Change EC2 Terminal Prompts To Make Servers Identifiable

We can use the terminal prompt to control the risk of working on the wrong server. Here’s how I made an improvement.

---

# Which is better?
[email protected]:~$
[email protected]:~$
# Or
[email protected]:~$
[email protected]:~$

It’s too easy to mix up terminal windows, especially when a busy dev is flitting from server to server. Nobody wants to break a production server, so let’s look at how we can make the prompt memorable.

If the prompt tells you which server you’re on then there’s another layer of defense between you and an honest (although embarassing) mistake.

Here’s what you’ll need to do this..

• An EC2 instance up and running
• Sudo privileges, or root access
• Some method of editing files, I’ll use sed, echo and tee but you could just use a text editor

This is what we’ll achieve

• Replace [email protected] with [email protected] for every user
• No user will be able to change the value of that prompt without a determined effort

Step 1

Review the AWS docs (or just follow these steps).

We need to edit the cloud.cfg file to tell AWS to preserve the hostname that we’ll use for a prompt.

The preserve_hostname was present in my cloud.cfg, so I used sed to replace the text.

sed -i 's/preserve_hostname: false/preserve_hostname: true/g' /etc/cloud/cloud.cfg

Step 2

Next, we need to set the hostname. It’s mentioned in the docs above, but here’s the command:

hostnamectl set-hostname go.getdane.co.uk

# or on your staging server
hostnamectl set-hostname go.staging.getdane.co.uk

Step 3. Here’s the hard bit (well, it took longer to figure out)

If you rebooted now, you’d see the prompt changes to go. Well thats no good if we have staging domain too, we’d always just see go when we really want go.getdane.co.uk or go.staging.getdane.co.uk.

We want this prompt to change for all users. It’s only fair that anyone logging in gets this new benefit and reduces their risk of error.

We also want to make sure this isn’t easily changed.

Check out PROMPT_COMMAND in the bash docs. The value we give to this is executed prior to issuing a prompt (every time). You’ll see in the docs that we’re interested in PS1, that’s what defines the relevant part of the prompt.

If you echo out PS1 you’ll see a crazy string that is interpreted to provide the prompt. Luckily there’s only one change to make here.

# From
\h # This provides everything up to the first dot of the hostname
# To
\H # We'll swap to this, which gives us the full hostname

So, my PS1 needs to look like this:

# I changed the second h to H, giving me the full hostname
\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\H\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$

If we swap PS1 with that value using PROMPT_COMMAND in /etc/bash.bashrc then we can ensure this is applied to all users.

Here’s the command I used, you could use nano or vi.

echo 'PROMPT_COMMAND="PS1=\"\[\e]0;\u@\h: \w\a\]${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\H\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$\""' | tee -a /etc/bash.bashrc

That’s it, get yourself a beer! Now, you can start a new terminal and you’ll see a prompt like this:

# Production server
[email protected]:~$

# Staging server
[email protected]:~$