user Archives • Linux Tips

How To: Boot An SSH User

Today’s article will be pretty simple, though it will have a limited scope, as we discuss how to boot an SSH user. The vast majority of my regular readers are ‘simple’ desktop users. This article shouldn’t apply to them; if it does, something may have gone terribly wrong.

I need to point out that if you’re doing this for security reasons, you’ve done something wrong – or you’re getting ready to fire someone and want to ensure any logged-in accounts have been disconnected.

What is this article about?

Well, let’s say you have a server and people are logged in via SSH. Let’s also say you want to disconnect one of them. That is, you want to boot an SSH user. If you want to boot an SSH user, it’s remarkably similar to sending that SSH user a message. You might want to read that article:

Send A Message To Another Logged In User

We’ll be using similar tools, though we’ll also be using tools from this article:

How To: Kill Processes By Their PID (Process ID)

I strongly suggest that you read both of the two links. That will save you some time and I’m going to gloss over some details because one of the great things about previous articles is that it means I don’t have to duplicate work. The onus is yours to read those articles so that you’re familiar with the subject.

In case you haven’t put two and two together, we’re going to boot an SSH user by killing their process ID (PID). It’s a lovely way to do so, perfectly graceful ideally, and will accomplish the goal of booting said SSH user.

This might be something you do when you take a privileged user down to HR to fire them. When they head to HR (and are then led out of the building by security), you can kill any processes they have ownership of, including any SSH sessions they have open. In this case, we’re just going to learn how to boot an SSH user.

So, put on your steel-toed boots ’cause we’re going to boot an SSH user!

(I do… I crack me up!)

Boot An SSH User:

You’re going to need an open terminal and you’re going to need to be connected to the same SSH server(s) as the user. As this is a bit of an advanced article, I’m going to assume you know what those things mean. If you don’t know what those things mean, you probably shouldn’t be operating a server, and definitely shouldn’t be operating a server that has multiple people connected to it.

If you remember (or clicked to read) the previous article, we are going to start with the ‘who’ command. You can simply try this command:

who -u

who -u

That has all the information you need and you’re looking for the PID as that’s what we’re going to be using to boot the SSH user. We’ll be killing their PID and logging them out immediately. You only need the first field to identify them by username and the sixth field to know the PID. So, you can just use this command:

who -u | awk '{print $1, $6}'

1	who -u \| awk '{print $1, $6}'

This will give you an output similar to this:

$ who -u | awk '{print $1, $6}'
kgiii 2253
test_user 174676

$ who -u | awk '{print $1, $6}'

kgiii 2253

test_user 174676

Using the first field to identify the user, you can see their PID. You can kill their process with the following command:

sudo kill -HUP <PID>

1	sudo kill -HUP <PID>

Or, for example:

sudo kill -HUP 174676

1	sudo kill -HUP 174676

If that doesn’t work, and sometimes a process can’t be killed, you can bring out the hammer and tell the kernel to drop the process. You should try the first way as it’s more graceful and using a hammer may result in a zombie process. But, if you want to bring out the hammer, you would use this command:

sudo kill -9 <PID>

1	sudo kill -9 <PID>

If the user is logged in more than once, you can boot the SSH user by adding the rest of their PIDs. You just simply add them to either of the two commands. For example:

sudo kill -HUP <PID> <PID> <PID>

1	sudo kill -HUP <PID> <PID> <PID>

And that’s how you boot multiple SSH users in one command.

Again, if you’re relying on this for security you’re doing it wrong. This should just be a part of the security process, such as when you’re letting someone go while they were still at their desk. You bring them to HR while security cleans their desk and IT make sure they’re logged out of any and every system. You then escort them out of the building.

Of course, there are probably other reasons why you might want to boot an SSH user. It’s your system, you can decide what to do. You shouldn’t be using this to boot an adversarial SSH connection because if it has reached that point you’re doing security very, very wrong.

Closure:

So, why not? We might as well have an article about how you boot an SSH user. It does not apply to much of my readership, but it’s certainly something worth knowing. It was also tied into a couple of earlier articles, including one from yesterday, so I figured I’d cover this subject while it was still fresh in my memory.

That memory ain’t what it once was! Welcome to old age!

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How To: Create A New User

Today’s article is a nice and easy one, where you’ll learn how to create a new user. It’s a skill everyone should have and it’s really not all that difficult. It’s a pretty basic skill, after all. Either way, it shouldn’t be all that long, nor all that difficult.

