What does this 'apt-get remove' instruction do?

What does the following code do?

I found it on internet so I copied it and pasted it into terminal but when I installed a new kernel it only detects the old one.

sudo apt-get remove --purge $(dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')

Let's break the code one by one (for my system):

$ dpkg -l 'linux-image-*'
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                          Version             Architecture        Description
+++-=============================-===================-===================-================================================================
un  linux-image-3.0               <none>              <none>              (no description available)
ii  linux-image-3.13.0-32-generic 3.13.0-32.57        amd64               Linux kernel image for version 3.13.0 on 64 bit x86 SMP
ii  linux-image-extra-3.13.0-32-g 3.13.0-32.57        amd64               Linux kernel extra modules for version 3.13.0 on 64 bit x86 SMP
ii  linux-image-generic           3.13.0.32.38        amd64               Generic Linux kernel image


$ dpkg -l 'linux-image-*' | sed '/^ii/!d'
ii  linux-image-3.13.0-32-generic                         3.13.0-32.57                                        amd64        Linux kernel image for version 3.13.0 on 64 bit x86 SMP
ii  linux-image-extra-3.13.0-32-generic                   3.13.0-32.57                                        amd64        Linux kernel extra modules for version 3.13.0 on 64 bit x86 SMP
ii  linux-image-generic                                   3.13.0.32.38                                        amd64        Generic Linux kernel image


$ uname -r | sed 's/\(.*\)-\([^0-9]\+\)/\1/'
3.13.0-32



$ dpkg -l 'linux-image-*' | sed '/^ii/!d' | sed "/$(uname -r | sed 's/\(.*\)-\([^0-9]\+\)/\1/')/d"
ii  linux-image-generic                                   3.13.0.32.38                                        amd64        Generic Linux kernel image



$ dpkg -l 'linux-image-*' | sed '/^ii/!d' | sed "/$(uname -r | sed 's/\(.*\)-\([^0-9]\+\)/\1/')/d" | sed 's/^[^ ]* [^ ]* \([^ ]*\).*/\1/'
linux-image-generic



$ dpkg -l 'linux-image-*' | sed '/^ii/!d' | sed "/$(uname -r | sed 's/\(.*\)-\([^0-9]\+\)/\1/')/d" | sed 's/^[^ ]* [^ ]* \([^ ]*\).*/\1/' | sed '/[0-9]/!d'
## No output

So this would do nothing:

$ sudo apt-get remove --purge $(dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d;s/^[^ ]* [^ ]* \([^ ]*\).*/\1/;/[0-9]/!d')

As you can see it was very close to removing my current kernel meta package linux-image-generic 3.13.0.32.38 amd64 Generic Linux kernel image (thanks to this last sed '/[0-9]/!d' line) , which is wrong.

In a nutshell, this piece of sed has several issues. You should avoid it to remove your older kernels.

EDIT:

I have already said that the sed command has issues. For the sake of extension i am going to remove the last two lines with the form that actually should be working in my current scenario.

The last two lines in a modified form:

sed -r 's/^[^ ]* *(([a-z]|-)*)(-[a-z]*) *(([0-9]|-)*)\..*/\1-\4\3/'

So:

$ dpkg -l 'linux-image-*' | sed '/^ii/!d;/'"$(uname -r | 
sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d' | 
sed -r 's/^[^ ]* *(([a-z]|-)*(-[a-z]*) *(([0-9]|\.|-)*)\..*/\1\-\4\3/'

Outpus:

linux-image-3.13.0.32-generic

Thats very dengerous!!

Now showing what this would do to my system, --dry-run ning the full apt-get remove command:

$ sudo apt-get remove --dry-run $(dpkg -l 'linux-image-*' | 
sed '/^ii/!d;/'"$(uname -r | sed "s/\(.*\)-\([^0-9]\+\)/\1/")"'/d' | 
sed -r 's/^[^ ]* *(([a-z]|-)*)(-[a-z]*) *(([0-9]|\.|-)*)\..*/\1\-\4\3/')


Reading package lists... Done
Building dependency tree       
Reading state information... Done
Note, selecting 'linux-image-3.13.0-32-generic' for regex 'linux-image-3.13.0.32-generic'
The following package was automatically installed and is no longer required:
  linux-headers-generic
Use 'apt-get autoremove' to remove it.
The following packages will be REMOVED:
  linux-generic linux-image-3.13.0-32-generic
  linux-image-extra-3.13.0-32-generic linux-image-generic
0 upgraded, 0 newly installed, 4 to remove and 0 not upgraded.
Remv linux-generic [3.13.0.32.38]
Remv linux-image-generic [3.13.0.32.38]
Remv linux-image-extra-3.13.0-32-generic [3.13.0-32.57]
Remv linux-image-3.13.0-32-generic [3.13.0-32.57]

As you see it will remove all my current kernels, also note that this is just a simulation to match my current scenario and the original sed can be improved.

** Note that the actual code will work if do a reboot after kernel upgrade as Oli already mentioned.

EDIT2:

Here is a simple yet robust script that will consider all situations and remove the actual older kernels:

#!/bin/bash
kernels=( $(grep -Po "^linux-image-[^-]+-[^-]+-generic\b" < <(dpkg --get-selections)) )
cur_rel=$(grep -Po ".*(?=-[a-z]*$)" < <(uname -r))

for kernel in "${kernels[@]}"; do
    ker_rel=$(grep -Po "[0-9].*(?=-[a-z]*)" <<< "$kernel")
    dpkg --compare-versions "$ker_rel" gt "$cur_rel" && { echo "Please Restart your computer first"; break; }
    dpkg --compare-versions "$ker_rel" lt "$cur_rel" && sudo apt-get remove "$kernel"
done

If you have any version that is newer than the current one this will give you a warning to restart you computer first. Also note that the older kernels are preserved due to a good reason which is if you somehow mess up your current kernel making your system unstable then you should be able to boot into any older kernel.

This is an effort to remove kernels that aren't the current one.

There are many bits of code out there that do something like this and this is it not a good example. It could easily remove newer-than-current kernels and therefore the meta-package that installs (linux-generic et al) if you had an upgrade and didn't reboot.