Amazon ec2 user-data, how does it work?
A very easy example for everyone's understanding.
If you want to create the file /tmp/testfile.txt when the machine gets started, you can simply add these two lines on the User data field.
#!/bin/bash
touch /tmp/testfile.txt
Remember to put the #!/bin/bash at the top before your commands.
When you run the instance (Linux AMI), you can see the User data field content at /var/lib/cloud/instance/user-data.txt
Sorry to post to such an old question, but this seems like the best place to put this additional piece of information.
Most all the AWS documents describe User Data as a property in which to put instance lifecycle startup scripting, that is, the stuff you want to run only when the instance first launches.
This is usually the case, but there has been at least one other person besides myself wanting to perform different scripting on restart, say to fix a broken key or something. And guess what... you can do that using User Data.
Here is the code and the link to the AWS document...
Content-Type: multipart/mixed; boundary="//"
MIME-Version: 1.0
--//
Content-Type: text/cloud-config; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="cloud-config.txt"
#cloud-config
cloud_final_modules:
- [scripts-user, always]
--//
Content-Type: text/x-shellscript; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Content-Disposition: attachment; filename="userdata.txt"
#!/bin/bash
/bin/echo "Hello World." >> /tmp/sdksdfjsdlf
--//
I can find no documentation on this formatting of User Data that allows this to happen. I've tried it out and it works. I have tried to see if it runs on every startup, and it does.
So, if you think you need to do this, I recommend that you backup. Make sure you have a copy of the original User Data, and use the code provided modified to suite, and remove the code upon the next time you stop the instance (to avoid multiple runs of the script).
The user-data is available to the instance with a simple HTTP request at this URL:
http://169.254.169.254/latest/user-data
Amazon EC2 does not put this user-data on the instance directly, though many AMIs have code that instructs the instance to download and process the user-data automatically.
See also:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/user-data.html