For what it’s worth, I tried adding an “Install nvm” step into my workflow as follows:

- name: Install nvm
  run: |
    curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.34.0/install.sh | bash
    export NVM_DIR="$HOME/.nvm"
    source "$NVM_DIR/nvm.sh"

The curl command runs fine (I tested it on its own first); but the three commands together fail with:

##[error]Process completed with exit code 3.

The full output of the step is below:

curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.34.0/install.sh | bash
  export NVM_DIR="$HOME/.nvm"
  source "$NVM_DIR/nvm.sh"
  shell: /bin/bash -e {0}
  % Total % Received % Xferd Average Speed Time Time Time Current
                                 Dload Upload Total Spent Left Speed
0 0 0 0 0 0 0 0 --:--:-- --:--:-- --:--:-- 0

100 13226 100 13226 0 0 92489 0 --:--:-- --:--:-- --:--:-- 92489

=> Downloading nvm from git to '/home/runner/.nvm'
Cloning into '/home/runner/.nvm'...

=> => Compressing and cleaning up git repository
=> Appending nvm source string to /home/runner/.bashrc

=> Appending bash_completion source string to /home/runner/.bashrc

N/A: version "12.1.0 -> N/A" is not yet installed.
You need to run "nvm install 12.1.0" to install it before using it.

=> You currently have modules installed globally with npm. These will no

=> longer be linked to the active version of Node when you install a new node

=> with nvm; and they may (depending on how you construct your $PATH)

=> override the binaries of modules installed with nvm:
/usr/local/lib

├── bower@1.8.8

├── grunt@1.0.4

├── gulp@4.0.2

├── n@6.0.1

├── now@16.1.1

├── parcel-bundler@1.12.3

├── typescript@3.5.3

├── webpack@4.39.2

└── webpack-cli@3.3.7

=> If you wish to uninstall them at a later point (or re-install them under your

=> nvm Nodes), you can remove them from the system Node as follows:
 $ nvm use system
 $ npm uninstall -g a_module

=> Close and reopen your terminal to start using nvm or run the following to use it now:
export NVM_DIR="$HOME/.nvm"

[-s "$NVM_DIR/nvm.sh"] && . "$NVM_DIR/nvm.sh" # This loads nvm

[-s "$NVM_DIR/bash_completion"] && . "$NVM_DIR/bash_completion" # This loads nvm bash_completion

##[error]Process completed with exit code 3.

Not entirely sure how to diagnose the “process completed with exit code 3” error?

Thanks @ethomson , I followed your suggestion and was able to get it working with the --install flag.