How to fix ERROR: Invalid interpolation format in Docker

# WRONG - Compose tries to interpolate $word as a variable
services:
  db:
    environment:
      - DB_PASSWORD=MyP@ss$word

# CORRECT - $$ produces a literal $
services:
  db:
    environment:
      - DB_PASSWORD=MyP@ss$$word

# Command with regex
services:
  node-exporter:
    command:
      # WRONG
      - '--path.sysfs=/host/sys'
      - '--collector.filesystem.ignored-mount-points=^/(sys|proc|dev|host|etc)($$|/)'
      # CORRECT - escape the $ in regex
      - '--collector.filesystem.ignored-mount-points=^/(sys|proc|dev|host|etc)($$|/)'

# .env
DB_PASSWORD=MyP@ss$$word

# .env - single quotes prevent interpolation
DB_PASSWORD='MyP@ss$word'

# docker-compose.yml
services:
  db:
    env_file:
      - ./secrets.env  # This file's contents go directly to container

# secrets.env - not processed by Compose interpolation
DB_PASSWORD=MyP@ss$word

ERROR: Invalid interpolation format for "environment" option
in service "web": "DATABASE_URL=postgres://user:pass$word@host/db"

# Find lines containing dollar signs
grep '\$' docker-compose.yml

# Check your .env file too
grep '\$' .env

# docker-compose.yml
services:
  db:
    environment:
      # Reference shell environment variable (no value assignment)
      - DB_PASSWORD

# Set in shell (no escaping needed)
export DB_PASSWORD='MyP@ss$word'

# Run docker-compose
docker-compose up

services:
  app:
    command: >
      sh -c 'for i in $$(seq 1 5); do echo $$i; done'

#!/bin/sh
# scripts/entrypoint.sh
for i in $(seq 1 5); do
  echo $i
done

COPY scripts/entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh
CMD ["/entrypoint.sh"]

services:
  app:
    build: .
    # No command needed - uses CMD from Dockerfile

# Basic substitution
image: ${REGISTRY}/myapp

# Default value if unset
image: ${REGISTRY:-docker.io}/myapp

# Default value if unset or empty
image: ${REGISTRY-docker.io}/myapp

# Error if unset (Compose v2+)
image: ${REGISTRY:?Registry must be set}

# Pattern substitution - will cause error
command: ${PATH/foo/bar}

# Substring extraction - will cause error
value: ${VAR:0:5}

# Array syntax - will cause error
args: ${@}

# Validate and show resolved configuration
docker-compose config

# If successful, you'll see the resolved YAML
# with all variables interpolated

# Show config and filter for your service
docker-compose config | grep -A 10 "db:"

db:
  environment:
    DB_PASSWORD: MyP@ss$word  # Single $ in output