Environment variable assignment in Docker Compose – colon way

by

The documentation itself says both methods are working:

You can use either an array or a dictionary.

Now let’s forgive Docker for failing to use the proper terminology (an array is actually a sequence in YAML, a dictionary is a mapping) and have a look from the YAML perspective:

A mapping is part of the YAML syntax and therefore parsed by the YAML parser, which enables a syntax-aware editor to do proper highlighting and such. Like the docs say, values like true and false will be converted to a boolean by YAML, so you need to be aware of that. Example from docs:

environment:
  RACK_ENV: development
  SHOW: 'true'
  SESSION_SECRET:

Would we not quote 'true', it would be parsed as a boolean value, which is not what we want.

Using a sequence on the other hand leaves the space of YAML syntax. The sequence itself is YAML, but the values are just parsed as one scalar each. For example, the first scalar value in the sequence here:

environment:
  - RACK_ENV=development
  - SHOW=true
  - SESSION_SECRET

Will be parsed by YAML as RACK_ENV=development. Docker will do post-processing to separate variable name from value. So using this method means that you are using two parsing steps, which makes it more difficult for a syntax-aware editor to properly highlight it. Also, you impose on the user the decision about where to use = and where :, which is not immediately obvious for people who don’t know YAML well. It can confuse people.

Looking at escaping, the true does not need to be quoted anymore. This is because it is in the middle of a YAML scalar and therefore is not parsed as standalone value. In fact, quoting it would make YAML treat the quotes as content.

This also means that if you need quoting (for example because you want to use escape sequences), you need to quote the whole scalar. For example, if you want to have a tab character inside your value, it’ll look like this:

environment:
  - "MY_VAR=some\tvalue"

It will not work if you only quote the part after =. Again, this may be confusing.

Conclusion: For me it seems that using a mapping is more consistent and confuses the user less, and it is therefore preferable. ymmv.

Related Contents:

  1. What’s the difference between parenthesis $() and curly bracket ${} syntax in Makefile?
  2. What’s the difference between placing “mut” before a variable name and after the “:”?
  3. What does the ampersand (&) before `self` mean in Rust?
  4. YAML: Do I need quotes for strings in YAML?
  5. Is there any way I can define a variable in LaTeX?
  6. How can I pass variable to ansible playbook in the command line?
  7. how to reference a YAML “setting” from elsewhere in the same YAML file?
  8. What is the difference between ${var}, “$var”, and “${var}” in the Bash shell?
  9. Allowed characters in Linux environment variable names
  10. no new variables on left side of :=
  11. Re-using environment variables in docker-compose.yml
  12. Using unset vs. setting a variable to empty
  13. When to use an object instance variable versus passing an argument to the method
  14. IntelliJ: How to auto-highlight variables like in Eclipse
  15. Use YAML with variables
  16. Difference between State, ST, IORef, and MVar
  17. In WebGL what are the differences between an attribute, a uniform, and a varying variable?
  18. Remove item from a Makefile variable?
  19. Why are variables “i” and “j” used for counters?
  20. How do I get a variable with the name of the user running ansible?
  21. Windows batch command to ignore case sensitivity in variables
  22. Error if don’t check if {{object.field}} exists
  23. Define multiple variables at once in Kotlin (e.g Java : String x,y,z;)
  24. Docker-Compose file has yaml.scanner.ScannerError
  25. What is the meaning of the ${0##…} syntax with variable, braces and hash character in bash?
  26. What is the meaning of a question mark in bash variable parameter expansion as in ${var?}?
  27. Non-declaration statement outside function body in Go
  28. Using variables in Nginx location rules
  29. How can I test that a variable is more than eight characters in PowerShell?
  30. What does variable names beginning with _ mean?
  31. When should I quote CMake variable references?
  32. Docker-compose env file not working
  33. setq and defvar in Lisp
  34. Multi-line string as part of sequence
  35. How do you print a dollar sign $ in Dart
  36. Multiple variable assignment in Swift
  37. In Java, should variables be declared at the top of a function, or as they’re needed?
  38. How to unset variables after use in .zshrc?
  39. Why does shell ignore quoting characters in arguments passed to it through variables? [duplicate]
  40. Windows 7 environment variable not working in path
  41. jmeter pass command line variables into a jmx
  42. Makefile variable initialization and export
  43. Why do underscore prefixed variables exist?
  44. Why there are two ways of declaring variables in Go, what’s the difference and which to use?
  45. What does [:] mean in groovy?
  46. Ansible to Conditionally Prompt for a Variable?
  47. ‘./docker-compose.yml’, service must be a mapping, not a NoneType
  48. Check if Windows batch variable starts with a specific string
  49. Ansible: check if variable equals string
  50. Why PHP variables start with a $ sign symbol?

Leave a Comment