240 lines
6.7 KiB
Plaintext
240 lines
6.7 KiB
Plaintext
---
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layout: "docs"
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page_title: "Variable Interpolation"
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sidebar_current: "docs-variable-interpolation"
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description: |-
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Learn about the Nomad's interpolation and interpreted variables.
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---
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# Variable Interpolation
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Nomad supports interpreting two classes of variables: node attributes and
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runtime environment variables. Node attributes are interpretable in constraints,
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task environment variables, and certain driver fields. Runtime environment
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variables are not interpretable in constraints because they are only defined
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once the scheduler has placed them on a particular node.
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The syntax for interpreting variables is `${variable}`. An example and a
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comprehensive list of interpretable fields can be seen below:
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```hcl
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task "docs" {
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driver = "docker"
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# Drivers support interpreting node attributes and runtime environment
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# variables
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config {
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image = "my-app"
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# Interpret runtime variables to inject the address to bind to and the
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# location to write logs to.
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args = [
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"--bind", "${NOMAD_ADDR_RPC}",
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"--logs", "${NOMAD_ALLOC_DIR}/logs",
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]
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port_map {
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RPC = 6379
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}
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}
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# Constraints only support node attributes as runtime environment variables
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# are only defined after the task is placed on a node.
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constraint {
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attribute = "${attr.kernel.name}"
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value = "linux"
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}
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# Environment variables are interpreted and can contain both runtime and
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# node attributes. These environment variables are passed into the task.
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env {
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"DC" = "Running on datacenter ${node.datacenter}"
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"VERSION" = "Version ${NOMAD_META_VERSION}"
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}
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# Meta keys are also interpretable.
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meta {
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VERSION = "v0.3"
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}
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}
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```
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## Node Variables <a id="interpreted_node_vars"></a>
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Below is a full listing of node attributes that are interpretable. These
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attributes are interpreted by __both__ constraints and within the task and
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driver.
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<table class="table table-bordered table-striped">
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<tr>
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<th>Variable</th>
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<th>Description</th>
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<th>Example Value</th>
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</tr>
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<tr>
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<td><tt>${node.unique.id}</tt></td>
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<td>36 character unique client identifier</td>
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<td><tt>9afa5da1-8f39-25a2-48dc-ba31fd7c0023</tt></td>
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</tr>
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<tr>
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<td><tt>${node.datacenter}</tt></td>
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<td>Client's datacenter</td>
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<td><tt>dc1</tt></td>
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</tr>
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<tr>
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<td><tt>${node.unique.name}</tt></td>
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<td>Client's name</td>
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<td><tt>nomad-client-10-1-2-4</tt></td>
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</tr>
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<tr>
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<td><tt>${node.class}</tt></td>
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<td>Client's class</td>
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<td><tt>linux-64bit</tt></td>
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</tr>
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<tr>
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<td><tt>${attr.<property>}</tt></td>
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<td>Property given by <tt>property</tt> on the client</td>
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<td><tt>${attr.cpu.arch} => amd64</tt></td>
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</tr>
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<tr>
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<td><tt>${meta.<key>}</tt></td>
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<td>Metadata value given by <tt>key</tt> on the client</td>
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<td><tt>${meta.foo} => bar</tt></td>
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</tr>
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</table>
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Below is a table documenting common node properties:
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<table class="table table-bordered table-striped">
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<tr>
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<th>Property</th>
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<th>Description</th>
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</tr>
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<tr>
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<td><tt>${attr.cpu.arch}</tt></td>
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<td>CPU architecture of the client (e.g. <tt>amd64</tt>, <tt>386</tt>)</td>
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</tr>
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<tr>
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<td><tt>${attr.cpu.numcores}</tt></td>
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<td>Number of CPU cores on the client</td>
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</tr>
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<tr>
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<td><tt>${attr.cpu.totalcompute}</tt></td>
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<td>
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<tt>cpu.frequency × cpu.numcores</tt> but may be overridden by <tt>client.cpu_total_compute</tt>
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</td>
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</tr>
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<tr>
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<td><tt>${attr.consul.datacenter}</tt></td>
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<td>The Consul datacenter of the client (if Consul is found)</td>
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</tr>
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<tr>
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<td><tt>${attr.driver.<property>}</tt></td>
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<td>See the [task drivers](/docs/drivers/index.html) for property documentation</td>
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</tr>
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<tr>
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<td><tt>${attr.unique.hostname}</tt></td>
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<td>Hostname of the client</td>
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</tr>
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<tr>
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<td><tt>${attr.unique.network.ip-address}</tt></td>
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<td>The IP address fingerprinted by the client and from which task ports are allocated</td>
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</tr>
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<tr>
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<td><tt>${attr.kernel.name}</tt></td>
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<td>Kernel of the client (e.g. <tt>linux</tt>, <tt>darwin</tt>)</td>
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</tr>
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<tr>
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<td><tt>${attr.kernel.version}</tt></td>
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<td>Version of the client kernel (e.g. <tt>3.19.0-25-generic</tt>, <tt>15.0.0</tt>)</td>
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</tr>
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<tr>
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<td><tt>${attr.platform.aws.ami-id}</tt></td>
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<td>AMI ID of the client (if on AWS EC2)</td>
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</tr>
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<tr>
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<td><tt>${attr.platform.aws.instance-type}</tt></td>
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<td>Instance type of the client (if on AWS EC2)</td>
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</tr>
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<tr>
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<td><tt>${attr.os.name}</tt></td>
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<td>Operating system of the client (e.g. <tt>ubuntu</tt>, <tt>windows</tt>, <tt>darwin</tt>)</td>
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</tr>
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<tr>
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<td><tt>${attr.os.version}</tt></td>
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<td>Version of the client OS</td>
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</tr>
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</table>
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Here are some examples of using node attributes and properties in a job file:
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```hcl
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job "docs" {
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# This will constrain this job to only run on 64-bit clients.
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constraint {
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attribute = "${attr.cpu.arch}"
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value = "amd64"
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}
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# This will restrict the job to only run on clients with 4 or more cores.
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# Note: you may also declare a resource requirement for CPU for a task.
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constraint {
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attribute = "${cpu.numcores}"
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operator = ">="
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value = "4"
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}
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# Only run this job on a memory-optimized AWS EC2 instance.
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constraint {
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attribute = "${attr.platform.aws.instance-type}"
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value = "m4.xlarge"
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}
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}
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```
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## Environment Variables <a id="interpreted_env_vars"></a>
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The following are runtime environment variables that describe the environment
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the task is running in. These are only defined once the task has been placed on
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a particular node and as such can not be used in constraints.
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Environment variables should be enclosed in brackets `${...}` for
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interpolation.
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### Dots in Variables <a id="dots_in_vars"></a>
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Starting in Nomad 0.9, task configuration interpolation requires variables to
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be valid identifiers. While this does not affect default variables or common
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custom variables, it is possible to define a variable that is not a valid
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identifier:
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```hcl
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env {
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"valid.name" = "ok"
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"invalid...name" = "not a valid identifier"
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}
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```
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The environment variable `invalid...name` cannot be interpolated using the
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standard `"${invalid...name}"` syntax. The dots will be interpreted as object
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notation so multiple consecutive dots are invalid.
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To continue supporting all user environment variables Nomad 0.9 added a new
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`env` variable which allows accessing any environment variable through index
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syntax:
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```hcl
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task "redis" {
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driver = "docker"
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config {
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image = "redis:3.2"
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labels {
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label1 = "${env["invalid...name"]}"
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label2 = "${env["valid.name"]}"
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}
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}
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}
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```
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<%= partial "envvars.html.md" %>
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