参考:[linux - How to convert string to integer in UNIX - Stack Overflow](https://onedrive.live.com/view.aspx?resid=8397E424B7DC017C%211760241&id=documents&wd=target%28%E7%BC%96%E7%A8%8B%2FLinux.one%7CC31B288E-EBC4-4484-916C-A9DF7E1F23D1%2FConvert%20string%20to%20integer%7CA7CC4C08-8721-4976-8C84-C3A92ED28499%2F%29
onenote:https://d.docs.live.net/8397e424b7dc017c/Onenote/学习/编程/Linux.one#Convert%20string%20to%20integer&section-id={C31B288E-EBC4-4484-916C-A9DF7E1F23D1}&page-id={A7CC4C08-8721-4976-8C84-C3A92ED28499}&object-id={E4E090AD-4615-4D43-8F73-AB0B43893AE4}&6A)

Q

1
2
3
4
A='3'
B='5'
C=A+B
# C is 'A+B', not 8

A

1
C=$(( d1 - d2 ))