```bash
_seed='chuang qian ming yue guang'
## para
_site='gmail.com'
_user='trydofor'
_size=20
## mark
mark=sha$_size:$_user:$_site
echo $mark
## sha20:trydofor:gmail.com
echo -n $mark:$_seed | sha1sum
## a53332b953a6fe12991d823af6fea849d9ea48a4
echo -n $mark:$_seed | sha1sum \
| xxd -r -p | base64
## pTMyuVOm/hKZHYI69v6oSdnqSKQ=
echo -n $mark:$_seed | sha1sum \
| xxd -r -p | base64 | cut -c -$_size
## pTMyuVOm/hKZHYI69v6o
```