SSH

stands for Secure Shell and is also known as Secure Socket Shell. SSH is a secure network communication protocol. The protocol is primarily used by network administrators to connect to remote devices securely, Uses Port 22

Use the command to find the path of your current working directory. Simply entering pwd will return the full current path – a path of all the directories that starts with a forward slash (/). For example, /home/username.

It has two acceptable options:

-L or –logical prints environment variable content, including symbolic links.

-P or –physical prints the actual path of the current directory.

To navigate through the Linux files and directories, use the command. Depending on your current working directory, it requires either the full path or the directory name.

Running this command without an option will take you to the home folder. Keep in mind that only users with sudo privileges can execute it.

Let’s say you’re in /home/username/Documents and want to go to Photos, a subdirectory of Documents. To do so, enter the following command:

Photos.

If you want to switch to a completely new directory, for example, /home/username/Movies, you have to enter cd followed by the directory’s absolute path:

cd /home/username/Movies

The command lists files and directories within a system. Running it without a flag or parameter will show the current working directory’s content.

To see other directories’ content, type ls followed by the desired path. For example, to view files in the Documents folder, enter:

/home/username/Documents

Here are some options you can use with the ls command:

ls -R lists all the files in the subdirectories.

ls -a shows hidden files in addition to the visible ones.

ls -lh shows the file sizes in easily readable formats, such as MB, GB, and TB.

Use the command to copy files or directories and their content. Take a look at the following use cases.

To one file from the current directory to another, enter cp followed by the file name and the destination directory. For example:

filename.txt /home/username/Documents

To copy files to a directory, enter the file names followed by the destination directory:

cp filename1.txt filename2.txt filename3.txt /home/username/Documents

To copy the content of a file to a new file in the same directory, enter cp followed by the source file and the destination file:

cp filename1.txt filename2.txt

To copy an entire directory, pass the -R flag before typing the source directory, followed by the destination directory:

cp -R /home/username/Documents /home/username/Documents_backup

mv command

The primary use of the is to move and rename files and directories. Additionally, it doesn’t produce an output upon execution.

Simply followed by the filename and the destination directory. For example, you want to move filename.txt to the /home/username/Documents directory:

mv filename.txt /home/username/Documents.

You can also use the mv command to rename a file:

mv old_filename.txt new_filename.txt

mkdir command

Use the command to create one or multiple directories at once and set permissions for each of them. The user executing this command must have the privilege to make a new folder in the parent directory, or they may receive a permission denied error.

Here’s the basic syntax:

For example, you want to create a directory called Music:

To make a new directory called Songs inside Music, use this command:

Music/Songs

The mkdir command accepts many options, such as:

-p or –parents create a directory between two existing folders. For example, mkdir -p Music/2020/Songs will make the new “2020” directory.

-m sets the file permissions. For instance, to create a directory with full read, write, and execute permissions for all users, enter mkdir -m777 directory_name.

-v prints a message for each created directory.

rmdir command

To permanently delete an empty directory, use the Remember that the user running this command should have sudo privileges in the parent directory.

For example, you want to remove an empty subdirectory named personal1 and its main folder mydir:

rmdir -p mydir/personal1

rm command

The command is used to delete files within a directory. Make sure that the user performing this command has write permissions.

Remember the directory’s location as this will remove the file(s) and you can’t undo it.

Here’s the general syntax:

filename

To remove multiple files, enter the following command:

rm filename1 filename2 filename3

Here are some acceptable options you can add:

• -i prompts system confirmation before deleting a file.
• -f allows the system to remove without a confirmation.
• -r deletes files and directories recursively.

touch command

The allows you to create an empty file or generate and modify a timestamp in the Linux command line.

For example, enter the following command to create an HTML file named Web in the Documents directory:

touch /home/username/Documents/Web.html

locate command

The can find a file in the database system.

Moreover, adding the -i argument will turn off case sensitivity, so you can search for a file even if you don’t remember its exact name.

