I frequently run into the issue described in Not enough free disk space when upgrading, because the boot partition created by the Ubuntu installation is too small. When cleaning up as described in the accepted answer, I found a bunch of things that look like they belong to older kernels.
$ uname -r
3.19.0-51-generic
$ ll /boot
total 96958
drwxr-xr-x 4 root root 3072 Apr 14 09:54 ./
drwxr-xr-x 23 root root 4096 Mar 2 10:08 ../
-rw-r--r-- 1 root root 1271904 Jan 22 04:40 abi-3.19.0-49-generic
-rw-r--r-- 1 root root 1271993 Feb 27 00:51 abi-3.19.0-51-generic
-rw-r--r-- 1 root root 177800 Jan 22 04:40 config-3.19.0-49-generic
-rw-r--r-- 1 root root 177800 Feb 27 00:51 config-3.19.0-51-generic
drwxr-xr-x 5 root root 1024 Apr 14 09:54 grub/
-rw-r--r-- 1 root root 9796008 Jan 21 11:09 initrd.img-3.19.0-31-generic
-rw-r--r-- 1 root root 32580082 Feb 17 13:02 initrd.img-3.19.0-49-generic
-rw-r--r-- 1 root root 32580270 Mar 2 10:08 initrd.img-3.19.0-51-generic
drwx------ 2 root root 12288 Apr 17 2015 lost+found/
-rw-r--r-- 1 root root 164216 Mar 6 2015 memtest86+.bin
-rw-r--r-- 1 root root 165892 Mar 6 2015 memtest86+.elf
-rw-r--r-- 1 root root 166396 Mar 6 2015 memtest86+_multiboot.bin
-rw------- 1 root root 3624223 Jan 22 04:40 System.map-3.19.0-49-generic
-rw------- 1 root root 3624551 Feb 27 00:51 System.map-3.19.0-51-generic
-rw------- 1 root root 6625856 Jan 22 04:40 vmlinuz-3.19.0-49-generic
-rw------- 1 root root 6627648 Feb 27 00:51 vmlinuz-3.19.0-51-generic
$ dpkg -l | grep 3.19.0
ii linux-generic 3.19.0.51.50 amd64 Complete Generic Linux kernel and headers
ii linux-headers-3.19.0-26 3.19.0-26.28 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-26-generic 3.19.0-26.28 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-28 3.19.0-28.30 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-28-generic 3.19.0-28.30 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-30 3.19.0-30.34 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-30-generic 3.19.0-30.34 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-31 3.19.0-31.36 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-31-generic 3.19.0-31.36 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-32 3.19.0-32.37 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-32-generic 3.19.0-32.37 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-33 3.19.0-33.38 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-33-generic 3.19.0-33.38 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-37 3.19.0-37.42 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-37-generic 3.19.0-37.42 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-42 3.19.0-42.48 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-42-generic 3.19.0-42.48 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-43 3.19.0-43.49 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-43-generic 3.19.0-43.49 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-49 3.19.0-49.55 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-49-generic 3.19.0-49.55 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-3.19.0-51 3.19.0-51.58 all Header files related to Linux kernel version 3.19.0
ii linux-headers-3.19.0-51-generic 3.19.0-51.58 amd64 Linux kernel headers for version 3.19.0 on 64 bit x86 SMP
ii linux-headers-generic 3.19.0.51.50 amd64 Generic Linux kernel headers
ii linux-image-3.19.0-49-generic 3.19.0-49.55 amd64 Linux kernel image for version 3.19.0 on 64 bit x86 SMP
ii linux-image-3.19.0-51-generic 3.19.0-51.58 amd64 Linux kernel image for version 3.19.0 on 64 bit x86 SMP
ii linux-image-extra-3.19.0-49-generic 3.19.0-49.55 amd64 Linux kernel extra modules for version 3.19.0 on 64 bit x86 SMP
ii linux-image-extra-3.19.0-51-generic 3.19.0-51.58 amd64 Linux kernel extra modules for version 3.19.0 on 64 bit x86 SMP
ii linux-image-generic 3.19.0.51.50 amd64 Generic Linux kernel image
ii linux-libc-dev:amd64 3.19.0-51.58 amd64 Linux Kernel Headers for development
It looks like there are some things left from older versions, like linux-headers-3.19.0-26 in the dpkg output. But there is also /boot/initrd.img-3.19.0-31-generic, which seems to be from a way older one, as mine is 3.19.0.51-generic currently.
Which of these things can I remove safely? Should I alsways apt-get purge them, or are there also things I can just rm?
