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190 lines
4.2 KiB
Markdown
190 lines
4.2 KiB
Markdown
# String literals
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A *string literal* represents a [string](types.md#type-string) value.
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> **Syntax**
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>
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> *expression* → *string*
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>
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> *string* → `"` ( *string_char*\* [*interpolation_element*][string interpolation] )* *string_char*\* `"`
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>
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> *string* → `''` ( *indented_string_char*\* [*interpolation_element*][string interpolation] )* *indented_string_char*\* `''`
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>
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> *string* → *uri*
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>
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> *string_char* ~ `[^"$\\]|\$(?!\{)|\\.`
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>
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> *indented_string_char* ~ `[^$']|\$\$|\$(?!\{)|''[$']|''\\.|'(?!')`
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>
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> *uri* ~ `[A-Za-z][+\-.0-9A-Za-z]*:[!$%&'*+,\-./0-9:=?@A-Z_a-z~]+`
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Strings can be written in three ways.
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The most common way is to enclose the string between double quotes, e.g., `"foo bar"`.
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Strings can span multiple lines.
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The results of other expressions can be included into a string by enclosing them in `${ }`, a feature known as [string interpolation].
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[string interpolation]: ./string-interpolation.md
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The following must be escaped to represent them within a string, by prefixing with a backslash (`\`):
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- Double quote (`"`)
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> **Example**
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>
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> ```nix
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> "\""
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> ```
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>
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> "\""
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- Backslash (`\`)
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> **Example**
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>
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> ```nix
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> "\\"
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> ```
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>
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> "\\"
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- Dollar sign followed by an opening curly bracket (`${`) – "dollar-curly"
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> **Example**
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>
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> ```nix
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> "\${"
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> ```
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>
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> "\${"
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The newline, carriage return, and tab characters can be written as `\n`, `\r` and `\t`, respectively.
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A "double-dollar-curly" (`$${`) can be written literally.
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> **Example**
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>
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> ```nix
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> "$${"
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> ```
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>
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> "$\${"
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String values are output on the terminal with Nix-specific escaping.
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Strings written to files will contain the characters encoded by the escaping.
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The second way to write string literals is as an *indented string*, which is enclosed between pairs of *double single-quotes* (`''`), like so:
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```nix
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''
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This is the first line.
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This is the second line.
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This is the third line.
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''
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```
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This kind of string literal intelligently strips indentation from
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the start of each line. To be precise, it strips from each line a
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number of spaces equal to the minimal indentation of the string as a
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whole (disregarding the indentation of empty lines). For instance,
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the first and second line are indented two spaces, while the third
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line is indented four spaces. Thus, two spaces are stripped from
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each line, so the resulting string is
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```nix
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"This is the first line.\nThis is the second line.\n This is the third line.\n"
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```
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> **Note**
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>
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> Whitespace and newline following the opening `''` is ignored if there is no non-whitespace text on the initial line.
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> **Warning**
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>
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> Prefixed tab characters are not stripped.
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>
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> > **Example**
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> >
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> > The following indented string is prefixed with tabs:
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> >
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> > <pre><code class="nohighlight">''
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> > all:
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> > @echo hello
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> > ''
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> > </code></pre>
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> >
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> > "\tall:\n\t\t@echo hello\n"
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Indented strings support [string interpolation].
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The following must be escaped to represent them in an indented string:
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- `$` is escaped by prefixing it with two single quotes (`''`)
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> **Example**
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>
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> ```nix
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> ''
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> ''$
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> ''
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> ```
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>
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> "$\n"
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- `''` is escaped by prefixing it with one single quote (`'`)
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> **Example**
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>
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> ```nix
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> ''
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> '''
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> ''
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> ```
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>
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> "''\n"
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These special characters are escaped as follows:
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- Linefeed (`\n`): `''\n`
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- Carriage return (`\r`): `''\r`
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- Tab (`\t`): `''\t`
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`''\` escapes any other character.
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A "double-dollar-curly" (`$${`) can be written literally.
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> **Example**
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>
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> ```nix
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> ''
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> $${
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> ''
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> ```
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>
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> "$\${\n"
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Indented strings are primarily useful in that they allow multi-line
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string literals to follow the indentation of the enclosing Nix
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expression, and that less escaping is typically necessary for
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strings representing languages such as shell scripts and
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configuration files because `''` is much less common than `"`.
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Example:
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```nix
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stdenv.mkDerivation {
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...
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postInstall =
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''
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mkdir $out/bin $out/etc
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cp foo $out/bin
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echo "Hello World" > $out/etc/foo.conf
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${if enableBar then "cp bar $out/bin" else ""}
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'';
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...
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}
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```
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Finally, as a convenience, *URIs* as defined in appendix B of
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[RFC 2396](http://www.ietf.org/rfc/rfc2396.txt) can be written *as
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is*, without quotes. For instance, the string
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`"http://example.org/foo.tar.bz2"` can also be written as
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`http://example.org/foo.tar.bz2`.
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