[Unit]
Description=Test for StandardOutput=append:

[Service]
ExecStartPre=sh -c 'printf "hello\n" > /tmp/test-exec-standardoutput-output'
ExecStartPre=sh -c 'printf "hello\nhello\n" > /tmp/test-exec-standardoutput-expected'
StandardInput=data
StandardInputText=hello
StandardOutput=append:/tmp/test-exec-standardoutput-output
StandardError=null
ExecStart=cat
ExecStart=cmp /tmp/test-exec-standardoutput-output /tmp/test-exec-standardoutput-expected
Type=oneshot