On boot, cannot type the password into (graphical) prompt in order to decrypt the laptop's disk and boot into the full desktop.

Cause: /boot fills up and newer kernels are either not installed or are installed buggily.

Workaround: reboot to grub, Ctrl-Alt-Del works, and select a different, working, kernel. So far, I've discovered that the oldest one (installed with original distro, presumably) works.

Most of the bulk space is taken up by initrd.img files

Suggested by Jez Cope on 2017-04-20:

sudo update-initramfs -k all -u

Note to previous: This does appear to successfully do what is intended: the initrd.img* files are rebuilt (they change sizes a little bit; hope it doesn't matter).

Suggested by Will Furnass on 2017-04-26:

Move the (suspect) initrd.img file away and create it fresh with

update-initramfs -c -k 3.19.whatever

(you have to specify the full filename suffix, eg 3.19.0-65-generic)

List kernels.

sudo dpkg -l "linux-image*"

(see man dkpg-query to interpret the output)

List packages owning each file in /boot/:

dpkg -S /boot/*

Nots to previous: the initrd.img* files and the vmlinux*.efi.signed files are not owned by any package. (On my current system) The only packages are memtest86+ and a selection of different versions of the Linux kernel: linux-image-*-generic

The initrd.img* files are cpio archives. to list their contents:

cpio -t < initrd.img.whatever

On my current system, those files are far too big for the cpio archive that the above command lists (the cpio archive is 30 blocks or 15KB). Following the cpio archive in the file is a gzip file:

{ cpio -t > /dev/null ; file - ; } < initrd.img-3.19.0-28-generic

of:

{ cpio -t > /dev/null ; zcat | file - ; } < initrd.img-3.19.0-28-generic

a cpio file! (it's cpio files all the way down).

Note to previous: After cpio reads the archive from the file, the file offset is left in place, at exactly the right point to read the gzip archive.

So to read the file listing of the inner cpio archive:

{ cpio -t > /dev/null ; zcat ; } < initrd.img-3.19.0-28-generic  | cpio -t