github.com/iaas-resource-provision/iaas-rpc@v1.0.7-0.20211021023331-ed21f798c408/website/docs/language/expressions/strings.html.md

---
layout: "language"
page_title: "Strings and Templates - Configuration Language"
---

# Strings and Templates

String literals are the most complex kind of literal expression in
Terraform, and also the most commonly used.

Terraform supports both a quoted syntax and a "heredoc" syntax for strings.
Both of these syntaxes support template sequences for interpolating values and
manipulating text.

## Quoted Strings

A quoted string is a series of characters delimited by straight double-quote
characters (`"`).

```
"hello"
```

### Escape Sequences

In quoted strings, the backslash character serves as an escape
sequence, with the following characters selecting the escape behavior:

| Sequence     | Replacement                                                                   |
| ------------ | ----------------------------------------------------------------------------- |
| `\n`         | Newline                                                                       |
| `\r`         | Carriage Return                                                               |
| `\t`         | Tab                                                                           |
| `\"`         | Literal quote (without terminating the string)                                |
| `\\`         | Literal backslash                                                             |
| `\uNNNN`     | Unicode character from the basic multilingual plane (NNNN is four hex digits) |
| `\UNNNNNNNN` | Unicode character from supplementary planes (NNNNNNNN is eight hex digits)    |

There are also two special escape sequences that do not use backslashes:

| Sequence | Replacement |
| ---      | ----        |
| `$${`    | Literal `${`, without beginning an interpolation sequence. |
| `%%{`    | Literal `%{`, without beginning a template directive sequence. |

## Heredoc Strings

Terraform also supports a "heredoc" style of string literal inspired by Unix
shell languages, which allows multi-line strings to be expressed more clearly.

```hcl
<<EOT
hello
world
EOT
```

A heredoc string consists of:

- An opening sequence consisting of:
    - A heredoc marker (`<<` or `<<-` — two less-than signs, with an optional hyphen for indented heredocs)
    - A delimiter word of your own choosing
    - A line break
- The contents of the string, which can span any number of lines
- The delimiter word you chose, alone on its own line (with indentation allowed for indented heredocs)

The `<<` marker followed by any identifier at the end of a line introduces the
sequence. Terraform then processes the following lines until it finds one that
consists entirely of the identifier given in the introducer.

In the above example, `EOT` is the identifier selected. Any identifier is
allowed, but conventionally this identifier is in all-uppercase and begins with
`EO`, meaning "end of". `EOT` in this case stands for "end of text".

### Generating JSON or YAML

Don't use "heredoc" strings to generate JSON or YAML. Instead, use
[the `jsonencode` function](/docs/language/functions/jsonencode.html) or
[the `yamlencode` function](/docs/language/functions/yamlencode.html) so that Terraform
can be responsible for guaranteeing valid JSON or YAML syntax.

```hcl
  example = jsonencode({
    a = 1
    b = "hello"
  })
```

### Indented Heredocs

The standard heredoc form (shown above) treats all space characters as literal
spaces. If you don't want each line to begin with spaces, then each line must be
flush with the left margin, which can be awkward for expressions in an
indented block:

```hcl
block {
  value = <<EOT
hello
world
EOT
}
```

To improve on this, Terraform also accepts an _indented_ heredoc string variant
that is introduced by the `<<-` sequence:

```hcl
block {
  value = <<-EOT
  hello
    world
  EOT
}
```

In this case, Terraform analyses the lines in the sequence to find the one
with the smallest number of leading spaces, and then trims that many spaces
from the beginning of all of the lines, leading to the following result:

```
hello
  world
```

### Escape Sequences

Backslash sequences are not interpreted as escapes in a heredoc string
expression. Instead, the backslash character is interpreted literally.

Heredocs support two special escape sequences that do not use backslashes:

| Sequence | Replacement |
| ---      | ----        |
| `$${`    | Literal `${`, without beginning an interpolation sequence. |
| `%%{`    | Literal `%{`, without beginning a template directive sequence. |


## String Templates

Within quoted and heredoc string expressions, the sequences `${` and `%{` begin
_template sequences_. Templates let you directly embed expressions into a string
literal, to dynamically construct strings from other values.

### Interpolation

A `${ ... }` sequence is an _interpolation,_ which evaluates the expression
given between the markers, converts the result to a string if necessary, and
then inserts it into the final string:

```hcl
"Hello, ${var.name}!"
```

In the above example, the named object `var.name` is accessed and its value
inserted into the string, producing a result like "Hello, Juan!".

### Directives

A `%{ ... }` sequence is a _directive_, which allows for conditional
results and iteration over collections, similar to conditional
and `for` expressions.

The following directives are supported:

* The `%{if <BOOL>}`/`%{else}`/`%{endif}` directive chooses between two templates based
  on the value of a bool expression:

    ```hcl
    "Hello, %{ if var.name != "" }${var.name}%{ else }unnamed%{ endif }!"
    ```

    The `else` portion may be omitted, in which case the result is an empty
    string if the condition expression returns `false`.

* The `%{for <NAME> in <COLLECTION>}` / `%{endfor}` directive iterates over the
  elements of a given collection or structural value and evaluates a given
  template once for each element, concatenating the results together:

    ```hcl
    <<EOT
    %{ for ip in aws_instance.example.*.private_ip }
    server ${ip}
    %{ endfor }
    EOT
    ```

    The name given immediately after the `for` keyword is used as a temporary
    variable name which can then be referenced from the nested template.

### Whitespace Stripping

To allow template directives to be formatted for readability without adding
unwanted spaces and newlines to the result, all template sequences can include
optional _strip markers_ (`~`), immediately after the opening characters or
immediately before the end. When a strip marker is present, the template
sequence consumes all of the literal whitespace (spaces and newlines) either
before the sequence (if the marker appears at the beginning) or after (if the
marker appears at the end):

```hcl
<<EOT
%{ for ip in aws_instance.example.*.private_ip ~}
server ${ip}
%{ endfor ~}
EOT
```

In the above example, the newline after each of the directives is not included
in the output, but the newline after the `server ${ip}` sequence is retained,
causing only one line to be generated for each element:

```
server 10.1.16.154
server 10.1.16.1
server 10.1.16.34
```

When using template directives, we recommend always using the "heredoc" string
literal form and then formatting the template over multiple lines for
readability. Quoted string literals should usually include only interpolation
sequences.