The `tail` command may be used for this. It gives you the last few (10 by default) lines of input as output (the "tail" of it).
With the `-n` flag you may specify exactly how much of the tail you'd like to have:
aws ec2 ... | sort | tail -n 1
To verify that it follows the right format, you may do
line="$( aws ec2 ... | sort | tail -n 1 )"
if [[ ! "$line" =~ ^amzn-ami-[0-9]{4}\.[0-9]{2}\.[a-z]-amazon-ecs-optimized\ ami-[0-9a-z]{8}$ ]]; then
echo "bad string"
exit 1
fi
If you only want _the last bit_ of the last line, use `cut` to cut it out:
line="$( aws ec2 ... | sort | tail -n 1 | cut -d ' ' -f 2 )"
The `-d ' ' -f 2` bit says "use space as the field delimiter and give me the second field".
Then the regular expression becomes shorter too:
if [[ ! "$line" =~ ^ami-[0-9a-z]{8}$ ]]; then
echo "bad string"
exit 1
fi