手写Systemd service 的经验不多，非常尴尬，踩了Systemd 2个坑，浪费了近1个小时，记录一下。
Python项目部署使用了virtualenv，添加一个Systemd service，发现启动失败，提示：
File "/usr/lib64/python2.7/subprocess.py", line 1327, in _execute_child
raise child_exception
OSError: [Errno 2] No such file or directory
忘记设置环境了，EnvironmentFile 里设置PATH：
EXAMPLE_ENV=/home/www/example_env
PATH=${EXAMPLE_ENV}:$PATH
发现无论如何都无效，Google一下才发现傻了：systemd: “Environment” directive to set PATH:
You can't use EnvVars in Environment directives. The whole Environment= will be ignored.
改成：
EXAMPLE_ENV=/home/www/example_env
PATH=/home/www/example_env/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/root/bin
终于好了。不过又遇到一个问题，因为：
ExecStart=${EXAMPLE_ENV}/bin/example ....
Systemd 提示：
Executable path is not absolute, ignoring:${EXAMPLE_ENV}/bin/example ...
咦，变量怎么没解析。Google 了下，改成下面这样就好了：
ExecStart=/usr/bin/env ${EXAMPLE_ENV}/bin/example
For each of the specified commands, the first argument must be either an absolute path to an executable or a simple file name without any slashes.