---
layout: docs
page_title: nomad var lock reference
description: |-
  The `nomad var lock` command locks a variable, only allowing the lock owner to modify it.
---

# `nomad var lock` command reference

The `var lock` command holds a lock on a [variable][].

## Usage

```plaintext
nomad var lock [options] <lock spec file reference> child...
nomad var lock [options] <path to store variable> [<variable spec file reference>] child
```

The lock command provides a mechanism for simple distributed locking. A lock
is created in the given variable, and only when held, is a child process invoked.

The lock command can be called on an existing variable or an entire new variable
specification can be provided to the command from a file by using an
@-prefixed path to a variable specification file. Items to be stored in the
variable can be supplied using the specification file as well.

Nomad lock launches its children in a shell. By default, Nomad will use the
shell defined in the environment variable SHELL. If SHELL is not defined,
it will default to /bin/sh. It should be noted that not all shells terminate
child processes when they receive SIGTERM. Under Ubuntu, /bin/sh is linked
to dash, which does not terminate its children. In order to ensure that
child processes are killed when the lock is lost, be sure to set the SHELL
environment variable appropriately, or run without a shell by setting -shell=false.

If [ACLs][] are enabled, this command requires the 'variables:write' capability
for the destination namespace and path.

## Restrictions

Variable paths are restricted to [RFC3986][] URL-safe characters that don't
conflict with the use of the characters `@` and `.` in template blocks. This
includes alphanumeric characters and the special characters `-`, `_`, `~`, and
`/`. Paths may be up to 128 bytes long. The following regex matches the allowed
paths: `^[a-zA-Z0-9-_~/]{1,128}$`

The keys for the items in a variable may contain any character, but keys
containing characters outside the set of Unicode letters, Unicode digits, and
the underscore (`_`) can not be read directly using dotted references in Nomad's
template engine. Instead, they require the use of the `index` template function
to directly access their values. This does not impact cases where the keys and
values are read using the `range` function.

Variable items are restricted to 64KiB in size. This limit is calculated by
taking the sum of the length in bytes of all of the unencrypted keys and values.

## General options

@include 'general_options.mdx'

## Lock options
- `-verbose`: Provides additional information via standard error to preserve
  standard output (stdout) for redirected output.

- `ttl`: Optional, TTL for the lock, time the variable will be locked. Defaults to 15s.

- `delay`: Optional, time the variable is blocked from locking when a lease is not renewed.
	Defaults to 15s.

- `max-retry`:Optional, max-retry up to this number of times if Nomad returns a 500 error
	while monitoring the lock. This allows riding out brief periods of
	unavailability without causing leader elections, but increases the amount of
	time required to detect a lost lock in some cases. Defaults to 5. Set to 0 to
	disable.

- `shell`: Optional, use a shell to run the command (can set a custom shell via
	the SHELL environment variable). The default value is true.

## Examples

Attempts to acquire a lock over the variable at path "secret/creds" for a time of
15s and executes  `nomad job run webapp.nomad.hcl` if it succeeds:

```shell-session
$ nomad var lock -ttl=15s secret/creds "nomad job run webapp.nomad.hcl"
```

The data can also be consumed from a file on disk by prefixing with the "@"
symbol. For example, you can store a variable using a specification created with
the `nomad var init` command.

```shell-session
$ nomad var lock secret/foo @spec.nv.json `nomad job run webapp.nomad.hcl`
```

[variable]: /nomad/docs/concepts/variables
[varspec]:  /nomad/docs/other-specifications/variables
[ACL Policy]: /nomad/docs/other-specifications/acl-policy#variables
[RFC3986]: https://www.rfc-editor.org/rfc/rfc3986#section-2
[ACL]: /nomad/docs/other-specifications/acl-policy#variables

