Tag Archives: Linux Basics

Linux

Intermediate Sed: Manipulating Streams of Text in a Linux Environment

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Introduction

The sed stream editor is a powerful editing tool that can make sweeping changes with very little input. In our previous article on sed, we discussed the basics of using sed to edit text.

This article will continue our introduction by examining some more advanced topics.

Supplying Multiple Editing Sequences

There are quite a few instances where you might wish to pass multiple commands to sed simultaneously. There are a few ways that this can be accomplished.

If you don’t already have the files at hand, let’s recreate our environment from last time so that we have some files to manipulate:

cd
cp /usr/share/common-licenses/BSD .
cp /usr/share/common-licenses/GPL-3 .
echo "this is the song that never ends
yes, it goes on and on, my friend
some people started singing it
not knowing what it was
and they'll continue singing it forever
just because..." > annoying.txt

Since sed operates through standard input and output, we can, of course, just string different calls to sed together through a pipeline (remember to escape the “&” since it means “the complete matched pattern” to sed): Continue reading Intermediate Sed: Manipulating Streams of Text in a Linux Environment →

Linux

The Basics of Using the Sed Stream Editor to Manipulate Text in Linux

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Introduction

The sed stream editor is a text editor that performs editing operations on information coming from standard input or a file. Sed edits line-by-line and in a non-interactive way.

This means that you make all of the editing decisions as you are calling the command and sed will execute the directions automatically. This may seem confusing or unintuitive, but it is a very powerful and fast way to transform text.

This tutorial will cover some basics operations and introduce you to the syntax required to operate this editor. You will almost certainly never replace your regular text editor with sed, but it will probably become a welcomed addition to your text editing toolbox. Continue reading The Basics of Using the Sed Stream Editor to Manipulate Text in Linux →

FreeBSD

How To Write a Simple Shell Script on a VPS (Part 3)

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Introduction

Conditional statements are indispensable to any useful scripting and full-featured programming language. On the third part of our Shell Script tutorial, you’ll learn all the “if’s” and “else’s” of bash, and how to use them to your advantage. This article has also been written assuming you have already configured your shell script folder according to the first tutorial.

The “if” command

A conditional statement is used to perform certain actions if a condition is true or false. With shell scripts, this is performed by the “if” command. It’s followed by an expression that’s going to be tested. This expression can also be the exit code of the execution of a command, a mathematical expression, beyond other various things. When working with exit codes, the command is pretty straightforward:

if ls folder
then
echo "Folder exists"
fi

If the folder exists, the echo command will run, because ls will have returned exit code 0, as in successful. Otherwise, if the folder doesn’t exist, the text will not be displayed. All “if” statements need to be followed by the “then” command, and ended with “fi”. If you’re not working with exit codes, and want to test a mathematical expression for example, you’ll need the “test” command. There are the following operators in shell script to compare numbers: Continue reading How To Write a Simple Shell Script on a VPS (Part 3) →

FreeBSD

How To Write a Simple Shell Script on a VPS (Part 2)

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Introduction

The second part of this tutorial will teach you more basic commands for shell scripts on a VPS. They are responsible for both displaying and gathering information to and from the user. This article has been written assuming you have already configured your shell script folder according to the first tutorial.

The echo Command

This command enables you to display information for the user. You can display simple text strings, variables, or both of them together. It has two parameters: -n, which makes the text displayed not be followed by a new line, and -e, which enables the following set of “code” inside the string:

\a   - Alert sound
\b   - Backspace character
\c   - Don’t display new line
\e   - Escape character
\n   - New line
\r   - Carriage return
\t   - Horizontal tab
\0xx - ASCII character
\\   - Backslash

For example, the following commands perform exactly the same: Continue reading How To Write a Simple Shell Script on a VPS (Part 2) →

FreeBSD

How To Write a Simple Shell Script on a VPS

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Introduction

This tutorial is aimed at teaching you how to write shell scripts for the most variety of purposes. Shell scripts can be used to run multiple commands, a single command with difficult and extensive arguments, or more user friendly interfaces for distributing your work. Essentially it makes your life easier by automating stuff you’d have to do manually without it.

