bash admin command braindump

sudo apt-get update sudo apt-get install openssh-server sudo ufw allow 22
mv f1 f2 rm -f dir
file dir structure:
/etc group pwd config
  ---/cron.hourly /apt ( 3rd party)
/var
   ---- /log /www 
/dev device usb, disk network interfaces.
/usr vs. /bin vs. /sbin user, systen , admin
/lib kernal model shared lib
/sys /proc resources


sudo groupadd /groupdel jqd-group sudo chown root:jqd-group /var/jqd ls -l /var | grep jqd

sudo useradd -m tony sudo passwd tony sudo usermod -aG jqd-group tony grep jqd-group /etc/group

chmod r,g o -- owner/group/other r,w,x,=0,4,2,1 chmod g-r chmod o+x chmod 777, 770, 664(no one can x)

ps aux |grep root pstree -p top kill 123 killall php-cgi  top i=mem t=task l=load/uptime uptime

tail -f /var/log/syslog & jobs  fg 1 bg 1 tar czf archive.tar.gz /var/* nohup
sudo apt-get update sudo apt-get install apache2 sudo service apache2 status/stop/start
lsblk ls /etc/apt/source.list ls /var/www/html cd /var/log zcat syslog.2.gz

cron 7 fields  5 20 * * 1-5 root /usr/bin/backup.sh min0-59 hr0-23 dom0-31 moy1-12 dow0-7 why 0?
usr cron 6 fields */5 8-16 * * 1 echo "test" ~/working/1.txt every 5min 8am-4pm any day m Monday

system cron /etc/crontab   my_cron with 7 fields no extension  cron.d cron.daily .hourly .weekly .monthly
user cron job   crontab -e -r -l  # edit remove list cannot add file must use editor /var/spool/cron/crontab