Running nx target only if file doesn't exist - nrwl-nx

Question

I have a project which has a build step, however, I need to make sure that the file firebase.config.json exists before running the build command.
With that, I have two NPM scripts:
// package.json
{
...,
"nx": {
"targets": {
"prepare": {
"outputs": ["firebase.config.json"]
},
"build": {
"outputs": ["dist"],
"dependsOn": [
{
"target": "prepare",
"projects": "self"
}
]
}
}
},
"scripts": {
"prepare": "firebase apps:sdkconfig web $FIREBASE_APP_ID_SHOP --json | jq .result.sdkConfig > firebase.config.json",
"build": "VITE_FIREBASE_CONFIG=$(cat ./firebase.config.json) vite build",
},
...
}
So with the above, every time I run nx build app it will first run prepare and build the firebase.config.json file.
However, every time I make a change to any of the source files inside my project, prepare re-runs even though the firebase.config.json is already present.
Is it possible for nx to only run a target if the file declared under outputs is not present?

Answer 1

If you are in a bash environment you can modify your prepare script to be the following (note the original command has been shortened with ellipses for readability).
// package.json
{
"scripts":{
"prepare": "CONFIG=firebase.config.json; [ -f \"$CONFIG\" ] || firebase apps:sdkconfig ... | jq ... > \"$CONFIG\""
}
}
The above prepare script will still run, but it should not spend any time reproducing the configuration file if it already exists.
CONFIG=firebase.config.json is just putting our file in a bash environment variable so we can use it in multiple places (helps prevent typos). [ -f "$CONFIG" ] will return true if $CONFIG holds a filename which corresponds to an existing file. If it returns true, it will short-circuit the || (OR) command.
If you want further verification of this technique, you can test this concept at the terminal with the command [ -f somefile.txt ] || echo "File does not exist". If somefile.txt does not exist, then the echo will run. If the file does exist, then the echo will not run.
A slightly-related side-note: while you clearly can do this all in the package.json configuration, if your nx workspace is going to grow to include other libraries or applications, I highly recommend splitting up all your workspace configuration into the default nx configuration files: nx.json, workspace.json, and the per-project project.json files for the sake of readability/maintainability.
Best of luck!