Proxies for curl
The fastest way to find out whether a set of addresses is worth paying for, before any of it reaches your collector.
Setting the address
One flag carries the address and the protocol together. Credentials go either in the same string or in a separate flag.
Quote the whole thing if the password contains characters the shell will try to interpret.
Environment variables are read selectively
The tool itself honours them. Other programs on the same machine may ignore them entirely, so verify per program.
Keep a no-proxy list for internal addresses, or internal calls start travelling through the intermediary.
Checking a whole list
Loop over the file, record the response code and the total time for each address, then sort by time. Three minutes and you know what you bought.
The slowest few addresses are what will hold up every run you do.
The first check of all
One request to an address-reporting service confirms the traffic actually goes where you think.
for a in $(cat pool.txt); do
curl -s -o /dev/null -w "$a %{http_code} %{time_total}\n" -x $a https://example.org/
done | sort -k3 -n | tail -5