Automatic env var substitution in linkerd config files

[kevholditch]

Support automatic replacement of environment variables in the linkerd.conf file when linkerd starts up.

This is useful in a number of scenarios when parts of the config are not known until runtime especially when running in a docker container (see my blog post for a real world scenario).

The format of the variable in the config file could be prefixed with a '$' for example:

interpreter:
kind: default
    transformers:
    – kind: io.l5d.specificHost
    host: $LOCAL_IP

Here $LOCAL_IP would be replaced with the value found at runtime in the environment variable LOCAL_IP.

[olix0r]

the way i've done this sort of thing in the past is passing in the config on stdin like:

./genconfig.sh |./linkerd -

where genconfig.sh has something like:

#!/bin/sh

set -eu

cat <<EOF
interpreter:
  kind: default
    transformers:
    – kind: io.l5d.specificHost
      host: $LOCAL_IP
EOF

[kevholditch]

If you see my blog post that is how I've done it except using envsubst. William tweeted saying it sounded like a reasonable request to have it built in natively.

At the moment the only solution when needing env vars in your config is to derive from the official container and do some config manipulation yourself which is a bit cumbersome.

[wmorgan]

Thanks for submitting this @kevholditch!