extract and output the changelog from a .deb file, when possible

changelog_from_deb.sh

changelog_from_deb () {
    # won't work when packages symlink their docs from another package from the same source;
    # you'll get "No changelog found."

    t="$(mktemp -d)"
    p="$(dpkg-deb -f "$1" Package)"
    fail=1
    dpkg-deb --fsys-tarfile "$1" | \
        tar -x --wildcards -C $t ./usr/share/doc/"$p"/changelog\* 2>/dev/null
    for f in changelog.Debian.gz changelog.gz; do
        if [ -e "$t/usr/share/doc/$p/$f" ]; then
            gzip -dc < "$t/usr/share/doc/$p/$f"
            fail=0
            break
        fi
    done
    rm -rf $t
    [ $fail -eq 0 ] || echo "No changelog found." >&2
    return $fail
}

Very handy - thanks Paul.
Quick hack based on the code above, that avoids the temp directory, and writes to stdout.

#!/bin/bash
if [ ! -f $1 ]; then
echo "Package not found." >&2
exit
fi

p="$(dpkg-deb -f "$1" Package)"
dpkg-deb --fsys-tarfile $1 | tar -xf - --to-stdout --wildcards ./usr/share/doc/"$p"/changelog* | gzip -d 2>/dev/null
if [ $? -ne 0 ]; then
echo "Changelog not found." >&2
fi

For .rpm that would be

rpm -qp --changelog "$1"