPU Q 820 @ 1.73GHz

One of my favorite usage of grep or egrep command to filter the output of the yum command/dpkg command/apt command/apt-get command:

dpkg --list | grep linux-image

yum search php | grep gd

apt search maria | egrep 'server|client'



How do I list just the names of matching files?

Use the -l option to list file name whose contents mention main():

$ grep -l 'main' *.c

OR

$ grep -Rl 'main' /path/to/project/dir/

Colors option

Finally, we can force grep to display output in colors, enter:

$ grep --color vivek /etc/passwd



--color

GREP_COLOR

grep --color=always

GREP_COLOR='1;35' grep --color=always 'vivek' /etc/passwd

GREP_COLOR='1;32' grep --color=always 'vivek' /etc/passwd

GREP_COLOR='1;37' grep --color=always 'root' /etc/passwd

GREP_COLOR='1;36' grep --color=always nobody /etc/passwd

Conclusion

In conclusion, theoption increase readability. For example, theenvironment variable and thecan be used as follows:In addition, to default red color now we can define colors using GREP_COLOR shell variable. The differnt color helps us massivly with visual grepping.

The grep command is a very versatile and many new Linux or Unix users find it complicated. Hence, I suggest you read the grep man page too. Let us summarize most import options:

Linux grep command options Description -i Ignore case distinctions on Linux and Unix -w Force PATTERN to match only whole words -v Select non-matching lines -n Print line number with output lines -h Suppress the Unix file name prefix on output -r Search directories recursivly on Linux -R Just like -r but follow all symlinks -l Print only names of FILEs with selected lines -c Print only a count of selected lines per FILE --color Display matched pattern in colors

If you enjoyed the grep tutorial, then you might like to read our “Regular Expressions in Grep” tutorial.



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