List all RPM packages installed from repo "X"

CentOS / RHEL / Fedora 22 or earlier

yum list installed | grep @epel

dnf list installed | grep @epel

As of RHEL6, as suggested in this stackoverflow answer, there's a more thorough method of doing this with yumdb:

yumdb search from_repo REPOID*

The repoid takes wild cards.

If we're going to cheat and pipe the output, then we can achieve pretty-print effect as well. Here's one using awk

yumdb search from_repo REPOID* |awk -F"\n" '{ RS=""; print $1 }'

Using egrep or grep -e

yumdb search from_repo REPOID* |egrep -v '(from_repo|^$)'

To list all available packages in a specified repository, do:

repoquery -a --repoid=REPONAME

dnf repo-pkgs <repoid> list installed

The command above uses DNF to list the packages installed from the <repoid>. Note repo-pkgs is just an alias of repository-packages.

From the man pages:

man dnf | grep "repository-packages.*list.*installed" -A 1