apt get install linux-source install linux-source-4.15.0
But my running kernel is 5.4.0-72-generic.
Why?
Cant it download the similar version as 5.4.0 if I download and install linux-source package? Makes no sense!!!

yes, obviously you can install linux-source-<version> the same way as your kernel itself.
But we have no idea what OS is it, how is it configured (the package management system) and how did you install that kernel.

Here is my log:
in brief:
ubuntu1804, kernel (default, comes with installation): 5.4
but apt install linux-source causes to download 4.15??


root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75# lsb_release --all
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 18.04.5 LTS
Release: 18.04
Codename: bionic
root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75# uname -r
5.4.0-73-generic
root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75# dpkg -l | grep linux-source
root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75# apt install linux-source
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
linux-hwe-5.4-headers-5.4.0-56 linux-hwe-5.4-headers-5.4.0-58 linux-hwe-5.4-headers-5.4.0-60
Use 'apt autoremove' to remove them.
The following additional packages will be installed:
linux-source-4.15.0
Suggested packages:
kernel-package libqt3-dev
The following NEW packages will be installed:
linux-source linux-source-4.15.0
0 upgraded, 2 newly installed, 0 to remove and 132 not upgraded.
Need to get 129 MB of archives.
After this operation, 146 MB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://mirror.arizona.edu/ubuntu bionic-updates/main amd64 linux-source-4.15.0 all 4.15.0-143.147 [129 MB]
Get:2 http://mirror.arizona.edu/ubuntu bionic-updates/main amd64 linux-source all 4.15.0.143.130 [2,484 B]
Fetched 129 MB in 11s (11.4 MB/s)
Selecting previously unselected package linux-source-4.15.0.
(Reading database ... 350420 files and directories currently installed.)
Preparing to unpack .../linux-source-4.15.0_4.15.0-143.147_all.deb ...
Unpacking linux-source-4.15.0 (4.15.0-143.147) ...
Selecting previously unselected package linux-source.
Preparing to unpack .../linux-source_4.15.0.143.130_all.deb ...
Unpacking linux-source (4.15.0.143.130) ...
Setting up linux-source-4.15.0 (4.15.0-143.147) ...
Setting up linux-source (4.15.0.143.130) ...
root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75#

root@ubuntu-desktop:/git.co/dev-learn/cuda/linux/cuda-by-example/p75# dpkg -l | grep linux-source
ii linux-source 4.15.0.143.130 all Linux kernel source with Ubuntu patches
ii linux-source-4.15.0 4.15.0-143.147 all Linux kernel source for version 4.15.0 with Ubuntu patches

If it were I, I'd open the Synaptic package manager and search for the correct version of the kernel sources.

If Synaptic is not installed, I'd install it. It's superior to the "software centers" that seem to be all the rage today.

Also, please surround any terminal output with "code" tags which become available when you click the "Advanced" button beneath the compose/edit post window. It makes terminal output much easier to read.

which kernel is installed? You need to install the same version of linux-source.
4.15 (not 5.4) is the default and that's why linux-source-4.15.0 was selected.

how you set the default?
I set the boot default in /etc/grub/default. Dont tell me I did not set there!

ok, I will take note of the code tag. I was sort of lazy.
Can synaptic used from cmdline??

I see following:
/usr/sbin/synaptic
Unable to init server: Could not connect: Connection refused

(synaptic:10774): Gtk-WARNING **: 04:23:32.835: cannot open display:

It is not you who can set the default kernel for a distro. And you cannot change it.

That is a different error, most probably root has not access right to your display.

I still do not understand why don't you take the version of your kernel and use that for source package?

Can you udescrube little bit more on how you can prep installation of linux-source-<version>???
I can't understand other nonsense posts in the forum.
Here is what I did:


apt install linux-source-`uname -r` -y
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package linux-source-5.4.0-74-generic
E: Couldn't find any package by glob 'linux-source-5.4.0-74-generic'
E: Couldn't find any package by regex 'linux-source-5.4.0-74-generic'

This is bit better, helps somewhat, gives list containing version closer to my running kernel but not exactly:

apt-cache search linux-source

linux-source - Linux kernel source with Ubuntu patches
linux-source-4.15.0 - Linux kernel source for version 4.15.0 with Ubuntu patches
linux-source-4.18.0 - Linux kernel source for version 4.18.0 with Ubuntu patches
linux-source-5.0.0 - Linux kernel source for version 5.0.0 with Ubuntu patches
linux-source-5.3.0 - Linux kernel source for version 5.3.0 with Ubuntu patches

and I was able to install 5.3.0 instead of 4.x
root@nonroot-SYS-7049GP-TRT:~# apt install linux-source-5.3.0
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
libncurses-dev | ncurses-dev kernel-package libqt3-dev
The following NEW packages will be installed:
linux-source-5.3.0
0 upgraded, 1 newly installed, 0 to remove and 245 not upgraded.
Need to get 135 MB of archives.
After this operation, 146 MB of additional disk space will be used.
Get:1 http://us.archive.ubuntu.com/ubuntu bionic-updates/main amd64 linux-source-5.3.0 all 5.3.0-74.70 [135 MB]
Fetched 135 MB in 21s (6,475 kB/s)
Selecting previously unselected package linux-source-5.3.0.
(Reading database ... 188974 files and directories currently installed.)
Preparing to unpack .../linux-source-5.3.0_5.3.0-74.70_all.deb ...
Unpacking linux-source-5.3.0 (5.3.0-74.70) ...
Setting up linux-source-5.3.0 (5.3.0-74.70) ...

The "Linux source" packages are really only necessary unless you are curious, or if some source-compiled package that you need to install needs to refer to the correct layout of some C-language struct that is exposed by some API-call.

Since they are quite "bulky," be sure to keep only the ones that you need ... and then, only if you are curious or if you otherwise actually need them. (If you are "simply curious," you can browse the source-code much easier online ...)