Glu

Glu is a simple config file generation command line tool. Glu refers to glueing variable scopes to template, using only JSON or YAML file. This makes your configuration configurable as much DRY as you can.

Its syntax is similar to those of other template engines, like Jinja, but it is more like a programming using JSON or YAML. Glu provides much robust syntax for generating configuration, and highly optimized for only generating configurations.

How to install?

Support any python version. If you are having an compatibility issue, please let me know.

$ pip install glu

If you want to install it globally, add sudo in front of the command.

Quick Start

You need to provide source files, which would be used as a variable scope, and a template file, to which variables of scope will be injected, or glued.

If your source files and template file is ready, just run:

$ glu -s source_file [source_file...] -t template_file -o output_file [-e]

For example,

$ glu -s /path/to/source_file.json\
         /path/to/yaml_source_file.yml\ 
      -t /path/to/template_file.json\
      -o /path/to/output_file.json\
      -e  # if you want to use environment variable

Grammar Basics

Rules for gluing is basically substitution. Variable from source file is registered as a global scope, and can be used in any places.

# source
{
  "name": "¯\_(ツ)_/¯"
}

# template
{
  "greeting": "Hello, {{ name }}"
}

# generated
{
  "greeting": "Hello, ¯\_(ツ)_/¯"
}

You can substitute variables of arbitrary type, and if variable is within the string, it will be interpolated like above. But be careful not to use non-string variable within the string, because it cannot be interpolated properly. (integer is OK)

If you want to access variable in another dictionary variable, you can use . for nested access. Dot (.) is evaluated generously, that is if foo.bar will be evaluated to undefined if foo is undefined or foo.bar is undefined, without throwing null reference exception. Dot will be used very frequently in your JSON code.

# scope
{
  "array": [1, 2, 3],
  "dictionary": {
    "foo": "¯\_(ツ)_/¯",
    "bar": "{{ array }}"
  }
}

# template
{
  "result": [
    "{{ dictionary }}",
    "{{ dictionary.bar }}"
  ]
}

# generated
{
  "result": [
    {
      "foo": "¯\_(ツ)_/¯",
      "bar": [1, 2, 3]
    },
    [1, 2, 3]
  ]
}

Operators

There are some operators that makes Glu robust. Operator interface is not stable yet, so keep in mind this operator might be changed in future, which you have to update them manually ¯\_(ツ)_/¯

Fallback operator looks like ??, which uses next variable if former variable is undefined. if evaluated value is undefined, Glu will raise error.

# source
{
  "score": { "math": 93 }
)

# template
{
  "score": "{{ score.science ?? score.history ?? score.math }}"
}

# generated
{
  "score": 93
}

Filter operator looks like |, which will apply filter in rhs to value in lhs. Let's first look at the string filters.

# source
{
  "foo": "This_isREALLYAmazing"
}

# template
{
  "display": "{{ foo | display }}",
  "camel_case": "{{ foo | camel_case }}",
  "pascal_case": "{{ foo | pascal_case }}",
  "snake_case": "{{ foo | snake_case }}",
  "lisp_case": "{{ foo | lisp_case }}",
  "uppercase": "{{ foo | uppercase }}",
  "uppercase_dash": "{{ foo | uppercase_dash }}",
  "capitalize_dash": "{{ foo | capitalize_dash }}",
}

# will be
{
    "display": "This Is Really Amazing",
    "camel_case": "thisIsReallyAmazing",
    "pascal_case": "ThisIsReallyAmazing",
    "snake_case": "this_is_really_amazing",
    "lisp_case": "this-is-really-amazing",
    "uppercase": "THIS_IS_REALLY_AMAZING",
    "uppercase_dash": "THIS-IS-REALLY-AMAZING",
    "capitalize_dash": "This-Is-Really-Amazing",
}

There is mute filter, which removes undefined fields from dictionary or array. mute_if_empty field behaves the same, but removes empty field

# source
{
    "empty_array": [],
    "empty_string": "",
    "empty_dict": {}
}

# template
{
    "foo": "{{ empty_array | mute_if_empty }}",
    "bar": "{{ empty_string | mute_if_empty }}",
    "zoo": "{{ empty_dict | mute_if_empty }}",
    "coz": "{{ undefined_variable | mute }}"
}

# generated
{}

Inject operator (<) let you inject the local scope when gluing variable, which will temporally override the global scope. Inject operator enables you to function-like or class-like behavior.

# source
{
  "Person": {
    "full_name": "{{ first_name }} {{ last_name }}"
  },
  "joe": {
    "first_name": "Alan",
    "last_name": "Joe"
  },
  "park": {
    "first_name": "Aiden",
    "last_name": "Park"
  }
}

# template
{
  "people": [
    "{{ Person < joe }}",
    "{{ Person < park }}"
  ]
}

# generated
{
  "people": [
    { "full_name": "Alan Joe" },
    { "full_name": "Aiden Park" }
  ]
}

You can use multiple operators on single expression, but its evaluation order can change any time, be careful to use so.

# This is possible
{
  "example": "{{ foo ?? bar < bar_scope ?? zec < zec_scope | snake_case | mute }}"
}

Above example will be evaluated as:

# This is psuedo code
filter(mute, 
  filter(snake_case,
    fallback(foo,
      fallback(
        inject(bar, bar_scope),
        inject(zec, zec_scope)))))

Predefined variables

Glu supports some variables which is useful when generating configurations.

Variable Name	Value
@date	Date string of format yyyymmdd
@timestamp	Integer value of unix timestamp
@uuid_x	Randomly issued UUID string. There are total 10 random uuid: @uuid_0 to @uuid_9
@env	Place where environment variable is stored, if --use-env option is set

Variables start with @ are all reserved, so please do not start your variable name with @.