Setting Up a Folder

Before you start writing your shell script, it’s a good practice to designate it a folder. It’s recommended that, for personal scripts, you do it over at ~/bin. To create that folder, run: Continue reading How To Write a Simple Shell Script on a VPS →

DNS

An Introduction to DNS Terminology, Components, and Concepts

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Introduction

DNS, or the Domain Name System, is often a very difficult part of learning how to configure websites and servers. Understanding how DNS works will help you diagnose problems with configuring access to your websites and will allow you to broaden your understanding of what’s going on behind the scenes.

In this guide, we will discuss some fundamental DNS concepts that will help you hit the ground running with your DNS configuration. After tackling this guide, you should be ready to set up your domain name with DigitalOcean or set up your very own DNS server.

Before we jump into setting up your own servers to resolve your domain or setting up our domains in the control panel, let’s go over some basic concepts about how all of this actually works.

Domain Terminology

We should start by defining our terms. While some of these topics are familiar from other contexts, there are many terms used when talking about domain names and DNS that aren’t used too often in other areas of computing.

Let’s start easy: Continue reading An Introduction to DNS Terminology, Components, and Concepts →

Linux

How To Set Up SSH Keys

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How To Set Up SSH Keys

About SSH Keys

SSH keys provide a more secure way of logging into a virtual private server with SSH than using a password alone. While a password can eventually be cracked with a brute force attack, SSH keys are nearly impossible to decipher by brute force alone. Generating a key pair provides you with two long string of characters: a public and a private key. You can place the public key on any server, and then unlock it by connecting to it with a client that already has the private key. When the two match up, the system unlocks without the need for a password. You can increase security even more by protecting the private key with a passphrase.

Step One—Create the RSA Key Pair

The first step is to create the key pair on the client machine (there is a good chance that this will just be your computer): Continue reading How To Set Up SSH Keys →

Linux

Add Swap on Ubuntu 14.04

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How To Add Swap on Ubuntu 14.04

Introduction

One of the easiest way of increasing the responsiveness of your server and guarding against out of memory errors in your applications is to add some swap space. Swap is an area on a hard drive that has been designated as a place where the operating system can temporarily store data that it can no longer hold in RAM.

Basically, this gives you the ability to increase the amount of information that your server can keep in its working “memory”, with some caveats. The space on the hard drive will be used mainly when space in RAM is no longer sufficient for data.

The information written to disk will be slower than information kept in RAM, but the operating system will prefer to keep running application data in memory and use swap for the older data. Overall, having swap space as a fall back for when your system’s RAM is depleted is a good safety net.

In this guide, we’ll cover how to create and enable a swap file on an Ubuntu 14.04 server.

Check the System for Swap Information

Before we begin, we will take a look at our operating system to see if we already have some swap space available. We can have multiple swap files or swap partitions, but generally one should be enough.

We can see if the system has any configured swap by typing: Continue reading Add Swap on Ubuntu 14.04 →

Linux

Use Bash’s Job Control to Manage Foreground and Background Processes

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Introduction

In this guide, we’ll talk about how bash, the Linux system, and your terminal come together to offer process and job control. In a previous guide, we discussed how the ps, kill, and nice commands can be used to control processes on your system.

This article will focus on managing foreground and background processes and will demonstrate how to leverage your shell’s job control functions to gain more flexibility in how you run commands.

Managing Foreground Processes

Most processes that you start on a Linux machine will run in the foreground. The command will begin execution, blocking use of the shell for the duration of the process. The process may allow user interaction or may just run through a procedure and then exit. Any output will be displayed in the terminal window by default. We’ll discuss the basic way to manage foreground processes below.

Starting a Process

By default, processes are started in the foreground. Until the program exits or changes state, you will not be able to interact with the shell.

Some foreground commands exit very quickly and return you to a shell prompt almost immediately. For instance, this command: Continue reading Use Bash’s Job Control to Manage Foreground and Background Processes →

Linux

Initial Server Setup with Debian 8

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Introduction

When you first create a new Debian 8 server, there are a few configuration steps that you should take early on as part of the basic setup. This will increase the security and usability of your server and will give you a solid foundation for subsequent actions.

