grep [pattern] [file_name]

grep -r [pattern] [directory_name]

locate [name]

find [/folder/location] -name [a]

find [/folder/location] -size [+100M]

ls

ls -a

pwd

mkdir [directory]

rm [file_name] 

rm -r [directory_name]

rm -rf [directory_name]

cp [file_name1] [file_name2]

cp -r [directory_name1] [directory_name2]

mv [file_name1] [file_name2]

ln -s /path/to/[file_name] [link_name]

touch [file_name]

more [file_name]

cat [file_name]

cat [file_name1] >> [file_name2]

head [file_name]

tail [file_name]

gpg -c [file_name]

gpg [file_name.gpg]

wc

dmesg

cat /proc/cpuinfo

free -h

lshw

lsblk

lspci -tv

lsusb -tv

dmidecode

hdparm -i /dev/disk

hdparm -tT /dev/[device]

badblocks -s /dev/[device]

cd ..

cd

cd /chosen/directory

tar cf [compressed_file.tar] [file_name]

tar xf [compressed_file.tar]

tar czf [compressed_file.tar.gz]

gzip [file_name]

scp [file_name.txt] [server/tmp]

rsync -a [/your/directory] [/backup/] 

id

last

who

w

groupadd [group_name]

adduser [user_name]

usermod -aG [group_name] [user_name]

sudo [command_to_be_executed_as_superuser]

userdel [user_name] 

usermod

yum list installed

yum search [keyword]

yum info [package_name]

yum install [package_name.rpm]

dnf install [package_name.rpm]

apt-get install [package_name]

rpm -i  [package_name.rpm]

rpm -e [package_name.rpm]

tar zxvf [source_code.tar.gz]
cd [source_code]
./configure
make
make install

ps

pstree

pmap

top

kill [process_id]

pkill [proc_name]

killall [proc_name]

bg

fg

fg [job]

lsof

ip addr show

ip address add [IP_address]

ifconfig

netstat -pnltu

netstat -nutlp

whois [domain]

dig [domain] 

dig -x host

dig -x [ip_address]

host [domain]

hostname -I

wget [file_name]

Ctrl + C

Ctrl + Z

Ctrl + W

Ctrl + U

Ctrl + K

Ctrl + Y

Ctrl + R

Ctrl + O

Ctrl + G

!!

exit

chmod 777 [file_name]

chmod 755 [file_name]

chmod 766 [file_name]

chown [user] [file_name]

chown [user]:[group] [file_name]

df -h

df -i

fdisk -l

du -ah

du -sh

findmnt

mount [device_path] [mount_point]

ssh user@host

ssh host

ssh -p [port] user@host

telnet host

The post Linux Commands for Beginners appeared first on iExpertify.

Related posts:

Linux vs Unix

Linux is a multi-operating system that can be accessed through numerous users concurrently. So, it has to be authenticated to ensure people from accessing other confidential files. Linux can also be used in mainframes and servers without any modifications. It uses the concept of ownership and permissions to enhance the security of the directories and files. 

Linux File Permissions

Types of permissions

Every directories and file in Linux have three basic permission types. They are discussed as follows:

#1 Read Permission

The read permission enables you to open and read a file. For a directory, the read permission enables the user to list the contents of the directory.

#2 Write Permission

The write permission allows the user to modify the file and write new data to the file. For a directory, the write permission allows the user to modify the content of the directory. The user can add, remove, or rename files that belong to a particular directory. 

#3 Execute Permissions

The execute permission allows the user to execute the file as a shell script or a program. For the directory, the execute permission enables the user to access the files in a directory and enter it by using the cd command but it does not allow to list the content.

Viewing the permissions

The view permission allows the user to check the directory or file in the GUI file manager or by reviewing the output using the command given below.

ls -l 

Permissions groups

Every directory and file on Linux is owned by a specific user and group and are defined separately as three user based permission groups. They are as follows:

# User

A user is a person who owns the directory or file. By default, the user who creates the file or directory will be the owner. 

