Configuration by conf.d structure:

All files in /etc/squid/conf.d are included in config (see also Squid ConfigIncludes.

Files can additionally be set up to generate or filter configuration content based on a comment in the first line:

• #! /some/command: The filename is put as first arg and the command is executed (shebang).
• #| /some/command: File is piped into the command. For environment variable expansion use #| envsubst (see docs
• An other first line: Just copies the file