How to 'apt-cache search'&'apt-file search' by distribut

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/etc/apt/sources.list:

deb http://http.us.debian.org/debian
stable main contrib non-free
deb http://http.us.debian.org/debian
[b:9a928ec131]unstable[/b:9a928ec131] main contrib non-free
deb-src http://http.us.debian.org/debian
[b:9a928ec131]stable[/b:9a928ec131] main contrib non-free
deb-src http://http.us.debian.org/debian
[b:9a928ec131]unstable[/b:9a928ec131] main contrib non-free
deb http://non-us.debian.org/debian-non-US stable/non-US main contrib
non-free
deb http://security.debian.org stable/updates main contrib non-free

What arguement should I use with 'apt-cache search' and 'apt-file
search' to specific the distribution?

Eg. If I want to search for source packages (or normal packages) in
unstable distribution, what would the complete command for 'apt-cache
search' and 'apt-file search'?

Thanks !
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