super + right = new tty
apt-get install net-tools -y
du -ahx | sort -rh | head -20
dpkg -l to check installed package
sudo hostnamectl set-hostname new_hostname set new host name
dpkg -l | grep sudo to check sudo
su to login as root
shasum name.vdi to create signature
APParmor vs SELinux: https://phoenixnap.com/kb/apparmor-vs-selinux
wc -l wc -w: -1 counts for lines amd -w counts for words
how to ssh into VB machine https://averagelinuxuser.com/ssh-into-virtualbox/
switch from NAT to Bridge Adapter
ip a or ifconfig or ip addr show to check the ip address of the vm
ssh username@ip -p 4242
why linux eat all my ram: https://www.linuxatemyram.com/
systemctl status ssh or sudo service ssh status to check ssh status
ssh-keygen -R hostIP to remove a specific host in ~/.ssh/known_hosts
apt-get install sudo -y
usermod -aG sudo username add user to the sudo group
/var/log touch sudo.log if needed.
sudo visudo or vim /etc/sudoers to edit the config file.
sudo visudo is more preferred since it validates the syntax upon saving.
sudo update-alternatives --config editor to change the default editor.
Default env_reset: resets the terminal environment to remove any user variables.
Default mail_badpass: mail notices to mailtouser when bad sudo password attempts encountered.
Default secure_path=...: restrict the PATH that will be used for sudo operations.
Defaults badpass_message="incorrect password!"
Defaults passwd_tries=3
Defaults logfile="/var/log/sudo.log"
Defaults log_input, log_output
Defaults requiretty
username ALL=(ALL:ALL) ALL
root ALL=(xx:xx) xx The first “ALL” indicates that this rule applies to all hosts.
root xx=(ALL:xx) xx This “ALL” indicates that the root user can run commands as all users.
root xx=(xx:ALL) xx This “ALL” indicates that the root user can run commands as all groups.
root xx=(xx:xx) ALL The last “ALL” indicates these rules apply to all commands.
%sudo ALL=(ALL:ALL) ALL group name must start with % in the group privilege section
apt-get install ufw -y to install ufw
sudo ufw enable
sudo ufw status
sudo ufw allow ssh ufw allow 4242
sudo ufw delete NUMBER to delete specific port according to the number fromsudo ufw status numbered
sudo ufw delete allow 22/tcp remove directly
sudo apt-get install libpam-pwquality to install the moudule
config: /etc/pam.d/common-password
/etc/login.defs: pw expire day
minlen=10
ucredit=-1: must contain atleast one uppercase, no credit for uppercase
lcredit=-1: lowercase and ^
dcredit=-1: digit and ^
maxerepeat=3: more than 3 consecutive identical characters
reject_username: must not contain username
difok=7: atleast 7 char not from the old password
enforce_for_root: rules apply to root
root old pw wont be compare, sodifok is not applied to root
chage man: https://linux.die.net/man/1/chage
pam-pwquality man: https://man.archlinux.org/man/pam_pwquality.8
sudo adduser new_user: create and setup a new user
sudo groupadd new_group: create a new group
sudo usermod -aG group_name user_name: add user to a group, -a to add the user to an existing supplementary group, -g will change the user primary group, -G will change the user primary group AND remove the user previous supplementary group.
getent: get entries from a database
getent group: show all group
groups user_name: check what groups a specific user belongs to
uname -a: print all infor of OS architecture and kernel version
or
hostnamectl | grep Kernel
hostnamectl | grep Architecture
Linux Architecture: https://www.interviewbit.com/blog/linux-architecture/
grep "physical id" /proc/cpuinfo | sort | uniq | wc -l count how many cpu on socket
grep '^core id' /proc/cpuinfo | sort | uniq | wc -l number of physical core
nproc total number processor
grep "^processor" /proc/cpuinfo | wc -l should have same number as nproc
Core and thread: https://www.guru99.com/cpu-core-multicore-thread.html
cpu, vcpu, core, logical core: https://superuser.com/questions/1257392/physical-vs-logical-vs-virtual-cores
cat /proc/meminfo for detail meminfo
free -m memory usage in MB
free -m | grep Mem | awk '{printf("%d/%dMB (%.2f%%)", $3, $2, $3/$2*100)}'
df -BG to check disk usage in Gb
df -BG | grep '^/dev/' include path start with /dev/ only since udev and tmpfs are actually located in RAM
udev, temps: https://askubuntu.com/questions/1150434/what-is-udev-and-tmpfs
/dev/ and subdir: https://unix.stackexchange.com/questions/18239/understanding-dev-and-its-subdirs-and-files /dev/loop*: https://linuxhint.com/dev-loop-linux/
df -BG | grep '^/dev/' | grep -v boot exclude the files mount on boot, they are used to boot the OS.
df -BG | grep '^/dev/' | grep -v boot | grep -v loop | awk '{fdg += $2} END {printf("%dG", fdg)}' full size in G
df -BM | grep '^/dev/' | grep -v boot | grep -v loop | awk '{udm += $3} END {printf("%dMB", udm)}' used space in MB
df -BM | grep '^/dev/' | grep -v boot | grep -v loop | awk '{fd += $2} {ud += $3} END {printf("(%.1f%%)", ud/fd*100)}' usage calculated in MB
vmstat 1 2 check the stat every 1s for 2 times
vmstat 1 2 | tail -1 only show the last line
vmstat | tail -1 | awk '{printf("%d%%", 100-$15)}': $15 is the 15th field, which is the % of cpu idle time, 100-cpu idle time will be cpu usage
top -bn1 | grep '^%Cpu' | awk '{printf("%.1f", 100 - $8)}'
top and vmstat: https://codeahoy.com/compare/top-vs-vmstat
cpu usage: https://www.baeldung.com/linux/get-cpu-usage
who -b | awk '{print($3, $4)}' who boot
https://www.cyberciti.biz/tips/linux-last-reboot-time-and-date-find-out.html
last -i to check who historial login
lsblk | grep "lvm" | check the line that contain lvm in lsblk
lsblk | grep "lvm" | wc -l count the number of result
if [ $(lsblk | grep "lvm" | wc -l) -eq 0 ]; then echo inactive; else echo active; fi if the count is 0, LVM is not active, else it is active.
-eq: comparison operator used in the [ ] construct
fi: keyword to close the if-else statement
https://askubuntu.com/questions/202613/how-do-i-check-whether-i-am-using-lvm
netstat -natp | grep ESTABLISHED | wc -l
https://www.computerhope.com/issues/ch001079.htm
users | wc -w
hostname -I IP
ip a | grep ether | awk '{print($2)}' MAC address
journalctl _COMM=sudo | grep COMMAND | wc -l
sudo crontab -u root -e: -u root specifies managing cron table for root, -e means edit
*/10 * * * */usr/local/bin/monitoring.sh
sudo cronstart sudo cronstop sudo service cron stop
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