Step One — Root Login

To log into your server, you will need to know your server’s public IP address and the password for the “root” user’s account. If you have not already logged into your server, you may want to follow the first tutorial in this series, How to Connect to Your Droplet with SSH, which covers this process in detail.

If you are not already connected to your server, go ahead and log in as the root user using the following command (substitute the highlighted word with your server’s public IP address):

  • ssh root@SERVER_IP_ADDRESS

Complete the login process by accepting the warning about host authenticity, if it appears, then providing your root authentication (password or private key). If it is your first time logging into the server, with a password, you will also be prompted to change the root password.

About Root

The root user is the administrative user in a Linux environment that has very broad privileges. Because of the heightened privileges of the root account, you are actually discouraged from using it on a regular basis. This is because part of the power inherent with the root account is the ability to make very destructive changes, even by accident.

The next step is to set up an alternative user account with a reduced scope of influence for day-to-day work. We’ll teach you how to gain increased privileges during the times when you need them.

Step Two — Create a New User

Once you are logged in as root, we’re prepared to add the new user account that we will use to log in from now on.

This example creates a new user called “demo”, but you should replace it with a user name that you like:

  • adduser demo

You will be asked a few questions, starting with the account password.

Enter a strong password and, optionally, fill in any of the additional information if you would like. This is not required and you can just hit “ENTER” in any field you wish to skip.

Step Three — Root Privileges

Now, we have a new user account with regular account privileges. However, we may sometimes need to do administrative tasks.

To avoid having to log out of our normal user and log back in as the root account, we can set up what is known as “super user” or root privileges for our normal account. This will allow our normal user to run commands with administrative privileges by putting the word sudo before each command.

Install Sudo

Debian 8 doesn’t come with sudo installed, so let’s install it with apt-get.

First, update the apt package index:

  • apt-get update

Then use this command to install sudo:

  • apt-get install sudo

Now you are able to use the sudo and visudo commands.

Grant Sudo Privileges

To add these privileges to our new user, we need to add the new user to the “sudo” group. By default, on Debian 8, users who belong to the “sudo” group are allowed to use the sudo command.

As root, run this command to add your new user to the sudo group (substitute the highlighted word with your new user):

  • usermod -a -G sudo demo

Now your user can run commands with super user privileges! For more information about how this works, check out this sudoers tutorial.

The next step in securing your server is to set up public key authentication for your new user. Setting this up will increase the security of your server by requiring a private SSH key to log in.

Generate a Key Pair

If you do not already have an SSH key pair, which consists of a public and private key, you need to generate one. If you already have a key that you want to use, skip to the Copy the Public Key step.

To generate a new key pair, enter the following command at the terminal of your local machine (ie. your computer):

  • ssh-keygen

Assuming your local user is called “localuser”, you will see output that looks like the following:

ssh-keygen output
Generating public/private rsa key pair.
Enter file in which to save the key (/Users/localuser/.ssh/id_rsa):

Hit return to accept this file name and path (or enter a new name).

Next, you will be prompted for a passphrase to secure the key with. You may either enter a passphrase or leave the passphrase blank.

Note: If you leave the passphrase blank, you will be able to use the private key for authentication without entering a passphrase. If you enter a passphrase, you will need both the private key and the passphrase to log in. Securing your keys with passphrases is more secure, but both methods have their uses and are more secure than basic password authentication.

This generates a private key, id_rsa, and a public key, id_rsa.pub, in the .ssh directory of the localuser‘s home directory. Remember that the private key should not be shared with anyone who should not have access to your servers!

Copy the Public Key

After generating an SSH key pair, you will want to copy your public key to your new server. We will cover two easy ways to do this.

Option 1: Use ssh-copy-id

If your local machine has the ssh-copy-id script installed, you can use it to install your public key to any user that you have login credentials for.

Run the ssh-copy-id script by specifying the user and IP address of the server that you want to install the key on, like this:

  • ssh-copy-id demo@SERVER_IP_ADDRESS

After providing your password at the prompt, your public key will be added to the remote user’s .ssh/authorized_keys file. The corresponding private key can now be used to log into the server.