# Group

The user group that owns the directory or file will not affect the actions of other users. All the users who belong to the group that owns the directory or file will have the same permission to access the file or directory.

# Other

The user who is not the owner of the directory or file and doesn’t belong to the same group of the directory or file. Simply, if we set the permission for the ‘other’ category, by default it will affect everyone. 

If you want to view the users on the system, you can view the user using the command below:

cat /etc/passwd

Similarly, you can view the group on the system by using the command below:

cat /etc/group

-rw-rw-r– is a code that represents the permissions given to the owner, user group, and the world.

Here, the ‘-’ represents the selected file. For the directory, it is denoted as ‘d’. 

The characters are simple to remember and understand.

• r- read permission
• w- write permission
• x- execute permission
• _- no permission

The first part of the code ‘rw-’ represents the owner can read the file, write the file, but cannot execute the file since the execute bit is set to ‘-’. Several Linux distributions such as CentOS, Ubuntu, Fedora, etc. will add users to the group of the same group name as the username. 

The second part of the code ‘rw-’ represents for the user group and group members can read the file, write the file.

The third part of the code ‘r–’ represents any user and the user can only read the file.

Changing file permission using chmod

With the help of the change mode ‘chmod’ command, we can set the permissions such as read, write, and execute on a directory or file for the owner, user, and the group.

chmod <permission-number> <file-name>

Here, the permission number is calculated by using the assigned values for r, w, and x. The basic permission number includes three digits. Some special cases can use four digits as a permission number. 

There are two ways to use the commands. They are as follows: 

1. Numeric mode

In a numeric mode, file permissions do not denote as characters but as a three-digit octal number. The following table provides the numbers for all permission types.

For example, see the command below.

$ chmod 764 sample

ls -l sample

rwxrw-r-- 1 iExpertify iExpertify 20 June 20 06:00 sample

chmod 764 and checking permission. In the above command, we have changed the file permissions to 764. 764 represents the following:

• The owner can read, write, and execute
• The user group can read and write
• Any user can only read

2. Symbolic mode

In this mode, we can change permissions for all three owners. We can modify the permissions of a specific owner. With the use of mathematical symbols, we can modify the file permissions.

Let’s see the example:

Current file

ls -l sample

-rw-rw-r-- 1 iExpertify iExpertify 22 2020-06-29 13:45 sample?

Setting permission to other users

$ chmod 0=rwx sample

$ ls -l sample

-rw-rw-rwx 1 iExpertify iExpertify 22 2020-06-29 13:45 sample

Adding execute permission to the user group

$ chmod g+x sample

$ ls -l sample

-rw-rwxrwx 1 iExpertify iExpertify 22 2020-06-29 13:45 sample?

Removing read permission for the user

$ chmod u-r sample

$ ls -l sample

--w-rwxrwx 1 iExpertify iExpertify 22 2020-06-29 13:45 sample

Changing ownership and group

For changing the ownership of a directory or file, use the command below:

chown user

If you want to change the user along with the group for a directory or file, use the command below

chown user: group filename

If you wish to change group owner only, use the command below

chgrp group_name filename

Here, chgrp represents for change group

Advanced permissions

The special permissions that are used to access directories or files are as following:

• _ – It represents there are no special permissions.
• d- It represents the directory.
• l-  It represents the symbolic link of a directory or a file.
• t- It represents the sticky bit permissions. It represents ‘t’ in the executable portion of all user permissions.
• s- It indicates the setuid or setgid permissions. It represents ‘s’ in the read portion of the owner or group permissions.

Setuid or setgid special permissions

The setuid or setgid permissions are used to assign the system to run an executable as the owner with the owner’s permissions. We can assign this permission by explicit defining permissions.

