# linux
Linux is a kernel. See https://xkcd.com/456/

[TPM](TPM) stuff

# learn
https://kernelnewbies.org/Linux_Kernel_Newbies is a good starting point for learning about the kernel.
https://kernelnewbies.org/Outreachyfirstpatch -> Write your first patch for the kernel

https://man7.org/tlpi/ -> The Linux Programming Interface -- Michael Kerrisk

https://github.com/PacktPublishing/Linux-Device-Driver-Development-Second-Edition -> Linux Device Driver Development, Second Ed. -- John Madieu




# kernel
``fs/`` → virtual file system implementation (generic fs code and toplevel implementation of fs syscalls), specific filesystems 
``mm/`` → virtual memory management *(``mmap, kmalloc, vmalloc``)* 
``kernel/`` → core kernel code, scheduler, primitives (``printk``), process syscalls *(``fork, exit``)* 
``net/`` → network generic code and protocol implementations; most syscalls in ``net/socket.c``

[Interactive map of the Linux kernel](https://makelinux.github.io/kernel/map/)


# xset
change xorg settings

## display power management (DPMS)
xset s off		Disable screen saver blanking
xset s 3600 3600 	Change blank time to 1 hour
xset -dpms		Turn off DPMS
xset s off -dpms 	Disable DPMS and prevent screen from blanking
xset dpms force off 	Turn off screen immediately
xset dpms force standby Standby screen
xset dpms force suspend	Suspend screen 



# Notes
__NOTE:__ This section is meant for tidbits of useful knowledge on linux, mostly commands.
Ideally, these notes will branch out over time, and I will refactor them into separate topics under Linux.



* On a systemd-based system you can restrict a process’s memory use with ``systemd-run --scope -p MemoryMax=500M --user program``
* To open a port in Linux with iptables:

```sh
# allow TCP ipv4 
iptables -I INPUT -p tcp --dport 3389 -j ACCEPT
# allow UDP ipv4
iptables -I INPUT -p udp --dport 3389 -j ACCEPT
# allow TCP ipv6
ip6tables -I INPUT -p tcp --dport 3389 -j ACCEPT
# allow UDP ipv6
ip6tables -I INPUT -p udp --dport 3389 -j ACCEPT
```

* To add swapfile to Ubuntu 20 server:
	* ``sudo fallocate -l 1G /swapfile``
	* ``sudo chmod 600 /swapfile``
	* ``sudo mkswap /swapfile``
	* ``sudo swapon /swapfile``
	* **Makes changes permanent:** ``echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab``
* **Java**:
	* ``apt-cache search openjdk`` to search for all available java packages.
	* ``update-alternatives --config java`` to select default java runtime
* To add a user ``sudo useradd -m username`` which will create user along with home directory.
* To list all users run ``cat /etc/passwd``
* To type unicode symbols in Linux, hold ``CTRL+SHIFT+U``, let go of all three, type the unicode code and press space.

* Generate a random password with ``tr -dc A-Za-z0-9_ < /dev/urandom | head -c 16 | xargs``

* Enable promiscuous mode on network adapter (for network tools under a VM) with ``chmod a+rw /dev/vmnet0``


* Run linux with bad memory (from pragmaticaddict)
	* Test with memtest86+
	* Use the memmap kernel option to mark the area as reserved
		* Example: memmap=0x121640a20$0x12410abb0

* Run command as system user: `su - targetuser -s /bin/bash`

* See keycodes easily with `xev | awk -F'[ )]+' '/^KeyPress/ { a[NR+2] } NR in a { printf "%-3s %s\n", $5, $8 }'`

* Parallelize with `NUM_PROCS=$(cat /proc/cpuinfo | awk '/^processor/{print $3}'| wc -l)`
`< $@ xargs -d '\n' -P $NUM_PROCS -I {} /bin/bash -c "{}"`

* View your webcam with mpv `av://v4l2:/dev/video0 --profile=low-latency`

* View speed of interface `cat /sys/class/net/<interface>/speed`

* Rename everything in cwd to lowercase `for f in *[[:upper:]]*; do mv -- "$f" "${f,,}"; done`


# MIME
Configure default applications for xdg-* at `~/.config/mimeapps.list`


# Links
* https://www.catb.org/~esr/writings/unix-koans/introduction.html -> Unix teachings of the Master Foo
* https://linux-audit.com/about/ -> linux articles, guides, security information
* https://beej.us/guide/
	- useful resources on unix & programming (sockets, C, GDB, …)
	- C guide
	- GDB

https://cb.vu/unixtoolbox.html

https://sergiu121.github.io/docs/index.html -> USO lab book starter on operating systems and (mostly) linux
https://ocw.cs.pub.ro/courses/uso/cursuri/start -> USO wiki, slides and resources

http://quotes.cat-v.org/programming/bumper-sticker-computer-science -> 20/20 hindsight CS quotes
https://perl.plover.com/ -> Lots of unix stuff: history, teachings, presentations on various topics.
https://djoffe.com/stuff/linuxref/ -> useful linux CLI tools with description

https://blog.robertelder.org/linux-hardware-debugging/ -> hardware debugging resource

https://0xax.gitbooks.io/linux-insides/content/ -> kernel internals; work in progress book

https://github.com/mk-fg/fgtk -> helpful public domain scripts

## News
* https://lwn.net/headlines/rss -> weekly linux news
* https://www.phoronix.com/ -> frequent kernel updates