Unfreeze PC at kernel level:
https://en.wikipedia.org/wiki/Magic_SysRq_key#Common_Usage

Show error and warning messages:
# dmesg -T –level=err,warn

Monitor message in real time:
# dmesg -Tx –follow

Test RAM:
# apt-get install memtester
# memtester 1024 5
(allocate 1024Mo and repeat 5 times)

# du -sh /var/*

Show disk usage with human readable values
# df -h
Show disk INODE usage
# df -1

Change directories permission:
$ find /path/to/base/dir -type d -exec chmod 755 {} +
Change files permission:
$ find /path/to/base/dir -type f -exec chmod 644 {} +

Owner:
$ chown owner_name * -R
Group:
$ chgrp group_name * -R

Test the command with -n
$ rsync -nav –delete-after /source/ /destination/
Execute command : -a for archive (keep files structure + recursive…), -v for verbose, –delete-after to delete files in backup directory if they do not exist in source (moved, renamed, …)
$ rsync -av –delete-after /source/ /destination/

Create user:
$ sudo useradd -m username
Set password:
$ sudo passwd username

Create group:
$ sudo addgroup goupname
Config file:
/etc/group
Add user to group:
$ sudo usermod -a -G groupname username

List installed packages:
sudo apt list –installed