Technical Information: General Linux Topics

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So, You're Ready for the Linux Desktop ...

• Firefox 64-bit downloads
• Interesting site on Linux performance topics (and some more here)
• Setting up GitLab Community Edition

qemu-img create -f qcow2 sda 18G

lvcreate -L 100G -n vmnamehome vgvms

See here for further details

• From display owner (where X is display number): xauth extract cookiefilename :X
• For remote user, get cookie file and: xauth merge cookiefilename

• Good article: https://www.digitalocean.com/community/tutorials/how-to-create-raid-arrays-with-mdadm-on-ubuntu-18-04
• NOTE: There seems to be issues with some hardware, where GPT partition tables are manipulated by some UEFI functionality, and so RAID volumes created on raw disk (without partition) are at risk of losing their superblock(s). Because of this, its generally recommended to utilize a partition instead of full-disk devices.
• Set up RAID5 with mdadm:

  mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdX1 /dev/sdY1 /dev/sdZ1

• Check status: cat /proc/mdstat
• Configure for availability after boot:
  (NOTE: this must be done after recovery status is complete)

  mdadm --detail --scan >> /etc/mdadm/mdadm.conf
  update-initramfs -u

• Scrub to prevent bit-rot: echo check > /sys/block/mdX1/md/sync_action
  (use cat /proc/mdstat to monitor)
• Tune re-build speed (https://www.cyberciti.biz/tips/linux-raid-increase-resync-rebuild-speed.html):
  To show limits: sysctl dev.raid.speed_limit_min ; sysctl dev.raid.speed_limit_max
  To increase speed, enter: sysctl -w dev.raid.speed_limit_min=value
  In /etc/sysctl.conf

  dev.raid.speed_limit_min = 50000
  ## good for 4-5 disks based array ##
  dev.raid.speed_limit_max = 2000000
  ## good for large 6-12 disks based array ###
  dev.raid.speed_limit_max = 5000000

• Sample LVM with mdadm RAID:

  pvcreate -M2 /dev/md0
  vgcreate -s 64 newvg /dev/md0
  lvcreate -L 128G -n newlv newvg
  mkfs -t ext4 /dev/newvg/newlv

• Ensure overlay module is loaded (modprobe overlay)
• Mount syntax looks like:

  mount -t overlay -o lowerdir=ldir,upperdir=udir,workdir=wdir overlayfs overlaydir

This uses dm-crypt LUKS support, which is all part of the kernel (I believe) (in Ubuntu, make sure the lvm2 package is installed) :

dd if=/dev/urandom of=.diskimg bs=1M count=1024
losetup --find --show /home/rk4n3/.diskimg
cryptsetup -c aes-xts-plain64:sha256 -s 512 -v -y luksFormat /dev/loopX
cryptsetup luksOpen /dev/loopX diskname
mkfs -t ext4 /dev/mapper/diskname
mount /dev/mapper/diskname /home/rk4n3/sec

Here is similar, but for burning an encrypted DVD:

dd if=/dev/zero of=dvd.img bs=1M count=4400
losetup --find --show dvd.img
cryptsetup -c aes-xts-plain64:sha256 -s 512 -v -y luksFormat /dev/loopX
cryptsetup luksOpen /dev/loopX dvdimg
mkisofs -l -r -allow-leading-dots -allow-lowercase -allow-multidot -J -joliet-long -o /dev/mapper/dvdimg dir2burn

Cleaning up after …

cryptsetup luksClose /dev/mapper/dvdimg
losetup -d /dev/loopX

Mounting …

cryptsetup luksOpen /dev/sr0 yourdvdname
mount -o ro /dev/mapper/yourdvdname /mount/point/dir

Run: xinput list
There will be a list that looks similar to:

