How to use grep command in Linux and Unix

The grep command is one of the most essential commands in Unix/Linux ecosystem, It is used to extract match and find the occurrence of string literals and patterns among the text files.

In this section we will focus on the important scenarios and provide the realtime explanation about the Various Grep command options offered in linux command line.

We now have a text file with Following content.

Below is the content of the file called intro.txt.

[vamshi@linuxcent grep]$ cat intro.txt
the first line is in lower case
THE SECOND LINE IS IN UPPER CASE


There following are Most popular Linux server Distributions:
1) Redhat Enterprise Linux
2) Ubuntu
3) Centos
4) Debian
5) SUSE Linux Enterprise Server
This is a Demonstration of Linux grep command

Let’s start with search of a string pattern in the given file

The grep command sample search syntax is

$ grep “string pattern” <filename>
$ cat < filename | * > | grep "string pattern"

Lets see the Demonstration

[vamshi@linuxcent grep]$ grep "Linux" intro.txt
There following are Most popular Linux server Distributions:
1) Redhat Enterprise Linux
5) SUSE Linux Enterprise Server
This is a Demonstration of Linux grep command

Searching for a pattern on multiple files using grep command

Lets see the Demonstration

[vamshi@linuxcent grep]$ grep "Linux" intro.txt intro-demo.txt
intro.txt:There following are Most popular Linux server Distributions:
intro.txt:1) Redhat Enterprise Linux
intro.txt:5) SUSE Linux Enterprise Server
intro.txt:This is a Demonstration of Linux grep command
intro-demo.txt:There following are Most popular Linux server Distributions:
intro-demo.txt:1) Redhat Enterprise Linux
intro-demo.txt:5) SUSE Linux Enterprise Server
intro-demo.txt:This is a Demonstration of Linux grep command

 

How to print the sequence number of matched pattern in file using Grep command in Linux?

Print the sequence number of lines that contain the matched string pattern can be achieved using -n Option:

[vamshi@linuxcent grep]$ grep -n "Linux" intro.txt
4:There following are Most popular Linux server Distributions:
5:1) Redhat Enterprise Linux
9:5) SUSE Linux Enterprise Server
11:This is a Demonstration of Linux grep command

The -b Option prints the subsequent starting offset index of the matching word from the lines within the given filename.

[vamshi@linuxcent grep]$ grep -b "Linux" intro.txt
66:There following are Most popular Linux server Distributions:
127:1) Redhat Enterprise Linux
184:5) SUSE Linux Enterprise Server
217:This is a Demonstration of Linux grep command

So far we have seen only the matching string pattern, but if you want to find the occurance of particular word then use grep -w Option

Lets us see the Demonstration below:

[vamshi@linuxcent grep]$ grep -w "Cent" intro.txt


And As expected here we have not seen any occurrence of the word Cent from the file although the word Centos was present, But it didnt match because we used -w option to match the word Cent.

But when we grep for the Word Linux then we can see the exact occurrence of the word and the matching lines are printed.

[vamshi@linuxcent grep]$ grep -w "Linux" intro.txt
There following are Most popular Linux server Distributions:
1) Redhat Enterprise Linux
5) SUSE Linux Enterprise Server
This is a Demonstration of Linux grep command

This can be used in conjunction with -o prints only completely matched word string pattern.

[vamshi@linuxcent grep]$ grep -w -o "Linux" intro.txt
Linux
Linux
Linux
Linux

How to use the Invert Option in Linux grep command

The -v option prints all the lines not matching the given string pattern.

[vamshi@linuxcent grep]$ grep -v "Linux" intro.txt
the first line is in lower case
THE SECOND LINE IS IN UPPER CASE


2) Ubuntu
3) Centos
4) Debian

 

How to print the names of all the files matching the string pattern using Linux Grep command.

Using the -l Option of grep command will print all the file names containing the matching string pattern/string literal

[vamshi@node02 grep]$ grep -l Linux *
intro-demo.txt
intro.txt

How to get the number of Count Matches using Grep command ?

In Linux Grep command we can use the Option -c to get the total occurance Count of the matching pattern/string

Lets see the Demonstration below

[vamshi@linuxcent grep]$ grep -c "Linux" intro.txt
4

How to Exclude specific Directories while searching with grep command ?

The --exclude-dir="Some Directory name" options skips the contents of the mention directory Lets see the Demonstration:

$ grep -w -E "data" * -R --exclude-dir="backup" --exclude-dir="adb-fastboot" --exclude-dir="IntelliJ-IDEA"

How to use Linux Grep commmand to ignore the case  OR perform Case Insensitive search ?

Linux Grep Command offers the -i to skip the case and perform the search.

