WTFM

Docker Compose Env Var Adventures

 Programming  300  2 mins

Let’s dive into the world of Docker Compose and environment variables, where hilarity ensues and variables come alive (or do they?).

We’ve all been there. Playing hide and seek with environment variables in Docker Compose. Let’s see how our dear friend MYVAR enjoys this grand game of Peekaboo!

Round 1: The Invisible Variable

You think you’re smart. You declare it confidently…

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$ cat compose.yml && docker compose run test 2> /dev/null  && docker compose down &> /dev/null
services:
  test:
    image: alpine
    command: echo MYVAR=$MYVAR
    environment:
      MYVAR: "asfd"
MYVAR=

Well, MYVAR decided to stay invisible. Maybe it’s hiding under a digital rock?

Round 2: Double Dollar Dilemma

“Ah, I’ll catch you with $$MYVAR!,” you exclaim.

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$ cat compose.yml && docker compose run test 2> /dev/null  && docker compose down &> /dev/null
services:
  test:
    image: alpine
    command: echo MYVAR=$$MYVAR
    environment:
      MYVAR: "asfd"
MYVAR=$MYVAR

Wait, now we’ve just given MYVAR an identity crisis. It’s still playing hard to get!

Round 3: Command-Wrapper Shenanigans

“I got this! Let’s add a sh -c wrapper…”

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$ cat compose.yml && docker compose run test 2> /dev/null  && docker compose down &> /dev/null
services:
  test:
    image: alpine
    command: sh -c 'echo MYVAR=$MYVAR'
    environment:
      MYVAR: "asfd"
MYVAR=

Nope. Still hiding. MYVAR seems to be a master of disguise.

Round 4: The Grande Reveal

“$MYVAR, you can run but you can’t hide from $$ and a sh -c wrapper!”

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$ cat compose.yml && docker compose run test 2> /dev/null  && docker compose down &> /dev/null
services:
  test:
    image: alpine
    command: sh -c 'echo MYVAR=$$MYVAR'
    environment:
      MYVAR: "asfd"
MYVAR=asfd

Victory at last! 🎉 The sneaky MYVAR is finally revealed!

See Also: