How can I find out the instance id of an ec2 instance from within the ec2 instance?

See the EC2 documentation on the subject.

Run:

wget -q -O - http://169.254.169.254/latest/meta-data/instance-id

If you need programmatic access to the instance ID from within a script,

die() { status=$1; shift; echo "FATAL: $*"; exit $status; }
EC2_INSTANCE_ID="`wget -q -O - http://169.254.169.254/latest/meta-data/instance-id || die \"wget instance-id has failed: $?\"`"

Here is an example of a more advanced use (retrieve instance ID as well as availability zone and region, etc.):

EC2_INSTANCE_ID="`wget -q -O - http://169.254.169.254/latest/meta-data/instance-id || die \"wget instance-id has failed: $?\"`"
test -n "$EC2_INSTANCE_ID" || die 'cannot obtain instance-id'
EC2_AVAIL_ZONE="`wget -q -O - http://169.254.169.254/latest/meta-data/placement/availability-zone || die \"wget availability-zone has failed: $?\"`"
test -n "$EC2_AVAIL_ZONE" || die 'cannot obtain availability-zone'
EC2_REGION="`echo \"$EC2_AVAIL_ZONE\" | sed -e 's:\([0-9][0-9]*\)[a-z]*\$:\\1:'`"

You may also use curl instead of wget, depending on what is installed on your platform.