Attribute selectionattrset . attrpath [ or expr ]none1
Function applicationfunc exprleft2
Arithmetic negation- numbernone3
Has attributeattrset ? attrpathnone4
List concatenationlist ++ listright5
Multiplicationnumber * numberleft6
Divisionnumber / numberleft6
Subtractionnumber - numberleft7
Additionnumber + numberleft7
String concatenationstring + stringleft7
Path concatenationpath + pathleft7
Path and string concatenationpath + stringleft7
[String and path concatenation]string + pathleft7
Logical negation (NOT)! boolnone8
Updateattrset // attrsetright9
Less thanexpr < exprnone10
Less than or equal toexpr <= exprnone10
Greater thanexpr > exprnone10
Greater than or equal toexpr >= exprnone10
Equalityexpr == exprnone11
Inequalityexpr != exprnone11
Logical conjunction (AND)bool && boolleft12
Logical disjunction (OR)bool `` bool
Logical implicationbool -> boolnone14

Attribute selection

Select the attribute denoted by attribute path attrpath from attribute set attrset. If the attribute doesn’t exist, return value if provided, otherwise abort evaluation.

An attribute path is a dot-separated list of attribute names. An attribute name can be an identifier or a string.

attrpath = name [ . name ]... name = identifier | string identifier ~ [a-zA-Z_][a-zA-Z0-9_'-]*

Has attribute

attrset ? attrpath

Test whether attribute set attrset contains the attribute denoted by attrpath. The result is a Boolean value.


Numbers are type-compatible: Pure integer operations will always return integers, whereas any operation involving at least one floating point number return a floating point number.

See also Comparison and Equality.

The + operator is overloaded to also work on strings and paths.

String concatenation

string + string

Concatenate two strings and merge their string contexts.

Path concatenation

path + path

Concatenate two paths. The result is a path.

Path and string concatenation

path + string

Concatenate path with string. The result is a path.


The string must not have a string context that refers to a store path.

String and path concatenation

string + path

Concatenate string with path. The result is a string.


The file or directory at path must exist and is copied to the store. The path appears in the result as the corresponding store path.


attrset1 + attrset2

Update attribute set attrset1 with names and values from attrset2.

The returned attribute set will have of all the attributes in e1 and e2. If an attribute name is present in both, the attribute value from the former is taken.


Comparison is

  • arithmetic for numbers
  • lexicographic for strings and paths
  • item-wise lexicographic for lists: elements at the same index in both lists are compared according to their type and skipped if they are equal.

All comparison operators are implemented in terms of <, and the following equivalencies hold:

a <= b! ( b < a )
a > bb < a
a >= b! ( a < b )


  • Attribute sets and lists are compared recursively, and therefore are fully evaluated.
  • Comparison of functions always returns false.
  • Numbers are type-compatible, see arithmetic operators.
  • Floating point numbers only differ up to a limited precision.

Logical implication

Equivalent to !b1 || b2.