[vamshi@linuxcent grep]$ grep -i Cent intro.txt
3)Centos

Lets search for the Work Upper from the file with -i option

[vamshi@linuxcent grep]$ grep Upper -i intro-demo.txt
THE SECOND LINE IS IN UPPER CASE

How to Grep for the lines before and After the pattern occurrence in a file

We can use the below options in Linux Grep command to extract the N number of lines matching the containing pattern before/After from the given file.. Lets look at the Options in detail

Generic Syntax:

grep -<A|B|C> “string pattern” filename*

Lets see the Demonstration as follows:

-A: Prints N number of lines After the pattern match and the line containing the match

[vamshi@linuxcent grep]$  grep -A2 docker /etc/passwd
dockerroot:x:996:992:Docker User:/var/lib/docker:/sbin/nologin
mysql:x:27:27:MariaDB Server:/var/lib/mysql:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin

-B: Prints N number of lines Before the pattern match and the line containing the match

[vamshi@linuxcent grep]$  grep -B2 docker /etc/passwd
apache:x:48:48:Apache:/usr/share/httpd:/sbin/nologin
builduser1:x:1002:1002::/home/builduser1:/bin/bash
dockerroot:x:996:992:Docker User:/var/lib/docker:/sbin/nologin

-C: Prints the line Containing the pattern and the N number of lines Before and After

[vamshi@linuxcent grep]$ grep -C2 docker /etc/passwd
apache:x:48:48:Apache:/usr/share/httpd:/sbin/nologin
builduser1:x:1002:1002::/home/builduser1:/bin/bash
dockerroot:x:996:992:Docker User:/var/lib/docker:/sbin/nologin
mysql:x:27:27:MariaDB Server:/var/lib/mysql:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin

Linux Grep command using the Regular Expression (regex) search pattern

How to use the Special Meta-characters in Grep Command

The Special Meta-Character ^ (caret) symbol to match expression at the start of a line.
Practical examples of grep command using meta-characters are as follows:

[vamshi@linuxcent grep]$ grep ^root /etc/services
rootd           1094/tcp                # ROOTD
rootd           1094/udp                # ROOTD

$ (dollar) symbol to match expression at the end of a line

[vamshi@linuxcent grep]$ grep "bash$" /etc/passwd
root:x:0:0:root:/root:/bin/bash
vagrant:x:1000:1000:vagrant:/home/vagrant:/bin/bash
vamshi:x:1001:1001::/home/vamshi:/bin/bash
jenkins:x:997:994:Jenkins Automation Server:/var/lib/jenkins:/bin/bash
builduser1:x:1002:1002::/home/builduser1:/bin/bash
linuxcent:x:1003:1004::/home/linuxcent:/bin/bash

Another Bonus regex for evading Empty lines from a text file using the Meta-characters ^ Carat and $ Dollar Symbols.

[vamshi@node02 grep]$ grep -Ev "^$" intro.txt
the first line is in lower case
THE SECOND LINE IS IN UPPER CASE
There following are Most popular Linux server Distributions:
1) Redhat Enterprise Linux
2) Ubuntu
3) Centos
4) Debian
5) SUSE Linux Enterprise Server
This is a Demonstration of Linux grep command

As the result of the above command observed Empty lines in our text file are now excluded with the inverted matching of Empty line pattern expression.

Digit Matching operation using Linux Grep Command

The Grep command Option -P offers the perl-regex pattern matching to perform some of the tricky pattern matching conditions, Lets see the Demonstration.

[vamshi@linuxcent grep]$ grep -P "[\W][\d][\d]/" /etc/services

Now we Demonstrate Numerical pattern matching in the following ways to match the exact occurrence of IPv4 address

[vamshi@linuxcent grep]$ ip a |grep -E "[0-9].[0-9].[0-9].[0-9]"
inet 127.0.0.1/8 scope host lo
inet 10.100.0.20/24 brd 10.100.0.255 scope global noprefixroute eth1

The similar result can be extracted using -P, Demonstrated as below:

[vamshi@linuxcent grep]$ ip a  | grep -P "[\d].[\d].[\d].[\d]"
inet 127.0.0.1/8 scope host lo
inet 10.100.0.20/24 brd 10.100.0.255 scope global noprefixroute eth1

Highlighting the Search patterns with Color codes

If you want to highlight the search pattern from the output we can make use of Color code Option is grep

[vamshi@linuxcent ~]$ ps -ef |grep java --color=yes

 

grep command and the Meta-characters with practical examples

In the Extended Regular Expression the metacharacters ?, \+, \{,  \|, \(, and \) enhances and takes the search string to manifest into a rich pattern

The square brackets [/code][ ][/code] includes a list of characters and it matches a single character in the list. Within the bracket, we can specify a  range, like [a-z] [0-9] [abcd], But it only matches a single character in the give list range.
The brackets can contain the carat ^ or the $ symbols and can be globbed with the wildcard characters for finding the repetitive pattern.

