2>&1 < /dev/null 什么意思?
# openssl s_client -showcerts -connect 192.168.1.104:8443 </dev/null
https://unix.stackexchange.com/questions/230887/what-does-dev-null-mean
https://unix.stackexchange.com/questions/230330/what-does-do
What are the shell's control and redirection operators?
https://unix.stackexchange.com/questions/159513/what-are-the-shells-control-and-redirection-operators