I) Command Lines

1.1) difference between “esc+shift+i” and “esc+shift+a”

“esc+shift+i” : input at the beginning of the line
“esc+shift+a” : input at the end of the line

1.2) how to use escape character to have computing on multiplication

val=`expr $a \* $b`
echo "a * b : $val"

1.3) file types in Linux

-f file 检测文件是否是普通文件（既不是目录，也不是设备文件），如果是，则返回 true。