On what occasion will inode change? I checked manual of `find` , and there's an option `-ctime` which specify the last modification time of inode. But on what occasion would inode change ? When is that option useful ?

To simplify:

* Any change in file contents changes both the `mtime` and the `ctime`.
* Any change in metadatas (permissions and other information shown by `stat`) changes only `ctime`.



When is it useful: I don't know… But for example, if you want an over-approximation for the time when the last link (`ln`) to the inode was created, you should check `ctime` not `mtime`.