Linux

Some reading:

• Filesystem Hierarchy Standard
• Unix VS Linux

/dev/null = null device = device file that discards all data written to it but reports that the write operation succeeded

Run multiple command:

• A ; B – Run A and then B, regardless of the success or failure of A
• A && B – Run B only if A succeeded
• A || B – Run B only if A failed

Get PID

ps ax | grep [NAME] | grep -v grep | awk '{print $1}'

Check IPs connecting to a machine

sudo netstat -ant | sed '1d' | awk '{print $5}' | cut -f1 -d':' | sort | uniq

Network Time Protocol (NTP) is a protocol used to synchronize computer system clock automatically over a networks.

sudo apt-get remove ntp ntpdate
sudo apt-get install ntp ntpdate

ULimit

doc

ulimit -n # current
ulimit -u unlimited # set to unlimited
ulimit -a # show all