Is an online Ubuntu terminal emulator available?

A perfect solution for your demand to "play around with some terminal commands from official Ubuntu packages" would be to use the LXD demo server that lets you try out the LXD container hypervisor, which is available for installation in the Ubuntu repositories. With LXD you can use a system container running on top of the bare metal installation of the Ubuntu operating system.

LXD demo server specifications are :

• Limit of 30 minutes per session
• Limit of 5 sessions per IP
• 2 shared CPU cores
• 512 MB of dedicated memory
• 10 GB of dedicated disk space
• IPv6 only network connectivity

Example from an Ubuntu 16.04 system container running on top of the Ubuntu 16.10 system :

cl@cl-uw-1:~$ lxc start uc-1604-1
cl@cl-uw-1:~$ lxc exec uc-1604-1 bash
root@uc-1604-1:~# xmodmap
The program 'xmodmap' is currently not installed. You can install it by typing:
apt install x11-xserver-utils
root@uc-1604-1:~#  

Example from performing the same on the evaluation LXD 16.04 LTS server operating system :

As you can see, you are able to play around with commands on original Ubuntu without having to install anything. This can be done (for free) from where ever you are just by using a web browser. Open the LXD demo server in the web browser, accept the terms of the service and start using it.

If you like and want to use LXD containers on your system, just execute : sudo apt install lxd
Learn more about LXD in the documentation and in the comprehensive blog post series from the the technical lead and upstream project leader for LXC / LXD at Canonical Ltd. - Stéphane Graber.

Weird that no one suggested it yet, but, instead of an emulator, why not play around with an actual Ubuntu machine? You can rent an Ubuntu virtual private server from several providers at a really cheap rate, and some of them will let you use their service for free for a limited time.

Some suggestions off the top of my head:

• Digital Ocean: the cheapest machine costs only $5/month with many choices of Linux operating systems (including Ubuntu 14.04 and 16.04, both in 32 and 64 bits flavors). If you are a student aged 13 or older, you can apply for a $50 credit (meaning you get to run your Ubuntu instance for free over the course of 10 months) here. If you are not, you can search around for coupons. You can access your instance through the web, all you need is an email address and a decent web browser (Chrome and Firefox work). Performance might be laggy depending on your connection but it's as good as it gets:

• Google Cloud Platform: I've never used myself, but I know they offer 2 months of VPS hosting as a trial, plus a myriad of other tools that might be worth taking a look at. They claim to have a web-accessible console, as well:

• Amazon Web Services: Amazon is known for offering a whole year of its free tier, which includes enough hours to run a single t2.micro instance uninterruptedly. There are several Ubuntu offerings for you to choose from. I have used it myself and also recommend, but they require a valid credit card upon signing up. They didn't charge me anything, though. I'm not sure if the console is available through the web, but you can always use PuTTY to login to your instance, assuming you are a Windows user. Linux and OS X users don't require any additional software.

If your goal is to learn the Ubuntu insides, all those options should offer you enough time to study and practice. The best part about using a VPS to learn is that, if you manage to break it, you can always destroy it and start a new one from the scratch.

There should probably be more providers with similar services available, but I'd advise you to stick with the most popular ones, since their communities are more likely to answer your questions and provide support.

Online, yes. Free? Most likely not (at least not forever). Either way, after you have a server, it's just a question of setting something like this up.

Getting a server:

What you're asking for is effectively a mini server or a VPS. These are computers that are hosted by companies like DigitalOcean, Amazon Web Services, Google Cloud Platform, and so on. These companies may have free trials or promo codes, but you need to remember that these are effectively services being offered to you, and these are companies. Being nice isn't in the business model because it just isn't profitable.

However, that doesn't mean they're not free. DigitalOcean has a referral program that allows you to get free server time when other people sign up using your unique code. AWS has a one-year free trial for their servers. You can probably find similar deals for other server hosts if you actually try looking. Though, these services are usually pretty cheap ($5/month minimum).

Sure, you could probably find some "free" VPS providers out there, but this is a good time to point out that there's no such thing as a free lunch. You might not be paying in money, but your VPS might be very restricted in what it can do, may be used in data collection, only allow you access at certain times (timesharing), not allow you to save your work, and so on. If you want a reputable install of Ubuntu that actually works the way you want, you're gonna have to pay for one, sorry. Or, alternatively, you have a good friend who is going to let you use some of their excess server capacity.

Remote Access Over Internet:

WARNING: Please make sure you read all of what is described in this section, and make sure you understand it. It's important.

What you're asking (full remote access, meaning shell and/or graphical, over HTTP/HTTPS) has the potential to be a huge security risk! If you give yourself this level of access to your system, others can get it and use various evil tricks, or snoop on your connection. I mention use of VNC later in this post, which can be downright dangerous unless you're extremely careful with your setup. I'm not responsible for your system getting pwned. Don't say I didn't warn you. Do things right, or tread very carefully.

I would personally advise re-evaluating your criteria and considering doing things the right way (Key-based SSH with port tunneling if needed + a strong firewall config) if at all possible. This means using the right software instead of bringing everything over HTTP.

Access over just a web browser is really something most people tend not to do for various reasons. The most-cited reason is that this is just pretty insecure all things considered. However, if you still want to use something over just a web browser, it's definitely possible.

If you're okay with just a direct console or shell, something like Webmin, ShellInABox, or Web Console would be a good choice. They're web-accessible and allow you pretty good levels of control over your machine. Note for something like this, though, you will need a HTTP server (like nginx) on your VPS.

If this is unacceptable and you don't want to host things on your machine, you can use something like webssh, but be wary of any security risks that may result over using something like this.

If you need X11 (a graphical interface for stuff like xmodmap, etc.), you're gonna have to do some more advanced stuff. If this is the case, you're going to have to build some sort of solution around X11 forwarding. If you can't use any computer-side software other than a web browser, this becomes more difficult. However, it's still possible to set up VNC and then use something like noVNC remotely, or by installing it on your own machine. But, this might introduce heavy lag into your system. It also offers a huge potential attack vector against your system, but you should have seen the giant warning at the top of this section.

Ideally, you'd just have a light SSH client (like PuTTY or similar) and X11 (like XMing) on a flash drive, and you can use this to establish a connection from anywhere, but you're already carrying a flash drive. At that point, it's probably best to just make a persistent drive.