Install / Reinstall Grub

For UEFI, assuming the system partition is sda2 and the EFI partition is sda1.

sudo mount /dev/sda2 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done
sudo cp /etc/resolv.conf /mnt/etc
modprobe efivars
sudo chroot /mnt
apt purge grub-common
# for secure boot enabled
apt install grub-efi-amd64-signed os-prober shim-signed
# for secure boot disabled
apt install grub-efi-amd64 os-prober
exit
sudo umount /mnt/boot/efi 
sudo umount -R /mnt

For BIOS, assuming the system partition is sda1:

sudo mount /dev/sda1 /mnt
for i in /dev /dev/pts /proc /sys; do sudo mount -B $i /mnt$i; done
sudo cp /etc/resolv.conf /mnt/etc
sudo chroot /mnt
apt purge grub-common
apt install grub-pc os-prober
exit
sudo umount -R /mnt

short brief from https://forums.linuxmint.com/viewtopic.php?t=320504

iptables log to file

Create new chain:

iptables -N DROP-LOG
iptables -A DROP-LOG -m limit --limit 1/second -j LOG --log-prefix "[iptables-drop] "
iptables -A DROP-LOG -j DROP

Create /etc/rsyslog.d/10-iptables.conf and add lines:

:msg,contains,"[iptables-drop] " /var/log/iptables.log
# stop logging anything that matches the last rule.
# Doing this will stop logging kernel log messages to the file
# normally containing kern.* messages (eg, /var/log/kern.log)
& stop

KVM add device

Add hostdev for VF in <devices>. In <source>/<address> use physical address for VF

<hostdev mode='subsystem' type='pci' managed='yes'>
     <source>        
         <address domain='0x0000' bus='0x5e' slot='0x0e' function='0x0'/>
    </source>      
    <address type='pci' domain='0x0000' bus='0x07' slot='0x01' function='0x0'/>  
</hostdev>

KVM install VM with vnc server

Install from img file (with disk size 15G and bridge name inner)

virt-install \
--name=bull \
--vcpus=8 \
--memory=24576 \
--disk path=/var/lib/libvirt/images/bull.qcow2,size=15 \
--cdrom /var/lib/libvirt/iso/debian-11.6.0-amd64-netinst.iso \
--network bridge=inner,mac=52:54:00:40:4a:5e \
--boot hd,uefi \
--machine q35 \
--osinfo detect=on,require=off \
--graphics vnc,listen=<IP_HOST_TO_LISTEN_TO>,password=<SECRET>

Install from PXE

virt-install \
--name=rhpxe \
--vcpus=4 \
--memory=2048 \
--network bridge=br0,mac=52:54:00:0e:27:FF \
--pxe \
--disk none \
--boot uefi \
--osinfo detect=on,require=off \
--graphics vnc,listen=<IP_HOST_TO_LISTEN_TO>,password=<SECRET> 

Delete old files

Find only executable
find

find /path/to/dir/ ( ( -type f -daystart -mtime +8 ) -o -type d -empty ) -delete

How to download all files (but not HTML) from a website using wget?

wget -A lst,meta,txt,txz,tgz,asc,md5 -m -p -E -k -K -np <URL>

elilo.conf

chooser=simple
# interactive mode
prompt
# delay when not in interactive mode (0.1 sec)
delay=100
# timeout when in interactive mode (0.1 sec)
timeout=100
default=vmlinuz
#
image=vmlinuz
label=vmlinuz
read-only
append="root=/dev/sdc2 vga=normal ro"

image=vmlinuz-generic
label=vmlinuz-generic
initrd=initrd.gz
read-only
append="root=/dev/sdc2 vga=normal ro"

image=vmlinuz-huge-5.15.27
label=huge-5.15.27
read-only
append="root=/dev/sdc2 vga=normal ro"

image=vmlinuz-generic-5.15.27
label=generic-5.15.27
initrd=initrd-5.15.27.gz
read-only
append="root=/dev/sdc2 vga=normal ro"

Show modules in mkinitrd

mkdir /tmp/initrd
/tmp/initrd
cp /boot/initrd.gz .
gunzip initrd.gz
cpio -id < initrd

# mkinitrd.conf.sample
# See "man mkinitrd.conf" for details on the syntax of this file
#
#SOURCE_TREE="/boot/initrd-tree"
#CLEAR_TREE="0"
#OUTPUT_IMAGE="/boot/initrd.gz"
#KERNEL_VERSION="$(uname -r)"
#KEYMAP="us"
MODULE_LIST="ext4:hv_vmbus:scsi_transport_fc:hv_storvcs:jbd2:mbchache"
#LUKSDEV="/dev/sda2"
#LUKSTRIM="/dev/sda2" # verify support with 'hdparm -I $dev | grep TRIM'
#LUKSKEY="LABEL=TRAVELSTICK:/keys/alienbob.luks"
ROOTDEV="/dev/sda2"
ROOTFS="ext4"
#RESUMEDEV="/dev/sda2"
#RAID="0"
#LVM="0"
#UDEV="1"
#MODCONF="0"
#MICROCODE_ARCH="/boot/intel-ucode.cpio"
#WAIT="1"

mkinitrd -F -k 5.15.63
or
/usr/share/mkinitrd/mkinitrd_command_generator.sh -k 5.15.63
cp /boot/initrd.gz /boot/efi/EFI/Slackware/
cp /boot/vmlinuz-generic-5.15.63 /boot/efi/EFI/Slackware/vmlinuz

cp /boot/initrd.gz /boot/efi/EFI/Slackware/initrd-5.15.63.gz
cp /boot/vmlinuz-generic-5.15.63 /boot/efi/EFI/Slackware/vmlinuz-5.15.63

Linux terminal tricks

cd -
pushd /folder
popd
sudo !! (repeat previous command without sudo)
! number (execute command with number from history command)

Check if process is running

PROCESS="netperf"
if pgrep -x "$PROCESS" > /dev/null; then
echo "$PROCESS Running"
else
echo "$PROCESS Not running"
fi