如何判断AMI为Ubuntu或Amazon Linux及自动获取EC2实例SSH用户名

Great question—relying on root_device_name to guess the SSH username is definitely fragile, since AMI configurations can vary (even official ones might shift over time, and custom AMIs often break this pattern). Let's walk through more reliable approaches:

These methods avoid the pitfalls of root device name checks:

• Parse AMI Names/Descriptions (Most Reliable for External Scripts)
  Official AWS AMIs follow consistent naming conventions that clearly indicate the OS. Use the AWS CLI or SDK to fetch AMI details with aws ec2 describe-images --image-ids <YOUR-AMI-ID>, then look at the Name or Description field:

  • If the name includes Amazon Linux (e.g., amzn2-ami-hvm-2.0.20240320.0-x86_64-gp2 or al2023-ami-2024.0.20240410.0-x86_64-kernel-6.1), the SSH username is ec2-user.
  • If it includes Ubuntu (e.g., ubuntu-jammy-22.04-amd64-server-20240422), the username is ubuntu.
    This works for most custom AMIs too, since many admins retain OS identifiers in the AMI name.

• Check AMI platform-details Field
  The describe-images response also includes a platform-details field that explicitly states the OS version. For example:

  • Amazon Linux 2 returns Amazon Linux 2
  • Ubuntu 22.04 returns Ubuntu Server 22.04 LTS
    You can parse this string directly to map to the correct SSH username.

• Fetch OS Info from Instance Metadata (Most Accurate Post-Launch)
  If you can run commands on the instance after it starts, pull OS details directly from the system itself via instance metadata or local files:

  # Get OS name from instance metadata dynamic data
  curl http://169.254.169.254/latest/dynamic/instance-identity/document | grep "osName"
  # Or read the OS release file
  cat /etc/os-release | grep PRETTY_NAME
  

  Match the output to ec2-user (Amazon Linux) or ubuntu (Ubuntu) — this is foolproof because it comes directly from the running OS.

To reliably determine an AMI's OS, use these methods instead of root device checks:

• Regex Matching on AMI Names
  Use regular expressions to match standardized AMI name patterns:

  • Amazon Linux 2023: ^al2023-ami-
  • Amazon Linux 2: ^amzn2-ami-
  • Ubuntu: ^ubuntu-
    This works for all official AMIs and most custom ones.

• AWS Marketplace Product Codes
  For AMIs from the AWS Marketplace, the product-codes field in describe-images will include the vendor's account ID. For example, Canonical (Ubuntu) uses 099720109477, while Amazon's official AMIs use their own internal codes. Note this only applies to Marketplace AMIs, not custom ones.

• Combine Virtualization & Root Device Type (Fallback)
  While root_device_name alone is unreliable, pairing it with virtualization-type (e.g., hvm) and AMI name hints can add extra validation — but this should be a last resort compared to the methods above.

Root device names like /dev/xvda or /dev/sda1 aren't tied to the OS. Custom Amazon Linux AMIs might use /dev/sda1, and some specialized Ubuntu AMIs could use /dev/xvda. This makes the check prone to false positives/negatives.

内容的提问来源于stack exchange，提问作者Yaroslav Bulatov