⎡ Virtual core pointer                    	id=2	[master pointer  (3)]
⎜   ↳ Virtual core XTEST pointer              	id=4	[slave  pointer  (2)]
⎜   ↳ ETPS/2 Elantech Touchpad                	id=12	[slave  pointer  (2)]
⎜   ↳ Logitech USB Receiver                   	id=15	[slave  pointer  (2)]
⎣ Virtual core keyboard                   	id=3	[master keyboard (2)]
    ↳ Virtual core XTEST keyboard             	id=5	[slave  keyboard (3)]
    ↳ Power Button                            	id=6	[slave  keyboard (3)]
    ↳ Video Bus                               	id=7	[slave  keyboard (3)]
    ↳ Video Bus                               	id=8	[slave  keyboard (3)]
    ↳ Power Button                            	id=9	[slave  keyboard (3)]
    ↳ Sleep Button                            	id=10	[slave  keyboard (3)]
    ↳ AT Translated Set 2 keyboard            	id=11	[slave  keyboard (3)]
    ↳ MSI WMI hotkeys                         	id=13	[slave  keyboard (3)]
    ↳ Logitech USB Receiver                   	id=14	[slave  keyboard (3)]

Run this command for the ID that matches trackpad device:
xinput set-prop 12 “Device Enabled” 0

At the command-line:

mount -t tmpfs -o size=512m tmpfs /home/someone

In the /etc/fstab:

tmpfs            /home/someone         tmpfs      nodev,uid=1801,gid=100,size=512M  0  0

export PS1='\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\ $'

On one machine:

nc -vvlnp 12345 > /dev/null

On the other:

dd if=/dev/zero bs=1M count=1K | nc -vvn x.x.x.x 12345

Epson ESC/POS command reference: https://reference.epson-biz.com/modules/ref_escpos/index.php?content_id=72
Nice article here: https://mike42.me/blog/2015-03-getting-a-usb-receipt-printer-working-on-linux …
Use dmesg to verify that usblp has seen the printer (lpX device should be in /dev/usb):

dmesg
[12724.994550] usb 8-4: new full-speed USB device number 5 using ohci-pci
[12725.168956] usb 8-4: New USB device found, idVendor=04b8, idProduct=0e03
[12725.168963] usb 8-4: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[12725.168968] usb 8-4: Product: TM-T20
[12725.168971] usb 8-4: Manufacturer: EPSON
[12725.168975] usb 8-4: SerialNumber: ....
[12725.175114] usblp 8-4:1.0: usblp1: USB Bidirectional printer dev 5 if 0 alt 0 proto 2 vid 0x04B8 pid 0x0E03

Here's PHP code for a simple output:

/* ASCII constants */
const ESC = "\x1b";
const GS="\x1d";
const NUL="\x00";
 
/* Output an example receipt */
echo ESC."@"; // Reset to defaults
echo ESC."E".chr(1); // Bold
echo "FOO CORP Ltd.\n"; // Company
echo ESC."E".chr(0); // Not Bold
echo ESC."d".chr(1); // Blank line
echo "Receipt for whatever\n"; // Print text
echo ESC."d".chr(4); // 4 Blank lines
 
/* Bar-code at the end */
echo ESC."a".chr(1); // Centered printing
echo GS."k".chr(4)."987654321".NUL; // Print barcode
echo ESC."d".chr(1); // Blank line
echo "987654321\n"; // Print number
echo GS."V\x41".chr(3); // Cut
exit(0);

export PS1=$'\e[32m\u262F ${USER}@\e[33m${SYS} \u2622\u25AC\u25B6\e[0m '

export PS1=$'\e[32m\u250F\u2501\e[33m ${USER}@${SYS} \e[31m\u2622\e[36m ${PWD}\n\e[32m\u2517\u2501\u276F\u276F\u276F\e[0m '

export PS1=$'\e[32m\u250F\u2501\e[33m ${USER}@$(uname -n) \e[31m\u2622\e[36m \w\n\e[32m\u2517\u2501\u276F\u276F\u276F\e[0m '

• Draw-to-recognize unicode characters: Shapecatcher
• Lookup: unicode character table

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