test command
例：
[ ! -z STRING ]
[ -f file ]
参考： http://www.computerhope.com/unix/test.htm
Built-in Variables(special variables)
http://johnstowers.co.nz/pages/bash-cheat-sheet.html
https://www.tutorialspoint.com/unix/unix-special-variables.htm
http://www.tldp.org/LDP/abs/html/internalvariables.html
Array
append to file
echo "hello world" | tee -a file
replace content in file
sed -i 's/original/new/g' file.txt