Input File Syntax

MOOSE input file syntax basically works like this:

# comment
[section] # inline-comment
  field1 = 'quoted-string'
  field2 = "quoted-string"
  field3 = "multi-line"
  field4 = unquoted_string # can't have whitespace

  field5 = 42 # integer
  field6 = 42.42 # floating point number
  field7 = true # boolean (false, on, off - case insensitive)

    foo = 42
  [] # close subsection

[] # close section [../] is also allowed, but deprecated


Brace Expressions

Brace expressions allow you to assign computed/calculated values to fields in your input files. Brace expressions have the following syntax:

${cmd [arg1] [arg2]...}

where arguments are whitespace delimited and are allowed to be brace expressions themselves. Brace expressions can appear as field values either unquoted or quoted (i.e. inside a string-value). MOOSE currently has four built-in brace-expression commands:

  • ${replace <var-name>}: looks up the parameter var-name in the input file searching from the current scope looking at consecutive parent scopes until one exists, substituting its value for the replacement expression.

  • ${raw <arg>...}: concatenates all arguments' values with no intervening spaces.

  • ${env <var-name>}: evaluates to the named environment variable.

  • ${fparse <arg>...}: takes a Function Parser math expression that can reference fields from the current input file and evaluates it with the function parser. Variables referencing input-file fields are looked up using the same procedure as with the replace command.


foo1 = 42
foo2 = 43
  num = 1
  bar = ${replace ${raw foo ${num}}}  # becomes 42
  bar2 = ${${raw foo ${num}}} # shortcut for above expression - becomes 42
  num = 2
  bar = ${${raw foo ${num}}} # becomes 43

a = ${fparse
      ${section1/bar} + foo1 / foo2
     } # becomes 42+42/43 or 42.976744...

Here are some important details about how brace-expressions are evaluated:

  • Brace expressions are evaluated in input-file order (i.e. lexical order) from top to bottom of the input file. Brace expressions must not depend on field values using brace expressions that occur later in the input file - although they can depend on field values without brace expressions that occur later in the input file.

  • Nested brace expressions are evaluated from the most deeply nested outward.

  • A single field value is allowed to have multiple brace expressions if (and only if) the field value is presented as a quoted string - e.g. foo = '${bla1} and ${bla2}'. These are evaluated from left to right.

  • If there are no arguments in the brace-expression beyond the "cmd" (e.g. ${foo}), then the replace command is implied: e.g. ${foo} means ${replace foo}.