In fact, I’ve previously covered some of this. Oddly, I’ve covered the more difficult stuff first. I’ve covered how to create a new user without a /home directory. I’ve also covered create a new user with sudo privileges. Oddly, I’ve never covered how to just plain create a new user. So, that’s what this article will explain.

The tool we’ll be using in this article is one you’ve used before, assuming you’ve been following the site. We’ll be using ‘adduser’ which the man page helpfully describes as:

adduser, addgroup – add a user or group to the system

As a tool, it does what it says on the tin. You can see that it’s also covering the ‘addgroup’ command. We won’t be covering that today, but it’s probably pretty obvious what it does. Hint: It is used to add groups!

Alas, we’ll just be using the ‘adduser’ bit, in this pretty simple article to follow. Anyhow, you never know when you’ll want to create a new user and Linux is very much a multi-user operating system – even if you don’t realize it. Between users and groups, you can do some pretty fancy stuff with permissions.

Create A New User:

This article requires an open terminal, like oh so many do. To crack open a terminal, just press CTRL + ALT + T and your default terminal should open. See? It’s magic!

Now, with your terminal open, you can create a new user with:

sudo adduser <new_user>

1	sudo adduser <new_user>

Next, you’ll be asked for a password. Don’t be fooled, they want your root/sudo password and not the intended password for the new user. That will come later, after the user is created. The application will tell you what it’s doing, such as creating the new user, creating the new user’s home directory, and copying the default files to the new user’s home directory.

After it’s done with that, it’ll ask you for some finishing information. You’ll be asked to type the password. That’s when you enter the password for the new user. You’ll be asked to confirm it to make sure you typed it properly. It’ll then confirm that it has set the password, ‘adduser’ is helpful like that.

At this point, you can opt to include more information. None of this is required information and you can leave the fields blank. But, if you want, you can fill the fields for things like the new user’s real name, their phone number(s), and even what room they are in.

Given that most of my readers are home users, you’re probably not going to need to add that information. Either way, when you’re done with that you will need to confirm the information. This is obvious, but you enter Y to confirm the data, or N to go back and edit something. When you finish that, you will have a new user account that you can use immediately.

Closure:

There you have it. You now have a new article. This one will have taught you how to create a new user. As I’ve mentioned, Linux is a multi-user operating system by design and, as such, you’ll eventually need to know how to create a new user. And… When that happens… You’ll either remember – or you’ll be able to search linux-tips.us for the answer! We’ve got well over 200 articles, so we’ve covered a lot of subjects.

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How To: Add A User To A Group

Today’s article is going to teach you how to add a user to a group. It’s not particularly difficult, but it’s something everyone should know. If you don’t know how to add a user to a group, this article is meant for you.

This article is going to make a few assumptions. The first is that you’re familiar with cat /etc/passwd which will tell you what groups a user belongs to. It also assumes that you’re familiar with cat /etc/group – where you can find supplementary information.

Furthermore, you should also be aware of the groups command, which is a handy command, probably worthy of its own article, that lets you know what groups you already belong to. So, there’s a bit you’re expected to know already, or at least be familiar with conceptually, but trust me when I say this is a very simple article and very straightforward.

Anyhow, if you’ve looked at those previous commands, you’ll see there are a lot of groups. Your user may or may not be a member of those groups, as the groups command will let you know. For myriad reasons, you may wish to add yourself or another user to different groups. Well, that’s what this article is actually about. It’s about showing you how to …

Add A User To A Group:

This article requires an open terminal, like many other articles on this site. If you don’t know how to open the terminal, you can do so with your keyboard – just press CTRL + ALT + T and your default terminal should open.

The command we’ll be using for this exercise is ‘usermod‘. You can get a head start by using ‘man usermod‘ but we really won’t be needing most of that information. In reality, we only care about a couple of the flags. Still, usermod is a pretty expansive command, with many options. Still, it defines itself simply as:

usermod – modify a user account

See? Pretty straightforward still. The command we actually want is:

sudo usermod -a -G <group_name> <user_name>

1	sudo usermod -a -G <group_name> <user_name>

The -a means append (add) the user. The -G means groups – so the -a -G means add a user to a group. You can verify the command worked (though, well, you really don’t need to – ’cause, assuming you did it properly it’ll work) with the following:

grep <user_name> /etc/group

1	grep <user_name> /etc/group

See? That’s it. You’ve learned how to add a user to a group – in under 500 words!

Closure:

Yup, it’s a nice and easy article for a skill you may need as you work with advanced group permissions. If you want to refine the permissions in your system, the sky’s the limit and it’s easy enough to add a user to a group if you need to. So, there’s another tool in your toolbox.

