open-nomad/website/source/docs/drivers/exec.html.md

layout	page_title	sidebar_current	description
docs	Drivers: Exec	docs-drivers-exec	The Exec task driver is used to run binaries using OS isolation primitives.

Isolated Fork/Exec Driver

Name: exec

The exec driver is used to simply execute a particular command for a task. However, unlike raw_exec it uses the underlying isolation primitives of the operating system to limit the task's access to resources. While simple, since the exec driver can invoke any command, it can be used to call scripts or other wrappers which provide higher level features.

Task Configuration

task "webservice" {
  driver = "exec"

  config {
    command = "my-binary"
    args    = ["-flag", "1"]
  }
}

The exec driver supports the following configuration in the job spec:

• command - The command to execute. Must be provided. If executing a binary that exists on the host, the path must be absolute. If executing a binary that is downloaded from an artifact, the path can be relative from the allocations's root directory.

• args - (Optional) A list of arguments to the command. References to environment variables or any interpretable Nomad variables will be interpreted before launching the task.

Examples

To run a binary present on the Node:

task "example" {
  driver = "exec"

  config {
    # When running a binary that exists on the host, the path must be absolute.
    command = "/bin/sleep"
    args    = ["1"]
  }
}

To execute a binary downloaded from an artifact:

task "example" {
  driver = "exec"

  config {
    command = "name-of-my-binary"
  }

  artifact {
    source = "https://internal.file.server/name-of-my-binary"
    options {
      checksum = "sha256:abd123445ds4555555555"
    }
  }
}

Client Requirements

The exec driver can only be run when on Linux and running Nomad as root. exec is limited to this configuration because currently isolation of resources is only guaranteed on Linux. Further, the host must have cgroups mounted properly in order for the driver to work.

If you are receiving the error:

* Constraint "missing drivers" filtered <> nodes

and using the exec driver, check to ensure that you are running Nomad as root. This also applies for running Nomad in -dev mode.

Client Attributes

The exec driver will set the following client attributes:

• driver.exec - This will be set to "1", indicating the driver is available.

Resource Isolation

The resource isolation provided varies by the operating system of the client and the configuration.

On Linux, Nomad will use cgroups, and a chroot to isolate the resources of a process and as such the Nomad agent must be run as root.

Chroot

The chroot is populated with data in the following directories from the host machine:

[
  "/bin",
  "/etc",
  "/lib",
  "/lib32",
  "/lib64",
  "/run/resolvconf",
  "/sbin",
  "/usr",
]

The task's chroot is populated by linking or copying the data from the host into the chroot. Note that this can take considerable disk space. Since Nomad v0.5.3, the client manages garbage collection locally which mitigates any issue this may create.

This list is configurable through the agent client configuration file.