To look for content that contains two or more words, use an asterisk (*). For example:

locate -i school*not

The command will search for files that contain the words school and note, whether they use uppercase or lowercase letters.

find command

Use the command to search for files within a specific directory and perform subsequent operations. Here’s the general syntax:

find [option] [path] [expression]

For example, you want to look for a file called notes.txt within the home directory and its subfolders:

find /home -name notes.txt

Here are other variations when using find:

• find -name filename.txt to find files in the current directory.
• find ./ -type d -name directoryname to look for directories.

grep command

Global Regular Expression Print. A Linux / Unix command-line tool is used to search for a string of characters in a specified file. The text search pattern is called a regular expression. When it finds a match, it prints the line with the result. The grep command is handy when searching through large log files

df command

The command is known as the “disk free” command gives the estimation of the total amount of disk memory space used by the input files and stored files in memory directories. It is used to measure and identify the memory usage of specific files and directories that take up a large sum of the disk memory usage. When we give a file name that is in the directory, the df will give the complete report of the memory usage of the disk in that directory. In this topic, we are going to learn about df Command in Linux.

du command

head command

The command allows you to view the first ten lines of a text. Adding an option lets you change the number of lines shown. The command is also used to output piped data to the CLI.

Here’s the general syntax:

[option] [file]

For instance, you want to view the first ten lines of note.txt, located in the current directory:

head note.txt

Below are some options you can add:

• -n or –lines prints the first customized number of lines. For example, enter head -n 5 filename.txt to show the first five lines of filename.txt.
• -c or –bytes prints the first customized number of bytes of each file.
• -q or –quiet will not print headers specifying the file name.

tail command

The command displays the last ten lines of a file. It allows users to check whether a file has new data or to read error messages.

Here’s the general format:

tail [option] [file]

For example, you want to show the last ten lines of the colors.txt file:

tail -n colors.txt

the command compares two contents of a file line by line. After analyzing them, it will display the parts that do not match.

Programmers often use the command to alter a program instead of rewriting the entire source code.

Here’s the general format:

[option] file1 file2

For example, you want to compare two text files – note.txt and note_update.txt:

diff note.txt note_update.txt

Here are some acceptable options to add:

-c displays the difference between two files in a context form.

-u displays the output without redundant information.

-i makes the diff command case insensitive.

tar command

The command archives multiple files into a  file – a common Linux format similar to ZIP, with optional compression.

Here’s the basic syntax:

tar [options] [archive_file] [file or directory to be archived]

For instance, you want to create a new TAR archive named newarchive.tar in the /home/user/Documents directory:

tar -cvf newarchive.tar /home/user/Documents

The tar command accepts many options, such as:

-x extracts a file.

-t lists the content of a file.

-u archives and adds to an existing archive file.

Check out the more practical examples to know more about the other functions.

chmod command

is a common command that modifies a file or directory’s read, write, and execute permissions. In Linux, each file is associated with three user classes – owner, group member, and others.

Here’s the basic syntax:

[option] [permission] [file_name]

For example, the owner is currently the only one with full permissions to change note.txt. To allow group members and others to read, write, and execute the file, change it to the -rwxrwxrwx permission type, whose numeric value is 777:

chmod 777 note.txt

This command supports many options, including:

-c or –changes displays information when a change is made.

-f or –silent suppresses the error messages.

-v or –verbose displays a diagnostic for each processed file.

chown command

The command lets you change the ownership of a file, directory, or symbolic link to a specified username.

Here’s the basic format:

[option] owner[:group] file(s)

For example, you want to make linuxuser2 the owner of filename.txt:

chown linuxuser2 filename.txt

jobs command

A job is a process that the shell starts. The command will display all the running processes along with their statuses. Remember that this command is only available in csh, bash, tcsh, and ksh shells.

This is the basic syntax:

[options] jobID

To check the status of jobs in the current shell, simply enter jobs to the CLI.

Here are some options you can use:

• -l lists process IDs along with their information.
• -n lists jobs whose statuses have changed since the last notification.
• -p lists process IDs only.

kill command

When you execute any of the three kill commands against a process, what’s really happening is that it’s sending a TERM signal to the process. This is a “nice” way of asking the process to close. It tells the process to wrap up what it’s doing, and then close when it finishes. That means a process has time to close out connections, stop writing to log files, and shut down gracefully.

There are a lot of different signals you can send to a process, but the only other ones you’re likely to need are TERM, and HUP.

ping command

The command is one of the most used basic Linux commands for checking whether a network or a server is reachable. In addition, it is used to troubleshoot various connectivity issues.

Here’s the general format:

ping [option] [hostname_or_IP_address]

For example, you want to know whether you can connect to Google and measure its response time:

ping google.com

wget command

The Linux command line lets you download files from the internet using the command. It works in the background without hindering other running processes.

The command retrieves files using HTTP, HTTPS, and FTP protocols. It can perform recursive downloads, which transfer website parts by following directory structures and links, creating local versions of the web pages.

