问题是权限问题.不允许用户使用BASH.

Problem was a permissions issue. the user was not allowed to use BASH.

已为Apache用户在/etc/passwd中将bin/false更改为bin/bash.

Had to change bin/false to bin/bash in /etc/passwd for apache user.

事后看来，仅添加bin/mogrify可能会更好

In hindsight might be better to just add bin/mogrify