[Unit]
Description=Test for StandardOutput=truncate:

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