Usage of env variable in docker compose run command

Question

Running the command docker-compose run -e TYPE=result mongo_db_backup should give me the value of the given TYPE variable:

mongo_db_backup:
  image: 'mongo:3.4'
  volumes:
    - '/backup:/backup'
  command: sh -c '$$(echo $TYPE)'

But instead I get the error The TYPE variable is not set. Defaulting to a blank string. What am I doing wrong

I'm getting the same error and a blank like. I don't understand why TYPE is empty as it is set in the command... — user3142695
– user3142695, Commented Sep 27, 2017 at 19:27
Can you change command to env | grep TYPE= and reply with what gets printed to the console? — Oluwafemi Sule
– Oluwafemi Sule, Commented Sep 27, 2017 at 19:28
try to use {} for env variable command: sh -c '$$(echo ${TYPE})' — Bukharov Sergey
– Bukharov Sergey, Commented Sep 27, 2017 at 19:40

Bruce P · Accepted Answer · 2018-10-18 17:39:30Z

11

It happens that Compose expands $TYPE before it gets to the inside of the container. Compose looks for the $TYPE environment variable in the shell or host environment and substitutes its value in.

This will work with the following terminal command:

docker-compose.yml

command: sh -c 'echo $TYPE'

terminal command

TYPE='hello world' docker-compose run web

When there is no $TYPE environment variable in the host machine, Compose sets the value of $TYPE to an empty string and outputs a warning.

Compose needs to be informed not to expand $TYPE since we want it expanded inside of the shell running in the container.

For this use

docker-compose.yml

command: sh -c "echo $$TYPE"

Prepending a dollar symbol to $TYPE escapes it.

Reference:

Variable Substitution in Compose

edited Oct 18, 2018 at 17:39

Bruce P

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answered Sep 27, 2017 at 19:50

Oluwafemi Sule

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