Docker Compose — Compose Config & Validate

Use docker compose config as your first debugging tool — it validates YAML, resolves variables, and shows the effective configuration.

Validate Only

docker compose config -q

No output = valid. Errors show the problem line.

Show Resolved Config

docker compose config

Shows the fully resolved compose file with:

• Variables substituted
• Override files merged
• Extensions expanded
• Defaults applied

Specific Inspections

docker compose config --services    # List all service names
docker compose config --volumes     # List all volume names
docker compose config --images      # List all images
docker compose config --profiles    # List all profiles

Dry Run (Compose v2.24+)

Test what would happen without actually doing it:

docker compose up --dry-run
docker compose down --dry-run

Output shows what Compose would do:

DRY-RUN MODE - Would create network myapp_default
DRY-RUN MODE - Would create container myapp-db-1
DRY-RUN MODE - Would create container myapp-api-1
DRY-RUN MODE - Would start container myapp-db-1
DRY-RUN MODE - Would start container myapp-api-1

Compare Configurations

# Save current config
docker compose config > /tmp/before.yml

# Make changes...

# Save new config
docker compose config > /tmp/after.yml

# Compare
diff /tmp/before.yml /tmp/after.yml

Verify Variable Substitution

# Show which variables are used
docker compose config | grep -E '\$\{|WARNING'

# If you see warnings about unset variables:
docker compose config 2>&1 | grep WARN

Hash Check

docker compose config --hash="*"

Shows a hash for each service config — useful to detect if a service needs recreation.

Debugging Checklist

1. docker compose config -q — Fix any YAML errors
2. docker compose config — Verify resolved values
3. docker compose config --services — Verify service names
4. docker compose up --dry-run — Test the plan
5. docker compose up -d — Execute

Summary

Command	Purpose
config -q	Validate only (quiet)
config	Show resolved configuration
config --services	List service names
config --images	List image references
up --dry-run	Preview actions
config --hash=*	Detect config changes

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Quick Reference

Item	Detail
config -q	Validate YAML syntax
config	Show fully resolved file
–services	List service names
–dry-run	Preview without executing
–hash	Detect config changes