Find Your Username In Linux

Today’s article is going to be fairly brief and easy, as it just covers how to find your username in Linux. For this exercise, we’ll be using a terminal and some basic commands. It shouldn’t be too stressful or difficult. In fact, it should be just the opposite.

When you open your terminal, you’re usually greeted with some information. In that information is typically your username. However, it’s possibly that this is no longer true. You could have it display anything you wanted, plus there’s a chance you’re logged in remotely and just can’t remember which terminal window is connected to which device.

So, there are some reasons why these commands exist. I mean, you should probably know your username. That’s not the kind of thing I forget, but I am getting older. Still, the commands exist and must exist for a reason.

My motto is that they wouldn’t have provided a path if they didn’t want you to get to the destination. (That’s not really my motto.) So, if there’s a command that’ll help you find your username in Linux, you might as well know it and know how to use it.

Find Your Username In Linux:

With that terminal open, let’s try echoing the $USER variable. It’s nice and easy, and it looks like this:

echo $USER

1	echo $USER

You can also use who and whoami as commands:

who

who

And:

whoami

whoami

There’s also w, which shows logged in users – so you may be able to deduce your username from that list. It just looks like a:

w

The ‘w’ command nice and handy, and has a bit more information about the user. It would look similar to this:

w command's output showing username — See? It’s pretty easy to see that there’s a user logged in – and more!

As you can see, there are a number of ways. I’m sure that I’m missing some. Feel free to chime in and add to the list. Basically, if you want to share it with the world, leave a comment. Otherwise, many readers know where to find me.

Closure:

And now you have another article. This one isn’t fancy, nor is it something you’re going to need all the time. However, it’s still a very basic and useful tool to add to your Linux toolbox. Things like these are the fundamentals. How to find your username in Linux is an absolute beginner move and a move that leads you forward to more knowledge.

How To: Create a New User Without a /home Directory

There are legit reasons to create a new user without a /home directory. Maybe you want a new user to have limited access to just a few things, or a dedicated user that runs a single application. This article will show you how.

See, I was reading a forum post on Linux.org about a member that had set up a bunch of Linux computers for some learning children. One of the things that came up was that the kids were already trying to guess the password so that they could install games.

That got me thinking about some security implications. What if they guessed the password and wanted to hide it? What if they used that password to create a new user, but one without a /home directory so that it wouldn’t be easily spotted by just using a file manager? It wouldn’t be impossible to find, but it’d not stand out immediately with a quick inspection. Besides, the new account’s password would remain the same even if the admin changed the password to the root account.

Well, if they get that advanced and guess that password, I kinda hope they read this article! Why? Because the world needs a little chaos and creativity! So, my fellow Linux.org forum user, this article is for you! Well, no… It’s for when the kids use a search engine to learn how to make a user account a little less obvious!

Create a New User Without /home:

This one will be short and easy, perfect for budding Linux users! There are two easy ways to create a user without automatically making a /home folder.

Both ways are done in the terminal, so you need to open it. To open the default terminal, use your keyboard and press CTRL + ALT + T.

Now, the first method is:

sudo useradd -M <new_username>

1	sudo useradd -M <new_username>

The second method is:

sudo useradd --no-create-home <new_username>

1	sudo useradd --no-create-home <new_username>

Those commands will both make a new user without a /home directory of their own. (Be sure to check ‘man useradd’ for more awesome things you can do.)

There are a couple more steps you can take, if you want. First, there’s no password assigned to the user you just created. So, let’s assign a password to them.

sudo passwd <new_username>

1	sudo passwd <new_username>

Follow the prompts to type in the password twice and you’re done with that step.

Next, the newly created user isn’t a member of sudoers – meaning it has no administrative rights. That’s easily fixed with the following command:

sudo adduser <new_username> sudo

1	sudo adduser <new_username> sudo

That command will make the new user a member of the sudoers group. Meaning they have administrative rights over the system. They can install software, remove software, delete files, create files, or even update the entire system.

The admin can still discover the new user by listing the users or poking around in the logs. However, the user won’t stand out immediately. There won’t be any new user folder in /home, so one may not have any reason to look. Additionally, changing the root password won’t matter. You’ll have to do something about the user they created.

And that, kids, is how you get started hiding stuff after you’ve discovered the root password! Use that account for your nefarious activities! You’re welcome!

Closure:

Nah, there are legit reasons why you’d want a user without a home folder. You may want that user to only access a limited set of applications or whatever. A keen admin would likely notice this fairly rapidly, so it’s not a great long-term strategy for hiding your game installing.