Here we shall see the demonstration of a few metacharacters conjunction into a single grep command

Using the Infix Operator | with the meta-characters

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

A quick practical look at the dmesg to find about the errors and warnings using grep command.

$ dmesg -H | grep -Ei “error|warn”

How to find the exact work match for the word error or err with a blank white space at the start of the word from dmesg?
$ dmesg -H | grep -Ei “(er)[r]{1,2}(or)”
$ dmesg -H | grep -Ei “(\We)[r]+(or)|(\Werr)”

Explanation: The grep search metacharacter regular expression matches the occurring of the string literals “error” with the minimum 1 time occurrence (error) of literal r and maximum 2 times.

The output only prints lines containing the word “error“ in a case insensitive grep search.

 

Another great practical example here we apply the same logic to enhance the metacharacter regex pattern to search for the occurrence of error with but without a trailing colon “:” symbol

dmesg -H | grep -Ei “(error)[:]{0}”

 

Linux Copy File Command for Files and Directories – cp Command Examples

Linux copy files command: cp is generally used for organizing the data on the Linux operating system, It copies the files and directories.

We shall take a deeper look at Linux cp command-utility in the section

In order to copy files and directories, you must have read permissions on the source file(s) and write permissions on the destination directory

How do I copy files under Linux operating systems?

How do I make a 2nd copy of a file on a Linux bash shell?

How can I copies files and directories on a Linux

Linux Copy File command Syntax

cp sourcefile destinationfile
cp sourcefile DESTDIR
cp sourcefile1 sourcefile2 DESTDIR
cp [OPTION] SOURCE DESTFILE
cp [OPTION] SOURCE DESTDIR

How to Copy a Directory if the destination does not exist?

To achieve this we can make use of the following cp command options -R or -r: Copy directories recursively.

Linux cp command Syntax with -R option:

cp -R SOURCE DESTINATION

If the destination doesn’t exist, it will be created.

It can also be used to Copy the contents Recursively

Lets see the demonstration as follows:

[vamshi@linuxcent ]$ cp -R dir1/ dir1-copy
[vamshi@linuxcent ]$ ls -l 
total 0
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:37 dir1-Recursive

Using the verbose Option -v to print the copy activity information onto the output screen.

Let’s use the -v flag to print the verbose information onto the screen.

How to Preserve the Source file and Directory permission?

Linux Copy command Syntax with -p option:

-p option preserves the mode, ownership and timestamps from the source to the destination

cp -p file1 file1-copy

Lets us see the Demonstration as Below

[vamshi@node02 cp-command]$ cp -Rp dir1/ dir1-copy
[vamshi@node02 cp-command]$ ls -ld dir1*
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:35 dir1-copy
drwxrwxr-x. 2 vamshi vamshi 6 Apr 11 06:37 dir1-Recursive

From the out we can conclude the the Linux copy command with -p Option preserves the original timestamps information and copies it to the destination

Linux cp command with Force copy -f Option, It forcefully overwrites the destination content
Sample Syntax:

cp -f file1 file1-copy

How to Copy Multiple files at once ?

Asterisk / wildcard (*) character is used to copy files multiple files with same pattern.

[vamshi@linuxcent ]$ cp -varpf file* DEST/
‘file10.txt’ -> ‘DEST/file10.txt’
‘file1.txt’ -> ‘DEST/file1.txt’
‘file2.txt’ -> ‘DEST/file2.txt’
‘file3.txt’ -> ‘DEST/file3.txt’
‘file4.txt’ -> ‘DEST/file4.txt’
‘file5.txt’ -> ‘DEST/file5.txt’
‘file6.txt’ -> ‘DEST/file6.txt’
‘file7.txt’ -> ‘DEST/file7.txt’
‘file8.txt’ -> ‘DEST/file8.txt’
‘file9.txt’ -> ‘DEST/file9.txt’

The options -p or -d enables preserving the links and can be used in conjunction with -R option to copy contents Recursively from the source directory.

How to Copy Files and Folders on Linux Using the cp Command recursively to Destination Directory

How to preserve the links with cp command?

