User data formats

User data that will be acted upon by cloud-init must be in one of the following types.

Cloud config data

Cloud-config is the simplest way to accomplish some things via user data. Using cloud-config syntax, the user can specify certain things in a human-friendly format.

These things include:

  • apt upgrade should be run on first boot

  • a different apt mirror should be used

  • additional apt sources should be added

  • certain SSH keys should be imported

  • and many more…


This file must be valid YAML syntax.

See the Cloud config examples section for a commented set of examples of supported cloud config formats.

Begins with: #cloud-config or Content-Type: text/cloud-config when using a MIME archive.


Cloud config data can also render cloud instance metadata variables using jinja templating. See Instance metadata for more information.

User data script

Typically used by those who just want to execute a shell script.

Begins with: #! or Content-Type: text/x-shellscript when using a MIME archive.

User data scripts can optionally render cloud instance metadata variables using jinja templating. See Instance metadata for more information.

Example script

Create a script file that contains the following:

echo "Hello World.  The time is now $(date -R)!" | tee /root/output.txt

Now run:

$ euca-run-instances --key mykey --user-data-file ami-a07d95c9

Kernel command line

When using the NoCloud datasource, users can pass user data via the kernel command line parameters. See the NoCloud datasource and Kernel command line documentation for more details.

Gzip compressed content

Content found to be gzip compressed will be uncompressed. The uncompressed data will then be used as if it were not compressed. This is typically useful because user data is limited to ~16384 [1] bytes.

MIME multi-part archive

This list of rules is applied to each part of this multi-part file. Using a MIME multi-part file, the user can specify more than one type of data.

For example, both a user data script and a cloud-config type could be specified.

Supported content-types are listed from the cloud-init subcommand make-mime:

$ cloud-init devel make-mime --list-types

Example output:


Helper subcommand to generate MIME messages

The cloud-init make-mime subcommand can also generate MIME multi-part files.

The make-mime subcommand takes pairs of (filename, “text/” mime subtype) separated by a colon (e.g., config.yaml:cloud-config) and emits a MIME multipart message to stdout.


Create user data containing both a cloud-config (config.yaml) and a shell script (

$ cloud-init devel make-mime -a config.yaml:cloud-config -a > userdata

Create user data containing 3 shell scripts:

  • - run every boot

  • - run once per instance

  • - run once

$ cloud-init devel make-mime -a -a -a

include file

This content is an include file.

The file contains a list of URLs, one per line. Each of the URLs will be read and their content will be passed through this same set of rules, i.e., the content read from the URL can be gzipped, MIME multi-part, or plain text. If an error occurs reading a file the remaining files will not be read.

Begins with: #include or Content-Type: text/x-include-url when using a MIME archive.


This content is boothook data. It is stored in a file under /var/lib/cloud and executed immediately. This is the earliest hook available. Note, that there is no mechanism provided for running only once. The boothook must take care of this itself.

It is provided with the instance id in the environment variable INSTANCE_ID. This could be made use of to provide a ‘once-per-instance’ type of functionality.

Begins with: #cloud-boothook or Content-Type: text/cloud-boothook when using a MIME archive.


This is a part-handler: It contains custom code for either supporting new mime-types in multi-part user data, or overriding the existing handlers for supported mime-types. It will be written to a file in /var/lib/cloud/data based on its filename (which is generated).

This must be Python code that contains a list_types function and a handle_part function. Once the section is read the list_types method will be called. It must return a list of mime-types that this part-handler handles. Since MIME parts are processed in order, a part-handler part must precede any parts with mime-types it is expected to handle in the same user data.

The handle_part function must be defined like:

def handle_part(data, ctype, filename, payload):
  # data = the cloudinit object
  # ctype = "__begin__", "__end__", or the mime-type of the part that is being handled.
  # filename = the filename of the part (or a generated filename if none is present in mime data)
  # payload = the parts' content

Cloud-init will then call the handle_part function once before it handles any parts, once per part received, and once after all parts have been handled. The '__begin__' and '__end__' sentinels allow the part handler to do initialisation or teardown before or after receiving any parts.

Begins with: #part-handler or Content-Type: text/part-handler when using a MIME archive.


 3def list_types():
 4    # return a list of mime-types that are handled by this module
 5    return(["text/plain", "text/go-cubs-go"])
 7def handle_part(data, ctype, filename, payload):
 8    # data: the cloudinit object
 9    # ctype: '__begin__', '__end__', or the specific mime-type of the part
10    # filename: the filename for the part, or dynamically generated part if
11    #           no filename is given attribute is present
12    # payload: the content of the part (empty for begin or end)
13    if ctype == "__begin__":
14       print("my handler is beginning")
15       return
16    if ctype == "__end__":
17       print("my handler is ending")
18       return
20    print(f"==== received ctype={ctype} filename={filename} ====")
21    print(payload)
22    print(f"==== end ctype={ctype} filename={filename}")

Also, this blog post offers another example for more advanced usage.

Disabling user data

Cloud-init can be configured to ignore any user data provided to instance. This allows custom images to prevent users from accidentally breaking closed appliances. Setting allow_userdata: false in the configuration will disable cloud-init from processing user data.