Option 2: Manually Install the Key

Assuming you generated an SSH key pair using the previous step, use the following command at the terminal of your local machine to print your public key (id_rsa.pub):

  • cat ~/.ssh/id_rsa.pub

This should print your public SSH key, which should look something like the following:

id_rsa.pub contents
ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAABAQDBGTO0tsVejssuaYR5R3Y/i73SppJAhme1dH7W2c47d4gOqB4izP0+fRLfvbz/tnXFz4iOP/H6eCV05hqUhF+KYRxt9Y8tVMrpDZR2l75o6+xSbUOMu6xN+uVF0T9XzKcxmzTmnV7Na5up3QM3DoSRYX/EP3utr2+zAqpJIfKPLdA74w7g56oYWI9blpnpzxkEd3edVJOivUkpZ4JoenWManvIaSdMTJXMy3MtlQhva+j9CgguyVbUkdzK9KKEuah+pFZvaugtebsU+bllPTB0nlXGIJk98Ie9ZtxuY3nCKneB+KjKiXrAvXUPCI9mWkYS/1rggpFmu3HbXBnWSUdf [email protected]

Select the public key, and copy it to your clipboard.

Add Public Key to New Remote User

To enable the use of SSH key to authenticate as the new remote user, you must add the public key to a special file in the user’s home directory.

On the server, as the root user, enter the following command to switch to the new user (substitute your own user name):

  • su – demo

Now you will be in your new user’s home directory.

Create a new directory called .ssh and restrict its permissions with the following commands:

  • mkdir .ssh
  • chmod 700 .ssh

Now open a file in .ssh called authorized_keys with a text editor. We will use nano to edit the file:

  • nano .ssh/authorized_keys

Now insert your public key (which should be in your clipboard) by pasting it into the editor.

Hit CTRL-X to exit the file, then Y to save the changes that you made, then ENTER to confirm the file name.

Now restrict the permissions of the authorized_keys file with this command:

  • chmod 600 .ssh/authorized_keys

Type this command once to return to the root user:

  • exit

Now you may SSH login as your new user, using the private key as authentication.

To read more about how key authentication works, read this tutorial: How To Configure SSH Key-Based Authentication on a Linux Server.

Step Five — Configure SSH

Now that we have our new account, we can secure our server a little bit by modifying its SSH daemon configuration (the program that allows us to log in remotely) to disallow remote SSH access to the root account.

Begin by opening the configuration file with your text editor as root:

  • nano /etc/ssh/sshd_config

Here, we have the option to disable root login through SSH. This is generally a more secure setting since we can now access our server through our normal user account and escalate privileges when necessary.

To disable remote root logins, we need to find the line that looks like this:

/etc/ssh/sshd_config (before)
#PermitRootLogin yes

You can modify this line to “no” like this if you want to disable root login:

/etc/ssh/sshd_config (after)
PermitRootLogin no

Disabling remote root login is highly recommended on every server!

When you are finished making your changes, save and close the file using the method we went over earlier (CTRL-X, then Y, then ENTER).

Reload SSH

Now that we have made our changes, we need to restart the SSH service so that it will use our new configuration.

Type this to restart SSH:

  • systemctl restart ssh

Now, before we log out of the server, we should test our new configuration. We do not want to disconnect until we can confirm that new connections can be established successfully.

Open a new terminal window. In the new window, we need to begin a new connection to our server. This time, instead of using the root account, we want to use the new account that we created.

  • ssh demo@SERVER_IP_ADDRESS

You will be prompted for the new user’s password that you configured. After that, you will be logged in as your new user.

Remember, if you need to run a command with root privileges, type “sudo” before it like this:

  • sudo command_to_run

If all is well, you can exit your sessions by typing:

  • exit

Where To Go From Here?

At this point, you have a solid foundation for your Debian 8 server. You can install any of the software you need on your server now.

Sumber: https://www.digitalocean.com/community/tutorials/initial-server-setup-with-debian-8