plowdown 取消了 -c 選項，不過多了 plowprobe。
plowprobe可以輸出的格式比 plowdown -c 多了很多
printf選項比較重要的的參數
--------------------------------------------------------
* %c probe function return status (0 for success, 13 for dead link, see list below)
* %f filename (can be empty string)
* %s filesize in bytes (can be empty string if not available). Note: it's often approxi‐mative.
Return Code
--------------------------------------------------------
0 Success.
1 Fatal error. Upstream site updated or unexpected result.
2 No available module (provided URL is not supported).
3 Network error. Mostly curl related.
4 Authentication failed (bad login/password).
5 Timeout reached (refer to -t/--timeout command-line option).
6 Maximum tries reached (refer to -r/--max-retries command-line option).
7 Captcha generic error.
8 System generic error.
10 Link alive but temporarily unavailable.
11 Link alive but requires a password.
12 Link alive but requires some authentication (private or premium link).
13 Link is dead.
14 Can't download link because file is too big (need permissions).
15 Unknown command line parameter or incompatible options.
Return code 跟plowdown一樣。 裡面比較重要的是0跟13, 0=> Ok, 13=> dead link。
