Improve your CAP dev container shell prompt

| 3 min read

In the current back to basics series on CAP Node.js we're using VS Code with a dev container, based on the definition in the repo for the series. The container image is based on this one:


The long prompt

It includes a non-root user node which has a nice default Bash shell prompt that looks like this:

node ➜ /workspaces/capb2b (compositions) $ _

I'm using the underscore (_) to mark the position of the cursor in the examples in this post.

I use VS Code's split terminals a lot, so often during an episode I'll be working in multiple split terminals, where width is at a premium. So nice as this shell prompt looks, I like to start typing my commands on a clean and empty line, while still having the information that's shown.

Finding the prompt definition

Rather than dig into the definition of the container image and find out where and how to change the default prompt (it's actually defined in the .bashrc file in a rather complex function called __bash_prompt) it was a lot easier and lighter-footed to just modify it on the fly.

The main Bash shell prompt is stored in an environment variable called PS1. Looking at this in the shell, we see quite a lot (but then again, it's a rich display of information that we see in the prompt):

node ➜ /workspaces/capb2b (compositions) $ echo $PS1
\[\]`export XIT=$? \ && [ ! -z "${GITHUB_USER}" ] && echo -n "\[\033[0;3
2m\]@${GITHUB_USER} "
|| echo -n "\[\033[0;32m\]\u " \ && [ "$XIT" -ne "
] && echo -n "\[\033[1;31m\]➜" || echo -n "\[\033[0m\]➜"`
1;34m\]\w `\ if [ "$(git config --get devcontainers-theme.hide-status 2>
!= 1 ] && [ "$(git config --get codespaces-theme.hide-status
!= 1 ]; then \ export BRANCH=$(git --no-optional-locks sy
mbolic-ref --short HEAD 2>/dev/null || git --no-optional-locks rev-parse
--short HEAD 2>/dev/null); \ if [ "${BRANCH}" != "" ]; then \ echo -n "
\ && if [ "$(git config --get de 2>/dev/null)
= 1 ] && \ git --no-optional-
locks ls-files --error-unmatch -m --directory --no-empty-directory -o --
exclude-standard ":/*" > /dev/null 2>&1; then \ echo -n " \[\033[1;33m\]
; \ fi \ && echo -n "\[\033[0;36m\]) "; \ fi; \ fi`
\[\033[0m\]$ \[\]

I've deliberately folded the output across multiple lines to make it display nicely in this post. Also, some of the control characters in this PS1 variable are not shown.

That $ near the end of the last line here is the final part of the prompt, i.e. this:

node ➜ /workspaces/capb2b (compositions) $ _

Modifying the prompt definition

So all we have to do is modify the value to add a newline (\n) before it. And that's pretty straightforward with Bash's shell parameter expansion, in particular with this form:


Trying it out in the current shell, like this, where we replace the current value of PS1 with one where we're effectively inserting a newline character (\n) before it:

export PS1=${PS1/\$ /\\n$ }

the prompt then suddenly looks like this, with the cursor on the second line ready to type:

node ➜ /workspaces/capb2b (compositions) 
$ _

Both the $ and \ symbols have special meanings so need to be escaped (by prefixing them with \) in this pattern substitution so that their raw values are used.

Persisting the modification

All we have to do now is make sure this tweak is made each time we start a shell in that dev container. To do that, let's just add it to the end of the .bashrc file in the user's directory, i.e. to /home/node/.bashrc. After this addition, the last few lines of the file look like this:

export EDITOR=vi
set -o vi
bind -x '"\C-l": clear'
export PS1=${PS1/\$ /\\n$ }

Now each new shell has the nice new prompt. This change has been added to the container image definition in the series' repo.

Update 16 Apr: Thanks to tech-wombat for pointing out in the comments to the video that this last update to the container image definition broke the build. It was down to the fact that the default Dockerfile shell (/bin/sh) was choking on the Bash flavoured substitutions in this line. I decided to separate it out and also make sure to escape the escape char too!

Good to know!