1. Cron Job Scheduling
  2. Awk
  3. Listening ports
  4. iptables
  5. DNS

Cron Job Scheduling

# second  minute  hour    day-of-month  month  day-of-week*
# (0-59)  (0-59)  (0-23)  (1-31)        (1-12) (0-7)
# Run every hour at 20 minutes past the hour
  0       20       *       *             *      *
# Run every Sunday at 3am
  0       *        3       *             *      0
# Run every 5 minutes
  0       */5      *       *             *      *

Awk

Kill a process using ps, grep and awk

ps -A | grep <name> | awk '{print $1}' | kill

Listening ports

netstat -tulpn

iptables

iptables firewall to block ports

/etc/iptables.up.rules

Check out /etc/network/if-pre-up.d/iptables, it'll have a line like:
/sbin/iptables-restore < /etc/iptables.up.rules

Use iptables -nL to see all firewall rules

DNS

To query a dns server, try this:

dig -x <dns server ip or hostname>

To test if dns can translate an ip try this

host google.com <dns server ip or host>