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You can use this one-liner to automatically clean up all old kernels (make sure to restart the computer before doing this if you have just updated the kernel):

sudo apt purge $( dpkg --list | grep -P -o "linux-(headers|image)-\d\S+" | grep -v $(uname -r | grep -P -o ".+\d") )

What it does is uninstall (apt purge) the installed packages (dpkg --list) whose names match "linux-headers-[number]" or "linux-image-[number]" (grep -P -o "linux-(headers|image)-\d\S+"), except (grep -v) those corresponding to the current kernel version (uname -r | grep -P -o ".+\d").

Sources:

• CommandLineFu.com - the original inspiration
• Ask Ubuntu - using apt’s "purge" rather than "remove")
• Ask Ubuntu - ensuring the entries returned could be safely removed
• Stack Overflow - using PCRE in grep (\d, \S, etc) and using the -P option
• Super User - using -o to make grep return only the matched content