nix/doc/manual/source/language/string-literals.md

String literals

A string literal represents a string value.

Syntax

expression → string

string → " ( string_char* interpolation_element )* string_char* "

string → '' ( indented_string_char* interpolation_element )* indented_string_char* ''

string → uri

string_char ~ [^"$\\]|\$(?!\{)|\\.

indented_string_char ~ [^$']|\$\$|\$(?!\{)|''[$']|''\\.|'(?!')

uri ~ [A-Za-z][+\-.0-9A-Za-z]*:[!$%&'*+,\-./0-9:=?@A-Z_a-z~]+

Strings can be written in three ways.

The most common way is to enclose the string between double quotes, e.g., "foo bar". Strings can span multiple lines. The results of other expressions can be included into a string by enclosing them in ${ }, a feature known as string interpolation.

The following must be escaped to represent them within a string, by prefixing with a backslash (\):

• Double quote (")

Example

"\""

"\""

• Backslash (\)

Example

"\\"

"\\"

• Dollar sign followed by an opening curly bracket (${) – "dollar-curly"

Example

"\${"

"\${"

The newline, carriage return, and tab characters can be written as \n, \r and \t, respectively.

A "double-dollar-curly" ($${) can be written literally.

Example

"$${"

"$\${"

String values are output on the terminal with Nix-specific escaping. Strings written to files will contain the characters encoded by the escaping.

The second way to write string literals is as an indented string, which is enclosed between pairs of double single-quotes (''), like so:

''
This is the first line.
This is the second line.
  This is the third line.
''

This kind of string literal intelligently strips indentation from the start of each line. To be precise, it strips from each line a number of spaces equal to the minimal indentation of the string as a whole (disregarding the indentation of empty lines). For instance, the first and second line are indented two spaces, while the third line is indented four spaces. Thus, two spaces are stripped from each line, so the resulting string is

"This is the first line.\nThis is the second line.\n  This is the third line.\n"

Note

Whitespace and newline following the opening '' is ignored if there is no non-whitespace text on the initial line.

Warning

Prefixed tab characters are not stripped.

Example

The following indented string is prefixed with tabs:

''
	all:
		@echo hello
''

"\tall:\n\t\t@echo hello\n"

Indented strings support string interpolation.

The following must be escaped to represent them in an indented string:

• $ is escaped by prefixing it with two single quotes ('')

Example

''
  ''$
''

"$\n"

• '' is escaped by prefixing it with one single quote (')

Example

''
  '''
''

"''\n"

These special characters are escaped as follows:

• Linefeed (\n): ''\n
• Carriage return (\r): ''\r
• Tab (\t): ''\t

''\ escapes any other character.

A "double-dollar-curly" ($${) can be written literally.

Example

''
  $${
''

"$\${\n"

Indented strings are primarily useful in that they allow multi-line string literals to follow the indentation of the enclosing Nix expression, and that less escaping is typically necessary for strings representing languages such as shell scripts and configuration files because '' is much less common than ". Example:

stdenv.mkDerivation {
...
postInstall =
  ''
    mkdir $out/bin $out/etc
    cp foo $out/bin
    echo "Hello World" > $out/etc/foo.conf
    ${if enableBar then "cp bar $out/bin" else ""}
  '';
...
}

Finally, as a convenience, URIs as defined in appendix B of RFC 2396 can be written as is, without quotes. For instance, the string "http://example.org/foo.tar.bz2" can also be written as http://example.org/foo.tar.bz2.