DIY Linux Summaries
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。
