在命令路径中为Systemd服务中的ExecStart使用变量

我正在尝试制作如下的systemd服务：

[Unit]
Description=Syslog

[Service]
Type=simple
Environment="TESTEXTSERVICESFILES=/opt/test/extservices"
Environment="TESTCONFDATA=/storage/test/conf"

ExecStartPre=/bin/echo ${TESTEXTSERVICESFILES}/syslog/bin/nxlog $TESTCONFDATA
ExecStart=/opt/test/extservices/syslog/bin/nxlog -c ${TESTCONFDATA}/syslog/nxlog.conf
#ExecStart=/${TESTEXTSERVICESFILES}/syslog/bin/nxlog -c ${TESTCONFDATA}/syslog/nxlog.conf


[Install]
WantedBy=multi-user.target

运行sudo systemctl daemon-reload之后；sudo systemctl start test-syslog; sudo systemctl status test-syslog '，我得到以下成功输出：

● test-syslog.service - TestSyslog
   Loaded: loaded (/usr/lib/systemd/system/test-syslog.service; enabled; vendor preset: disabled)
   Active: deactivating (stop-sigterm) since Fri 2018-02-23 10:15:09 UTC; 11ms ago
  Process: 9474 ExecStart=/./opt/test/extservices/test-syslog/bin/nxlog -c ${TESTCONFDATA}/test-syslog/nxlog.conf (code=exited, status=0/SUCCESS)
  Process: 9471 ExecStartPre=/bin/echo /.${TESTEXTSERVICESFILES}/test-syslog/bin/nxlog $TESTCONFDATA (code=exited, status=0/SUCCESS)
 Main PID: 9474 (code=exited, status=0/SUCCESS)
   CGroup: /system.slice/test-syslog.service
           └─9478 /./opt/test/extservices/test-syslog/bin/nxlog -c /storage/test/conf/test-syslog/nxlog.conf

Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: test-syslog.service: control process exited, code=exited status=0
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: test-syslog.service got final SIGCHLD for state start-pre
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: About to execute: /./opt/test/extservices/test-syslog/bin/nxlog -c ${TESTCONFDATA}/test-syslog/nxlog.conf
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: Forked /./opt/test/extservices/test-syslog/bin/nxlog as 9474
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: test-syslog.service changed start-pre -> running
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: Job test-syslog.service/start finished, result=done
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: Started Test Syslog.
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: Child 9474 belongs to test-syslog.service
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: test-syslog.service: main process exited, code=exited, status=0/SUCCESS
Feb 23 10:15:09 lt-x260-1606.test.local systemd[1]: test-syslog.service changed running -> stop-sigterm

此处服务已成功启动。但是，当我注释第一个ExecStart指令并取消注释第二个ExecStart指令时，我会以失败告终：

● test-syslog.service - Test Syslog
   Loaded: loaded (/usr/lib/systemd/system/test-syslog.service; enabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Fri 2018-02-23 10:11:44 UTC; 11ms ago
  Process: 9243 ExecStart=/$TESTEXTSERVICESFILES/test-syslog/bin/nxlog -c $TESTCONFDATA/test-syslog/nxlog.conf (code=exited, status=203/EXEC)
  Process: 9239 ExecStartPre=/bin/echo /.${TESTEXTSERVICESFILES}/test-syslog/bin/nxlog $TESTCONFDATA (code=exited, status=0/SUCCESS)
 Main PID: 9243 (code=exited, status=203/EXEC)

Feb 23 10:11:44 lt-x260-1606.test.local echo[9239]: /./opt/test/extservices/test-syslog/bin/nxlog /storage/test/conf

这次服务无法启动，就像它不想启动以$ {TESTEXTSERVICESFILES}变量开头的进程一样。有人知道即使两种情况下的命令行都一样，为什么它不起作用吗？