To use it, enter the following command:

[option] [url]

For example, enter the following command to download the latest version of WordPress:

wget https://wordpress.org/latest.zip

uname command

The or unix name command will print detailed information about your Linux system and hardware. This includes the machine name, operating system, and kernel. To run this command, simply enter into your CLI.

Here’s the basic syntax:

[option]

These are the acceptable options to use:

• -a prints all the system information.
• -s prints the kernel name.
• -n prints the system’s node hostname.

top command

displays critical information about your systems, like CPU and RAM utilization, details about running processes, and more. You can press the shortcut keys to change how information is displayed on the dashboard. The command is one of the stalwarts of Unix-like operating systems. Its utilitarian display is packed with useful information about your system’s running processes and resource usage.

history command

With , the system will list up to 500 previously executed commands, allowing you to reuse them without re-entering. Keep in mind that only users with sudo privileges can execute this command. How this utility runs also depends on which Linux shell you use.

To run it, enter the command below:

history [option]

This command supports many options, such as:

• -c clears the complete history list.
• -d offset deletes the history entry at the OFFSET position.
• -a appends history lines.

man command

The command provides a user manual of any commands or utilities you can run in Terminal, including the name, description, and options.

It consists of nine sections:

• Executable programs or shell commands
• System calls
• Library calls
• Games
• Special files
• File formats and conventions
• System administration commands
• Kernel routines
• Miscellaneous

To display the complete manual, enter:

man [command_name]

For example, you want to access the manual for the ls command:

man ls

Enter this command if you want to specify the displayed section:

man [option] [section_number] [command_name]

For instance, you want to see section 2 of the ls command manual:

man 2 ls

echo command

The command is a built-in utility that displays a line of text or string using the standard output. Here’s the basic syntax:

[option] [string]

For example, you can display the text Hostinger Tutorials by entering:

“Hostinger Tutorials”

This command supports many options, such as:

• -n displays the output without the trailing newline.
• -e enables the interpretation of the following backslash escapes:

• \a plays sound alert.
• \b removes spaces in between a text.

• \c produces no further output.
• -E displays the default option and disables the interpretation of backslash escapes.

zip, unzip commands

Use the command to compress your files into a file, a universal format commonly used on Linux. It can automatically choose the best compression ratio.

The command is also useful for archiving files and directories and reducing disk usage.

To use it, enter the following syntax:

zip [options] zipfile file1 file2….

For example, you have a file named note.txt that you want to compress into archive.zip in the current directory:

zip archive.zip note.txt

On the other hand, the unzip command extracts the zipped files from an archive. Here’s the general format:

unzip [option] file_name.zip

So, to unzip a file called archive.zip in the current directory, enter:

unzip archive.zip

hostname command

Run the command to know the system’s hostname. You can execute it with or without an option. Here’s the general syntax:

[option]

There are many optional flags to use, including:

• -a or –alias displays the hostname’s alias.
• -A or –all-fqdns displays the machine’s Fully Qualified Domain Name (FQDN).
• -i or –ip-address displays the machine’s IP address.

For example, enter the following command to know your computer’s IP address:

hostname -i

useradd, userdel commands

Linux is a multi-user system, meaning more than one person can use it simultaneously. is used to create a new account, while the command allows you to add a Only those with root privileges or sudo can run the command.

When you use the useradd command, it performs some major changes:

• Edits the /etc/passwd, /etc/shadow, /etc/group, and /etc/gshadow files for the newly created accounts.
• Creates and populates a home directory for the user.
• Sets file permissions and ownerships to the home directory.

Here’s the basic syntax:

useradd [option] username

To set the password:

passwd the_password_combination

For example, to add a new person named John, enter the following command simultaneously:

useradd John

passwd 123456789

To delete a user account, use the userdel command:

userdel username

apt-get command

is a command line tool for handling Advanced Package Tool (APT) libraries in Linux. It lets you retrieve information and bundles from authenticated sources to manage, update, remove, and install software and its dependencies.

Running the command requires you to use sudo or root privileges.

Here’s the main syntax:

apt-get [options] (command)

These are the most common commands you can add to apt-get:

• update synchronizes the package files from their sources.
• upgrade installs the latest version of all installed packages.
• check updates the package cache and checks broken dependencies.

nano, vi, jed commands

Linux allows users to edit and manage files a text editor, such as and come with the operating system, while has to be installed.

The denotes keywords and can work with most languages. To use it, enter the following command:

nano [filename]

vi uses two operating modes to work – insert and command. insert is used to edit and create a text file. On the other hand, the command performs operations, such as saving, opening, copying, and pasting a file.

