CRUD for content environment variables

[nealrichardson]

No description provided.

[kellobri]

User Research Anecdote: Folks on the Posit Data Science team are loving the updateServerEnvVars() function in rsconnect added in 2023

rsconnect gains two new functions for understanding and updating the environment variables that your apps currently use. listServerEnvVars() will return a data frame of applications, with a envVars list-column giving the names of the environment variables used by each application. updateServerEnvVars() will update all applications that use a specific environment variable with the current value of that environment variable

[tdstein]

Resolved by #236