The character that represents the setuid or setgid is ‘s’. To set the setuid or setgid bit on file1.sh, use the command below:

chmod g+s file1.sh

Be careful while using setuid or setgid permissions. If you assign the permissions incorrectly, then your system goes to intrusion.

Sticky bit special permissions

The sticky bit can be useful in a shared environment because when it is assigned to the permissions on a directory it sets permissions for the file owner only to rename or delete the file.

The character for the sticky bits is ‘t’. To set the sticky bits on a directory, use the command below:

chmod+t dir1

Tip

• The file /etc/group contains all the groups defined in the system
• You can use the command “groups” to find all the groups you are a member of
• You can use the command newgrp to work as a member a group other than your default group
• You cannot have 2 groups owning the same file.
• You do not have nested groups in Linux. One group cannot be sub-group of other
• x- eXecuting a directory means Being allowed to “enter” a dir and gain possible access to sub-dirs
• There are other permissions that you can set on Files and Directories which will be covered in a later advanced tutorial

Summary:

• Linux being a multi-user system uses permissions and ownership for security.
• There are three user types on a Linux system viz. User, Group and Other
• Linux divides the file permissions into read, write and execute denoted by r,w, and x
• The permissions on a file can be changed by ‘chmod’ command which can be further divided into Absolute and Symbolic mode
• The ‘chown’ command can change the ownership of a file/directory. Use the following commands: chown user file or chown user:group file
• The ‘chgrp’ command can change the group ownership chrgrp group filename
• What does x – eXecuting a directory mean? A: Being allowed to “enter” a dir and gain possible access to sub-dirs.

The post Linux File Permissions -chmod appeared first on iExpertify.

What are Regular Expressions?

Grep Regex is one of the most popular command-line utilities to find and search strings in a text file.

Using the grep command with regular expressions makes it even more powerful.

Regular expressions come in the picture when you want to search for a text containing a particular pattern.

It simplifies your search operation by searching the patterns on each line of the file.

Types of Regular expressions

For ease of understanding let us learn the different types of Regex one by one.

Basic Regular expressions

Some of the commonly used commands with Regular expressions are tr, sed, vi and grep. Listed below are some of the basic Regex.

Let’s create a sample test.txt file with the following content:

cat test.txt

Output:

This record ends in 2021
2021 is the start of the record

2021
2020

This record ends in 2020
2020 is the start of the record

For example, find all the lines which end with the word 2021:

grep "2021$" test.txt

You should see the following output:

This record ends in 2021

2021

Next, find all the lines which start and end with the word 2021:

grep "^2021$" test.txt

You should see the following output:

2021

For example, find all the lines which end with the word 2020 or 2021(The dot allows any single character in the place):

grep "202.$" test.txt

You should see the following output:

This record ends in 2021

2021

This record ends in 2020

2020

Now, lets display all the lines that start with the string balaram:

grep "^2021" test.txt

You should see the following output:

2021 is the start of the record
2021

Next, find the number of blank lines in the file test.txt:

grep "^$" test.txt

You should see the following output:

<3 empty lines>

Interval Regular expressions

These expressions tell us about the number of occurrences of a character in a string. They are

Example:

Filter out all lines that contain character ‘p’

We want to check that the character ‘p’ appears exactly 2 times in a string one after the other. For this the syntax would be:

cat sample | grep -E p\{2}

Note: You need to add -E with these regular expressions.

Extended regular expressions

These regular expressions contain combinations of more than one expression. Some of them are:

Example:

Searching for all characters ‘t’

Suppose we want to filter out lines where character ‘a’ precedes character ‘t’

We can use command like

cat sample|grep "a\+t"

The regular expression (\) used to search for special characters.