To use vi on a file, enter:

vi [filename]

jed has a drop-down menu interface that allows users to perform actions without entering keyboard combinations or commands. Like vi, it has modes to load modules or plugins to write specific texts.

To open the program, simply enter jed to the command line.

alias, unalias commands

allows you to create a shortcut with the same functionality as a command, file name, or text. When executed, it instructs the shell to replace one string with another.

To use the command, enter this syntax:

Name=String

For example, you want to make k the alias for the kill command:

k=’kill’

On the other hand, the unalias command deletes an existing alias.

Here’s what the general syntax looks like:

unalias [alias_name]

su command

The switch user or command allows you to run a program as a different user. It changes the administrative account in the current log-in session. This command is especially beneficial for accessing the system through SSH or using the GUI display manager when the root user is unavailable.

Here’s the general syntax of the command:

su [options] [username [argument]]

When executed without any option or argument, the su command runs through root privileges. It will prompt you to authenticate and use the sudo privileges temporarily.

Here are some acceptable options to use:

• -p or –preserve-environment keeps the same shell environment, consisting HOME, SHELL, USER, and LOGNAME.
• -s or –shell lets you specify a different shell environment to run.
• -l or –login runs a login script to switch to a different username. Executing it requires you to enter the user’s password.

htop command

Linux utility for displaying crucial information about the system's processes. It can be considered as a Linux counterpart of Windows Task Manager. htop is more of an interactive program as it supports mouse and keyboard operations for switching between values and tabs.

ps command

The command displays the information on the active processes in Linux. This command is useful when you want to manage any process, and prior information to that process is required.

The functionality of the command mainly relies on the number of options supported by it.

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• Enter the command to clean the Terminal screen.

• Press the button to autofill after entering a command with an argument.

• Use to terminate a running command.

• Press to pause a working command.

• Use to freeze your Terminal temporarily.

• Press to undo the Terminal freeze.

• Use to move to the beginning of the line.

• Press to bring you to the end of the line.

Print working directory

shows current directory you are in

Copy file

stands for copy. This command is used to copy files or group of files or directory. It creates an exact image of a file on a disk with different file name. command require at least two filenames in its arguments. Syntax:

Command Options accepts a wide array of options, which makes it an extremely versatile command. Options start with one or two dashes

The most basic use of is typing the command followed by the URL. https://www.geeksforgeeks.org This should display the content of the URL on the terminal. The URL syntax is protocol dependent and multiple URLs can be written as sets like: http://site. {one, two, three}.com URLs with numeric sequence series can be written as:

ifconfig

. command without any argument displays the details of all the active interfaces. This command also displays the assigned ip address of active interfaces. There could be interfaces that are active but they may not have been assigend an IP address.

ipaddress

When the - i is used with the hostname command it will display this output. It can also be used to display the of the host: hostname -I It will simply display the of the host in the terminal.

netstat - tulpn | grep LISTEN

Handy command to check listening and ports and application on Linux : $ netstat - tulpn | grep LISTEN All the netstat options in the command are explained in the next sections. 3. List all UDP ports To list all the UDP port connections, use the following syntax. $ netstat -au 4. List Listening Ports

A tool that is used for displaying network socket-related information on a Linux system. The tool displays more detailed information than the netstat command which is used for displaying active socket connections.

The basic command without any options simply lists all the connections regardless of the state they are in.

You can retrieve a list of both listening and non-listening ports using the -a option as shown below.

To display listening sockets only, use the -l flag as shown.

a command line interface used to set up and maintain tables for the Netfilter firewall for IPv4, included in the Linux kernel. The firewall matches packets with rules defined in these tables and then takes the specified action on a possible match. Tables is the name for a set of chains. Chain is a collection of rules.

In the Linux operating system, this is a utility or tool to manage the firewall rules in the environment. Similarly, we have one more tool to manage the firewall rules on the Linux, i.e., firewalld. These are used to manage setup and examine the IP packets in the Linux kernel. It will help to restrict unwanted packets in the environment.

The command known as the “disk free” command gives the estimation of the total amount of disk memory space used by the input files and stored files in memory directories. Use this command option to view it as a HUMAN easy read.

It is used to measure and identify the memory usage of specific files and directories that take up a large sum of the disk memory usage. When we give a file name that is in the directory, the df will give the complete report of the memory usage of the disk in that directory. In this topic, we are going to learn about df Command in Linux.

ps aux | Grep thiscommand

shows process command

command accepts a vast number of options that can be used to display a specific group of processes and different information about the process, but only a handful are needed in day-to-day usage. is most frequently used with the following combination of options

he command displays the information on the active processes in Linux. This command is useful when you want to manage any process, and prior information to that process is required. The syntax of the command is provided below: $<OPTIONS>. The functionality of the " " command mainly relies on the number of options supported by it.