Using the Options -p preserves the links and -r Option copies the content recussively same as -R Option and -v prints the verbose information

[vamshi@node02 Linux-blog]$ cp -varpf Redhat-Distro/ /tmp/DEST
‘Redhat-Distro/’ -> ‘/tmp/DEST’
‘Redhat-Distro/Fedora’ -> ‘/tmp/DEST/Fedora’
‘Redhat-Distro/Fedora/fedora.txt’ -> ‘/tmp/DEST/Fedora/fedora.txt’
‘Redhat-Distro/Centos’ -> ‘/tmp/DEST/Centos’
‘Redhat-Distro/Centos/centos.txt’ -> ‘/tmp/DEST/Centos/centos.txt’
‘Redhat-Distro/Centos/CentOS-versions’ -> ‘/tmp/DEST/Centos/CentOS-versions’
‘Redhat-Distro/Centos/CentOS-versions/centos7.txt’ -> ‘/tmp/DEST/Centos/CentOS-versions/centos7.txt’
‘Redhat-Distro/Centos/CentOS-versions/centos6.1.txt’ -> ‘/tmp/DEST/Centos/CentOS-versions/centos6.1.txt’
‘Redhat-Distro/Centos/README-CentOS’ -> ‘/tmp/DEST/Centos/README-CentOS’
‘Redhat-Distro/README-Redhat-Distro’ -> ‘/tmp/DEST/README-Redhat-Distro’
‘Redhat-Distro/RHEL-Versions’ -> ‘/tmp/DEST/RHEL-Versions’
‘Redhat-Distro/RHEL-Versions/redhat5.txt’ -> ‘/tmp/DEST/RHEL-Versions/redhat5.txt’
‘Redhat-Distro/RHEL-Versions/redhat8.txt’ -> ‘/tmp/DEST/RHEL-Versions/redhat8.txt’
‘Redhat-Distro/redhat.txt’ -> ‘/tmp/DEST/redhat.txt’

How to make a symbolic link with Linux cp command to files ?

As we know that ln command us useful to create symboic links, But the Linux copy command Syntax can do that to files with -s Option which creates Symbolic links:

cp -s SOURCE DESTINATION

Linux copy command Syntax with Softlink with Demonstration:

[vamshi@linuxcent ~]$ ls -l total 0
-rw-rw-r--. 1 vamshi vamshi 0 Apr 11 06:39 file1.txt

lrwxrwxrwx. 1 vamshi vamshi 9 Apr 11 06:39 file2.txt -> file1.txt

Linux cp command with interactive prompt using -i option

Sample Syntax:

cp -i file1 file1-copy

Also you can make it a best practice to setup alias alias for cp command.
The best practice is enable options -av

cp -av SOURCE DESTINATION
export cp="cp -av"

How can i copy the hidden files ?

To Copy the hidden files we can use cp command with -a option,lets us see in a practical example.

$ cp -av source/ destination/
‘source/.config1’ -> ‘destination/source/.config1’
‘source/.config2’ -> ‘destination/source/.config2’
‘source/.config3’ -> ‘destination/source/.config3’

Generally the hidden files in Linux are prefixed with a dot . So we can also use the wildcard character *, and copy them, below is another pracctical example

[vamshi@linuxcent cp-command]$ cp -av source/.conf* destination/
‘source/.config1’ -> ‘destination/.config1’
‘source/.config2’ -> ‘destination/.config2’
‘source/.config3’ -> ‘destination/.config3’

How to Copy a File from One Location to Another With a Different Name on Linux Using the cp Command

Assuming we have a couple of users on our linux server called Alice and Bob

[alice@linuxcent ~]$ sudo cp -avrpf /home/alice/djangoproject1/ /home/bob/
‘djangoproject1/’ -> ‘/home/bob/djangoproject1’
‘djangoproject1/__init__.py’ -> ‘/home/bob/djangoproject1/__init__.py’
‘djangoproject1/asgi.py’ -> ‘/home/bob/djangoproject1/asgi.py’
‘djangoproject1/settings.py’ -> ‘/home/bob/djangoproject1/settings.py’
‘djangoproject1/urls.py’ -> ‘/home/bob/djangoproject1/urls.py’
‘djangoproject1/wsgi.py’ -> ‘/home/bob/djangoproject1/wsgi.py’
‘djangoproject1/__pycache__’ -> ‘/home/bob/djangoproject1/__pycache__’
‘djangoproject1/__pycache__/__init__.cpython-36.pyc’ -> ‘/home/bob/djangoproject1/__pycache__/__init__.cpython-36.pyc’
‘djangoproject1/__pycache__/settings.cpython-36.pyc’ -> ‘/home/bob/djangoproject1/__pycache__/settings.cpython-36.pyc’

How to backup files using cp command?

The linux cp command offer the option --backup to backup the data files, below is the command.

cp --backup source destination