[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

● 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

Here the service has started successfully. But when I comment the first ExecStart directive and uncomment the second one I get as failure :

● 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