The aux command in Linux is a command line utility that displays all running processes. The output of this command is a list of all running processes, including the process ID, the user who started the process, the command that started the process, and the amount of CPU time the process has used.The ps aux command is useful for troubleshooting and for monitoring process activity.

The command is used to send a signal to the service to shut down immediately. Typically, this command is used when a program is not responding and requires immediate shutdown. It will automatically shut down if the kill command is used; however, an unresponsive program will not obey a kill command.

If you encounter a stubborn process that refuses to close, you can send this command instead of a signal instead, which will close the process immediately. Add the (its corresponding number in the list above) option to your command to send a KILL signal to a process.

pkill -f (filename/command name )

The Command is used to kill a command by the command's name. You may be used to killing a process with the kill command. However, we can use the command to do the same task. The only difference is that the command works only with the command names rather than the process IDs

systemctl start

systemctl stop

Starts a process

You can use the command to manage services and control when they start. Restart a service After editing the /etc/ssh/sshd_config file, use the restart command to make the service pick up the new settings: $ sudo restart sshd You can verify the service is running by using the status subcommand:

Dmesg

command let's look at the kernel buffer you peer into the hidden world of the Linux startup processes. Review and monitor hardware device and driver.

Vmstat

command (short for virtual memory statistics) is a built-in monitoring utility in Linux. The command is used to obtain information about memory, system processes, paging, interrupts, block I/O, disk, and CPU scheduling. Users can observe system activity virtually in real-time by specifying a sampling period

Pidstat

command is used for monitoring individual tasks currently being managed by the Linux kernel. It writes to standard output activities for every task selected with option -p or for every task managed by the Linux kernel if option -p ALL has been used.

MTR

a networking tool that combines ping and traceroute to diagnose a network

Mpstat

is a command that is used to report processor related statistics. It accurately displays the statistics of the CPU usage of the system. It displays information about CPU utilization and performance. It initializes the first processor with CPU 0, the second one with CPU 1, and so on.

IOstat

command is used for monitoring system input/output device loading by observing the time the devices are active in relation to their average transfer rates.

Free

command outputs a summary of RAM usage, including total, used, free, shared, and available memory and swap space. The command helps monitor resource usage and allows an admin to determine if there's enough room for running new programs. In this tutorial, you will learn to use the free command in Linux.Dec 29, 2021

Ifconfig

command to assign an address to a network interface and to configure or display the current network interface configuration information

Sar

command, or System Activity Reporter, is part of the sysstat package. It captures a set of statistical information such as CPU load, memory paging, memory utilization, swap usage, network I/O, and much more.

Ss

 used to dump socket statistics. It allows showing information similar to netstat. It can display more TCP and state information than other tools.

a command-line memory reporting tool that gives a user diverse reports on memory usage on a Linux system. There is one unique thing about this command, unlike other traditional memory reporting tools, it reports PSS

(Proportional Set Size ), a more meaningful representation of memory usage by applications and libraries in a virtual memory setup.

Dig @ server or Dig Google. Com

command stands for Domain Information Groper. It is used for retrieving information about DNS name servers. It is basically used by network administrators. It is used for verifying and troubleshooting DNS problems and to perform DNS lookups

Mkdir

Creates a directory

SSH

Secure shell connection using port 22

Method 1: Use 'sudo -i' to become root user or superuser in Linux.

Method 2: Use 'sudo -s' to become root user or superuser in Linux.

Method 3: Use 'sudo su -' to become root user or superuser in Linux.

Method 4: Use 'su - root' to become root user or superuser in Linux.

Ways to Become root user or Superuser in Linux

How to get to the Grub menu

Hold down shift key while system boots up

Where are the kernel models located?

Etc/modules use nano to open it. Nano/etc/modules

SS -l

To display listening sockets only, use the -l flag as shown.

SS -a

displaying active socket connections.

Display active socket connections
Listing Listening and Non-listening Ports You can retrieve a list of both listening and non-listening ports using the?

HUP

or

$ kill -HUP 1234

The other handy kill signal is , which instructs a daemon to reload its configuration. This can be useful when you’ve updated a service’s configuration files but need to reload the process for the new changes to take effect. Send the signal by using the following command syntax.