Scenario

When creating a CVM, you can configure an instance by specifying custom data. During the first launch of the CVM, the custom data will be passed into the CVM in text format and be executed. If you purchase multiple CVMs at a time, all CVMs will execute the custom data upon their first launch.
This article describes how to pass a Shell script when launching a Linux CVM for the first time.

Notes

• Linux operating systems that support custom data include:
  • 64-bit operating system: CentOS 6.8 64-bit or later versions, Ubuntu Server 14.04.1 LTS 64-bit or later versions, and suse42.3x86_64
  • 32-bit operating system: CentOS 6.8 32-bit or later versions
• A command can be executed by passing text only when a CVM is launched for the first time.
• Custom data must be Base64 encoded and then passed. For a compatible format, encode the custom data in Linux environment.
• Execute the custom data as root. Therefore, the sudo command is not required in the script. The root user can access all the files you created. If you need to grant other users with the access permission, modify the permission in the script.
• During launch, executing custom data tasks will increase the startup time of the CVM. Please wait a few minutes until the tasks are completed, and then test whether the tasks are executed successfully
• In this example, Shell script must start with #! and the path to the interpreter reading the script (usually /bin/bash).

Directions

Writing a Shell script

1. Run the following command to create a Shell script named “script_text.sh”.

   vi script_text.sh

2. Press i to switch to the editing mode, enter the following and save the “script_text.sh” script.

   #!/bin/bash
   echo "Hello Tencent Cloud."

   Shell script must start with #! and the path to the interpreter reading the script (usually /bin/bash). For more information on Shell script, see BASH Programming of the Linux Documentation Project (tldp.org) (http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO.html).

Encoding the script with Base64

1. Run the following command to encode the “script_text.sh” script with Base64.

   # Base64 encoded script
   base64 script_text.sh

   You will see the following information:

   # Encoded result
   IyEvYmluL2Jhc2gKZWNobyAiSGVsbG8gVGVuY2VudCBDbG91ZC4iCg==

2. Run the following command to verify the Base64 encoded result of the script.

   # Decode the returned result with Base64 and verify whether it is the command to be executed.
   echo "IyEvYmluL2Jhc2gKZWNobyAiSGVsbG8gVGVuY2VudCBDbG91ZC4iCg==" | base64 -d

Passing the text

You can launch an instance through multiple methods, and here we introduce two of them. Choose a method according to your requirements:

• Using the official website or the console
• Using API

Using the official website or the console

1. Refer to Creating Instances to purchase an instance, and click Advanced Settings in “4. Security Group and CVM”, as shown below:
   
2. In Advanced Settings, enter the returned result of Base64 encoded script in the Custom Data text box, as shown below:
   For example, the Base64 encoded result of the script_text script is IyEvYmluL2Jhc2gKZWNobyAiSGVsbG8gVGVuY2VudCBDbG91ZC4iCg==.
   
3. Create an CVM instance as prompted by the page.

   Tencent Cloud CVM executes the script using the open-source software cloud-init. For more information about cloud-init, see cloud-init's official website.

Using API

When creating a CVM by using API, you can pass the text by assigning the value of the encoded result returned in Base64 encoded script to the UserData parameter of the RunInstances API.
The following is a sample CVM creation request with UserData:

https://cvm.tencentcloudapi.com/?Action=RunInstances
  &Version=2017-03-12
  &Placement.Zone=ap-guangzhou-2
  &ImageId=img-pmqg1cw7
  &UserData=IyEvYmluL2Jhc2gKZWNobyAiSGVsbG8gVGVuY2VudCBDbG91ZC4iCg==
  &<Common request parameters>