Let’s create a sample test.txt file with the following contents:

cat test.txt

Output:

1.1.1.1
1a1a1a1
1b1c1d1

Now, search for all the lines which matches the pattern “1.1.1.1“:

grep "1.1.1.1" test.txt

This command does not show the proper result as “.” matches any single character:

1.1.1.1
1a1a1a1
1b1c1d1

You can use the regular expression “\” to resolve this issue:

grep "1\.1\.1\.1" test.txt

Output:

1.1.1.1

Brace expansion

The syntax for brace expansion is either a sequence or a comma separated list of items inside curly braces “{}”. The starting and ending items in a sequence are separated by two periods “..”.

Some examples:

In the above examples, the echo command creates strings using the brace expansion.

Let’s create a sample test.txt file with the following contents:

cat test.txt

Output:

apple
appple
appppple

Now, search for all the lines which match a character “p” two times:

grep -E "ap{2}l" test.txt

You should see the following output:

apple

Next, search for all the lines which match a character “p” two or more times:

grep -E "ap{2,}l" test.txt

You should see the following output:

apple
appple
appppple

Next, search for all the lines which match a character “p” two or three times:

grep -E "ap{2,3}l" test.txt

You should see the following output:

apple
appple

Square bracket expansion

The regular expression [] can be used to match any one character found within the bracket group.

search for all the lines which match any range character found within the “test” group.

grep "test[x-z]" test.txt

You should see the following output:

testx
testy
testz

The post Grep Regex Example Linux appeared first on iExpertify.

Just as the output of a command can be redirected to a file, so can the input of a command be redirected from a file. As the greater-than character > is used for output redirection, the less-than character < is used to redirect the input of a command.

We may want a file to be the input for a command that normally wouldn’t accept a file as an option. This redirecting of input is done using the “<” (less-than symbol) operator.

Below is an example of sending a file to somebody, using input redirection.

> mail john.dba@organization.com < sample_ddl

This reads a bit more difficult than the beginner’s cat file | mail someone, but it is of course a much more elegant way of using the available tools.

File Descriptors (FD)

In Linux/Unix, everything is a file. Regular file, Directories, and even Devices are files. Every File has an associated number called File Descriptor (FD).

Your screen also has a File Descriptor. When a program is executed the output is sent to File Descriptor of the screen, and you see program output on your monitor. If the output is sent to File Descriptor of the printer, the program output would have been printed.

Whenever you execute a program/command at the terminal, 3 files are always open, viz., standard input, standard output, standard error.

Following is a complete list of commands which you can use for redirection −

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The post Linux vs Unix appeared first on iExpertify.

Pipelines (|)

We often want to take the output of one command and pass it on to another. The standard way of performing this is using a pipeline, referred to as pipe, or pipeing the output.

The pipe is a vertical bar ‘|’. On a standard US keyboard this shares the same ‘\’ key, but is sometimes located above the RETURN key. On a standard UK keyboard this is normally found at the bottom left of the keyboard and is typed by using shift and the ‘\’ key. On other European keyboards this may be on one of the number keys (e.g. Alt-Gr 6).

The first command to run is listed first followed by the pipe symbol and then followed by the second command. Any output from the first command is then used as input to the second command. We can search, sort, slice, dice, and transform data stored in files in many different ways. A pipe (also called a pipeline) is a powerful shell feature that allows you to p ump the output of one program directly into another.

For example to sort a basic directory listing by name the ls command is piped through the sort command.

ls | sort

The output can be passed through a number of commands by using a pipe through each one. The full command string is referred to as a pipeline.

ls | sort | more

In the below example, we use sort and uniq command to sort a file and print unique values.

$ sort record.txt | uniq 

This will sort the given file and print the unique values only.

Use ls and find to list and print all lines matching a particular pattern in matching files. This command select files with .parm extension in the given directory and search for pattern like “program” in the below example and print those which have word table1 in them.

$ ls -l | find ./ -type f -name "*.parm" -exec grep "table1" {} \;

The command below would take the output from the cat command (listing the contents of a file). and send them to the grep command. The grep command searches for each occurrence of the word High in it’s input. The grep command output would now consist of each line within the file that contained the word High. This output is now sent to the wc command where the wc command with the -l option prints the total number of lines contained in it’s input. 

cat sample.text | grep "High" | wc -l

The post Piping – using | symbol appeared first on iExpertify.

