# String interpolation

String interpolation is a language feature where a [string], [path], or [attribute name][attribute set] can contain expressions enclosed in `${ }` (dollar-sign with curly brackets).

Such a construct is called *interpolated string*, and the expression inside is an [interpolated expression](#interpolated-expression).


> **Syntax**
>
> *interpolation_element* → `${` *expression* `}`

## Examples

### String

Rather than writing

```nix
"--with-freetype2-library=" + freetype + "/lib"
```

(where `freetype` is a [derivation expression]), you can instead write


```nix
"--with-freetype2-library=${freetype}/lib"
```

The latter is automatically translated to the former.

A more complicated example (from the Nix expression for [Qt](http://www.trolltech.com/products/qt)):

```nix
configureFlags = "
  -system-zlib -system-libpng -system-libjpeg
  ${if openglSupport then "-dlopen-opengl
    -L${mesa}/lib -I${mesa}/include
    -L${libXmu}/lib -I${libXmu}/include" else ""}
  ${if threadSupport then "-thread" else "-no-thread"}
";
```

Note that Nix expressions and strings can be arbitrarily nested;
in this case the outer string contains various interpolated expressions that themselves contain strings (e.g., `"-thread"`), some of which in turn contain interpolated expressions (e.g., `${mesa}`).

To write a literal `${` in an regular string, escape it with a backslash (`\`).

> **Example**
>
> ```nix
> "echo \${PATH}"
> ```
>
>     "echo ${PATH}"

To write a literal `${` in an indented string, escape it with two single quotes (`''`).

> **Example**
>
> ```nix
> ''
>   echo ''${PATH}
> ''
> ```
>
>     "echo ${PATH}\n"

`$${` can be written literally in any string.

> **Example**
>
> In Make, `$` in file names or recipes is represented as `$$`, see [GNU `make`: Basics of Variable Reference](https://www.gnu.org/software/make/manual/html_node/Reference.html#Basics-of-Variable-References).
> This can be expressed directly in the Nix language strings:
>
> ```nix
> ''
>   MAKEVAR = Hello
>   all:
>   	@export BASHVAR=world; echo $(MAKEVAR) $${BASHVAR}
> ''
> ```
>
>     "MAKEVAR = Hello\nall:\n\t@export BASHVAR=world; echo $(MAKEVAR) $\${BASHVAR}\n"

See the [documentation on strings][string] for details.

### Path

Rather than writing

```nix
./. + "/" + foo + "-" + bar + ".nix"
```

or

```nix
./. + "/${foo}-${bar}.nix"
```

you can instead write

```nix
./${foo}-${bar}.nix
```

### Attribute name

<!--
FIXME: these examples are redundant with the main page on attribute sets.