Linked environment variables

This document describes the mechanism to link one environment variables onto others and apply the same value changes.

The goal is to link environment variables together and apply all changes to both once linked.

Design choices:

• When link is being established, variable value is not synced

• Link is not reflexive: if A is linked to B, all changes made to A are applied to B, but direct changes to B are not applied to A unless if this link is also configured

• A module display shows the environment changes applying to linked environment variables

This mechanism applies to all environment variable management modulefile commands and module sub-commands:

• append-path

• prepend-path

• pushenv

• remove-path

• setenv

• unsetenv

Note

Link is not effective when a value is set to the environment variable through the ::env Tcl global array.

linked_envvars configuration option

New configuration option linked_envvars is made to configure these links between environment variables.

• Items are separated by colon character

• One variable can be linked to several ones, either through:

  • one item with several variables: A&B&C

  • several items: A&B:A&C

Implementation

• Define an internal state (current_envvar) to indicate the name of the environment variable currently modified

• Set a trace to execute mirrorEnvVarChange procedure at the end of the execution of environment variable management commands

• No trace setup if linked_envvars is empty

• mirrorEnvVarChange procedure:

  • get currently modified environment variable name through current_envvar state

  • get environment variable change command to execute through command-string trace argument

  • replace current_envvar in this command string by linked environment variable name

  • execute resulting command string in currently active interpreter

  • do this for each linked environment variable, including recursively linked ones: trace is triggered one and handle all environment variables to avoid cycle loop

Note

No other environment variable change can occur between the definition of the current_envvar state and the execution of the mirrorEnvVarChange trace procedure. The link mechanism cannot apply to Modules internal environment variables like __MODULES_SHARE_*.

Note

Environment variable name argument is always positioned before any value, thus replacing the first occurrence found of this name will accurately change the name argument on all environment variable change commands.