The > symbol is used to redirect the output of a command.

Redirecting stdout, stdin and stderr (> <)

Unless a command is piped into another the output normally goes to the standard output (stdout) which is normally the screen. The input is normally taken from standard input (stdin) which is normally the keyboard. To automate processes it is sometimes required to change this so that for example the output from a command is sent to a file or the printer, or the input to a command is taken from a file. This is done by redirecting the stdout and stdin streams.

Redirect command output (stdout) into a file

command > filename

Redirect error output (stderr) to a file

command 2> filename

It is also possible to write both stdout and the standard error stream to the same file. This can be achieved by redirecting the error data stream to the stdout data stream using 2>&1.

command >output.file 2>&1

Using a Temporary File

Sometimes the output would have to be redirected to a temporary file and then renamed to the required name. the sort from file1 cannot be directly stored in file1

sort file1 >/temp/tmp$$
mv /tmp/tmp$$ file1

The file ending in $$ will actually be created by the system with a unique number. This is useful for temporary files as it prevents you overwriting a temporary file in use by a different process.

The >> symbol

The single greater-than (>) can be replaced by double greater-than symbol (>>) if you would like the output to be appended to the file rather than to overwrite the file.

The tee command

Sometimes it is necessary for someone to monitor the output of the command but also for it to be duplicated into a file for logging purposes or to perform further processing on.

The tee command is used within a pipeline and whatever input it receives it both puts outputs into a file and forwards on the pipeline.

command | tee file1

The line above will take any output from the command and put a copy in file1 as well as sending it to the screen. This could be combined further by allowing the output to be further processed by another command. The tee command can be used any number of times in a pipeline. If you want tee to append to a file rather than overwrite it the -a option is used.

command1 | tee file1 | command2

For example, let us use cat, grep, tee and wc command to read the particular entry from user and store in a file and print line count.

$ cat result.txt | grep "iExpertify" | tee -a file2.txt | wc -l

This command select iExpertify and store them in file2.txt and print total number of lines matching iExpertify

The post Redirect output – using > symbol appeared first on iExpertify.

Wildcards (also referred to as meta characters) are symbols or special characters that represent other characters. You can use them with any command such as ls command or rm command to list or remove files matching a given criteria, receptively.

There are three main wildcards in Linux:

• An asterisk (*) – matches one or more occurrences of any character, including no character.
• Question mark (?) – represents or matches a single occurrence of any character. so, it matches exactly one charecter
• Bracketed characters ([ ]) – matches any occurrence of character enclosed in the square brackets. It is possible to use different types of characters (alphanumeric characters): numbers, letters, other special characters etc.

We use these wildcards typically in ls and rm commands for example to list all txt or html files in the folder:

ls *.txt *.html

to delete all txt files in the folder:

rm *.txt

Using first example we can get details of all files that start with a through c and have 3 digits followed by .txt

file [a-c][0-9][0-9][0-9].txt

In the second example, we delete 2020 files that can start with any letter or number and the second to fifth character as 2020

rm ?2020*.txt

In the last example, to list all files that start with a or c and have exactly 3 characters after them and the third of those is a numeric, use

ls [ac]??[0-9].txt

The post Using wildcards in Linux appeared first on iExpertify.

if you trying to delete a directory and all files under it, rmdir will fail with error message:

rmdir: failed to remove '<directory>': Directory not empty

The option is t remove using rm command with -rf option. r is for recursive and f is to force and ignore warnings and errors. There is no undo option, so be very careful.

rm -rf dirToDelete

using the rm -ir generates a message for each file or directory and deletion happens based on y or n that we can type in.

The post How to remove non empty directory? appeared first on iExpertify.

These are a list of the most commonly used linux commands

The post Simple Linux commands appeared first on iExpertify.