[data/values] support reading values from environment variables or passed-in flags

[nimakaviani]

the ytt user may wish to provide data values in the form of environment variables or by passing values through the command line. this is particularly beneficial when providing secrets.

As a user of ytt I expect it to provide me with either of the options above to provide or override values.

[cppforlife]

per our previous discussions, potential UX may look like this:

• multiple values are read from env vars
  • ytt tpl ... --data-values-env BAR_
  • BAR_FOO_KEY1_KEY2 -> Foo.Key1.Key2
• single value is read from command arg
  • ytt tpl ... --data-value-arg Foo.Key1.Key2=myval (where myval is YAML unmarshable value)
• single value is read from file
  • ytt tpl ... --data-value-file Foo.Key1.Key2=path (where path is some file whose contents will be loaded as a string)

[cppforlife]

as a side note, i was thinking one can "override" default values by specifying it in a dedicated overlay file, which brings an interesting question of how strict should --data-value-arg Foo.Key1.Key2=myval be? for example what if Foo.Key1.Key2 does not exist in the original values file?

[nimakaviani]

I feel like the behavior should be merge here if the key does not exist but the values are supplied to the command line explicitly

[cppforlife]

if the key does not exist but the values are supplied to the command line explicitly

only concern i have here is catching typos. that's why overlay functionality is conservative until it's marked as ok to be missing.

[emayssat]

I want this feature ;-)
I would like to read values passed on the command line or in the environment.
Any ETA for this feature?

[cppforlife]

@emayssat thanks for bumping this up. ill think more about what this feature should look like this week.

[cppforlife]

@emayssat @afcastano ive added b076047 as a first cut for this feature. try it out. examples:

export YTT_key__env=123
ytt template -R -f . \
  --data-value key.subkey="value" \
  --data-value key.boolean=true \
  --data-values-env YTT \
  --data-value-file key.file=/path

values are yaml encoded.

above keys should be specified in configuration data values, otherwise overlaying will fail, eg

#@data/values
---
key:
  subkey:
  boolean:
  env:
  file:

[cppforlife]

@emayssat @afcastano let me know if this works out for yall. have a good weekend.

[cppforlife]

included in v0.8.0