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---
title: 'Shell script for gathering imdsv2 instance metadata on AWS ec2'
description: 'An ansible fact I wrote'
date: '2024-10-12'
tags:
- ansible
- aws
---
## Introduction
I wrote a shell script to gather ec2 instance metadata with an ansible fact.
## The script
I am using POSIX `/bin/sh` because I wanted to support a variety of operating systems. Besides that, the only dependency is `curl`:
``` shell
#!/bin/sh
set -eu
metadata() {
local METHOD=$1
local URI_PATH=$2
local TOKEN="${3:-}"
local HEADER
if [ -z "${TOKEN}" ]; then
HEADER='X-aws-ec2-metadata-token-ttl-seconds: 21600' # request a 6 hours token
else
HEADER="X-aws-ec2-metadata-token: ${METADATA_TOKEN}"
fi
curl -sSfL --request "${METHOD}" \
"http://169.254.169.254/latest${URI_PATH}" \
--header "${HEADER}"
}
METADATA_TOKEN=$(metadata PUT /api/token)
KEYS=$(metadata GET /meta-data/tags/instance "${METADATA_TOKEN}")
PREFIX='{'
for KEY in $KEYS; do
VALUE=$(metadata GET "/meta-data/tags/instance/${KEY}" "${METADATA_TOKEN}")
printf '%s"%s":"%s"' "${PREFIX}" "${KEY}" "${VALUE}"
PREFIX=','
done
printf '}'
```
## Bonus version without depending on curl
Depending on curl can be avoided. If you are willing to use netcat instead and be declared a madman by your colleagues, you can rewrite the function with:
``` shell
metadata() {
local METHOD=$1
local URI_PATH=$2
local TOKEN="${3:-}"
local HEADER
if [ -z "${TOKEN}" ]; then
HEADER='X-aws-ec2-metadata-token-ttl-seconds: 21600' # request a 6 hours token
else
HEADER="X-aws-ec2-metadata-token: ${METADATA_TOKEN}"
fi
printf "${METHOD} /latest${URI_PATH} HTTP/1.0\r\n%s\r\n\r\n" \
"${HEADER}" \
| nc -w 5 169.254.169.254 80 | tail -n 1
}
```
## Deploying an ansible fact
I deploy the script this way:
``` yaml
- name: 'Deploy ec2 metadata fact gathering script'
copy:
src: 'ec2_metadata.sh'
dest: '/etc/ansible/facts.d/ec2_metadata.fact'
owner: 'root'
mode: '0500'
register: 'ec2_metadata_fact'
- name: 'reload facts'
setup: 'filter=ansible_local'
when: 'ec2_metadata_fact.changed'
```
## Conclusion
It works, is simple and I like it. I am happy!
|
