justfile grammar

Justfiles are processed by a mildly context-sensitive tokenizer and a recursive descent parser. The grammar is mostly LL(1), although an extra token of lookahead is used to distinguish between export assignments and recipes with parameters.

tokens

BACKTICK   = `[^`\n\r]*`
COMMENT    = #([^!].*)?$
DEDENT     = emitted when indentation decreases
EOF        = emitted at the end of the file
INDENT     = emitted when indentation increases
LINE       = emitted before a recipe line
NAME       = [a-zA-Z_][a-zA-Z0-9_-]*
NEWLINE    = \n|\r\n
RAW_STRING = '[^'\r\n]*'
STRING     = "[^"]*" # also processes \n \r \t \" \\ escapes
TEXT       = recipe text, only matches in a recipe body

grammar syntax

|   alternation
()  grouping
_?  option (0 or 1 times)
_*  repetition (0 or more times)
_+  repetition (1 or more times)

grammar

justfile      : item* EOF
item          : recipe
              | alias
              | assignment
              | export
              | eol
eol           : NEWLINE
              | COMMENT NEWLINE
alias         : 'alias' NAME ':=' NAME
assignment    : NAME ':=' expression eol
export        : 'export' assignment
expression    : value '+' expression
              | value
value         : NAME '(' sequence? ')'
              | STRING
              | RAW_STRING
              | BACKTICK
              | NAME
              | '(' expression ')'
sequence      : expression ',' sequence
              | expression ','?
recipe        : '@'? NAME parameter* ('+' parameter)? ':' dependencies? body?
parameter     : NAME
              | NAME '=' value
dependencies  : NAME+
body          : INDENT line+ DEDENT
line          : LINE (TEXT | interpolation)+ NEWLINE
              | NEWLINE
interpolation : '{{' expression '}}'