Ask questionsProposal: Adding support for environment vars

I'm planning to integrate flaggy into an existing project, replacing the currently used urfave/cli package. This existing program also provides the option to pass in options via environment variables. The env part now could be solved on its own. But I was thinking if it wasn't a cool feature for flaggy itself?

It could look something like this:

flaggy.String(&stringFlag, "f", "flag", "A test string flag")

If --flag was not passed to the program, we would use the env var FLAG if it is set.


flaggy.String(&stringFlag, "f", "flag", "A test string flag")

With a prefix we would look for MYAPP_FLAG.

Precedence should be clear:

  1. The passed --flag
  2. ENV var FLAG
  3. Default value of the flag

Lowercase flags would always relate to UPPERCASE env vars. Hyphens would be replaced with underscores. CamelCase will be split into separate words. So --someFlag would relate to SOME_FLAG.

By default the feature should be disabled, as I can see it could cause problems in some situations, where flags are named like common bash vars, e.g. HOME, TERM, etc

I'm not sure about subcommand flags. For my use case I would only need to add support for global flags - for functionality that would trigger special functionality on a global level, like MYAPP_DEBUG=1. But IF we want that feature as well for subcommand flags, I guess it will make sense to include the subcommand name into the env var name, to avoid conflicts with flags from other subcommands.

flaggy.String(&stringFlagA, "a", "flagA", "A test string flag (A)")
subcommand := flaggy.NewSubcommand("subcommandExample")
subcommand.String(&stringFlagB, "b", "flagB", "A test string flag (B)")

Corresponding env vars would be:


Happy to work on this. Just wanted to check if you were interested in this feature or would reject it.

I also do like the way urfave/cli has support for custom env vars per flag. But I don't see how we could implement this in a backwards compatible way into flaggy. The function signature of all flag functions would change.


Answer questions udondan

One thing to keep in mind is, that with var myFlag = os.Getenv("MY_FLAG"), the help message would show the value of the env var as default value. This might be confusing, since it is a not a fixed default.

On the other hand this might also be used to the users advantage, e.g.

var flagLang = os.Getenv("LANG")
var flagTime = os.Getenv("LC_TIME")

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