Essential Linux Commands (still updating)

gdm: How to stop & restart gdm

Go out from X by Ctrl+Alt+F1 and login to a console first and

$sudo service gdm stop
or
$sudo /etc/init.d/gdm stop

This works for most services in /etc/init.d/ with start/stop/restart.

blkid: How to get UUIDs of hard disks to tweak /etc/fstab

$sudo blkid
$sudo blkid -s UUID

hdparm: Find out Hard disk Spec (brand, model, size, etc.)

$sudo hdparm -I /dev/sda

Changing a target of symlink

instead of

$ rm -f symlink
$ ln -s target symlink

when a target is a file

$ ln -sf new_target symlink

not to mind the target is a directory or a file then

$ ln -sfn new_target_dir symlink

mdadm: Multiple Device Admin (Linux Software RAID)

$ cat /proc/mdstat
$ sudo mdadm /dev/md0 -r /dev/sda1
$ sudo mdadm /dev/md0 -a /dev/sda1

grub: Change the installed hard disk (MBR change, for example, into sda)

$ sudo grub-install /dev/sda
$ sudo update-grub

grub: Change the default boot kernel/OS

$ fgrep menuentry /boot/grub/grub.cfg

copy menuentry text that you want to boot by default,

$ sudo vi /etc/default/grub

GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 4.2.0-16-generic"

(when your new kernel doesn't work properly && grub is already updated with the new kernel's entry, the text of GRUB_DEFAULT above works well)

$ sudo update-grub