Template functions

Available template functions include:

• Config Engine's custom functions

  Several custom functions were added for Config Engine. They are documented on this page.

• Containerlab's template functions functions

• Golang's text/template functions

  Functions from Golang's standard library

Config Engine's functions

A couple of custom functions can be used in Config Engine. They are documented below and the source can be found here

👉 Data validation

These functions can be used in a template to indicate required input parameters.

• expect tests if the input is a certain type or in some cases even within an allowable range. If the input matches the expected type, the function returns and will not affect your template. If the input does not match, an error will be raised and template execution will be stopped.

  Expect supports the following formats:

  • a simple type: str, string, int
  • and IP address with mask, IP/mask
  • a numeric range, e.g. 0-100
  • a regular expression

  Usage: expect <value> <format>

• optional takes exactly the same parameters as expect. The only difference is that template execution will continue if the value is not present. Typically optional would be used in conjunction with default

  Usage: optional <value> <format>

• default returns a default value if a value is not available

  Usage: default <default-value> <value>

👉 String manipulation

There are three string manipulation functions:

• contains <substr> <value> tests is substr is present in the value.
• join <separator> <values> joins a list of values into a single string, using the specified seperator
• split <seperator> <value> splits a string into a list

👉 IP functions

• ip <value> returns only the IP address from a string that contains an IP/mask
• ipmask <value> returns only the mask from a string that contains an IP/mask

👉 Enhanced standard functions

• index was adapted to support pipelines. The indexes can either follow the value, or be before the value (supporting pipe). Negative indexes are allowed and will be the offset from the length.

  This is an extension of golang's standard index function:

  index returns the result of indexing its first argument by the following arguments. Thus index x 1 2 3 is, in Go syntax, x[1][2][3]. Each indexed item must be a map, slice, or array.

• slice returns the result of Golang's slice that supports pipelines. It will attempt the standard slice function, but if that fails, it attempts an alternative implementation, the the first 2 parameters are indexes followed by the value. Negative indexes are allowed and will be the offset from the length.

  The standard slice function

  slice returns the result of slicing its first argument by the remaining arguments. Thus slice x 1 2 is, in Go syntax, x[1:2], while slice x is x[:], slice x 1 is x[1:], and slice x 1 2 3 is x[1:2:3]. The first argument must be a string, slice, or array.

Golang's template functions

Golang's text/template includes several standard functions and comparison operators. This section repeats the text/template documentation.

• call - Returns the result of calling the first argument, which must be a function, with the remaining arguments as parameters. Thus call .X.Y 1 2 is, in Go notation, dot.X.Y(1, 2) where Y is a func-valued field, map entry, or the like. The first argument must be the result of an evaluation that yields a value of function type (as distinct from a predefined function such as print). The function must return either one or two result values, the second of which is of type error. If the arguments don't match the function or the returned error value is non-nil, execution stops.

• html - Returns the escaped HTML equivalent of the textual representation of its arguments. This function is unavailable in html/template, with a few exceptions.

• js - Returns the escaped JavaScript equivalent of the textual representation of its arguments.

• len - Returns the integer length of its argument.

• print - An alias for fmt.Sprint

• printf - An alias for fmt.Sprintf

• println - An alias for fmt.Sprintln

• urlquery - Returns the escaped value of the textual representation of its arguments in a form suitable for embedding in a URL query. This function is unavailable in html/template, with a few exceptions.

👉 Boolean operators

• and returns the boolean AND of its arguments by returning the first empty argument or the last argument. That is, and x y behaves as if x then y else x. Evaluation proceeds through the arguments left to right and returns when the result is determined.

• not returns the boolean negation of its single argument.

• or returns the boolean OR of its arguments by returning the first non-empty argument or the last argument, that is, or x y behaves as if x then x else y. Evaluation proceeds through the arguments left to right and returns when the result is determined.

👉 Comparison

Go includes standard comparison functions

Operator	Result
eq	eq arg1 arg2 returns the boolean truth of arg1 == arg2
ne	ne arg1 arg2 returns the boolean truth of arg1 != arg2
lt	lt arg1 arg2 returns the boolean truth of arg1 < arg2
le	le arg1 arg2 returns the boolean truth of arg1 <= arg2
gt	gt arg1 arg2 returns the boolean truth of arg1 > arg2
ge	ge arg1 arg2 returns the boolean truth of arg1 >= arg2