You have a Debian, Ubuntu or similar installation, and your /var partition keeps getting full when you upgrade. You check /var/cache/apt/archives and find it full of the usual mix of current and old versions of downloaded packages. Somehow, they haven't been deleted after installation, or you keep downloading them but not installing them. You now need a quick and clever way of cleaning up this mess, so only the latest version of each package remains.

Solution

Here's an appropriate bash invocation:

find /var/cache/apt/archives/ -name '*deb' |
perl -ne 'print "$1\n" if /([^\/_]+)_/'|
sort -u |
while read a; do
    find /var/cache/apt/archives/ -name "${a}_*" | 
    sort -r | tail -n+1 | xargs -rp sudo rm -v;
done

For each package name, this will delete all but the latest .debpackage. Obviously, if there's only one version of a package, it's not deleted. You will be asked to confirm each deletion.

Discussion

Debian package names can't contain an underscore (_). Everything that comes after the underscore in a .deb filename is version (and platform) information. The first three lines of the script locate all packages and get their unique name stems. By using find rather than using something like ls /var/cache/apt/archives/*.deb, we avoid failure in case there are too many packages for the command line¹. It's also a very bad idea to parse ls output.

The second line finds everything between the last / and first _ of each line and prints that out. This is the package name.

The third line collects all of the package names, sorts them and prints out unique names so we don't duplicate any effort.

Then, the while…done loop reads each of these stems, locates all the archived files matching the stem, sorts them in reverse order (i.e. most recent package first) and deletes all but the first one. It does this by running sudo rm -v via xargs The -p option prompts before executing the command and -r doesn't execute if the argument list is empty (i.e. nothing to delete).