Send systemd service stdout/stderr to a log file with StandardOutput=append and log rotation caveats

Documented steps

1. Default: stdout/stderr go to the journal. To also/instead write a plain file, in [Service]: StandardOutput=append:/var/log/myapp/out.log StandardError=append:/var/log/myapp/err.log `append:` (systemd 240+) opens O_APPEND; `file:` truncates-at-open semantics differ (opens without append; multiple writers clobber); `truncate:` (systemd 254+) truncates at each service start.
2. Create the directory with correct ownership via LogsDirectory=myapp (creates /var/log/myapp owned by the service user, sets $LOGS_DIRECTORY) instead of ExecStartPre mkdir hacks: LogsDirectory=myapp StandardOutput=append:/var/log/myapp/out.log
3. Keep journal metadata AND identify lines: set SyslogIdentifier=myapp so journal entries are tagged; `journalctl -t myapp` filters by that tag.
4. Rotation: systemd does not rotate these files. Add a logrotate config with copytruncate (the app holds the fd; a rename-based rotate would keep writing the old inode): /var/log/myapp/*.log { daily rotate 14 compress copytruncate }
5. Alternative that avoids rotation entirely: keep logs in the journal and cap it (SystemMaxUse= in journald.conf), or forward: `ExecStart=/bin/sh -c 'exec /usr/local/bin/myapp 2>&1 | tee -a /var/log/myapp/out.log'` only if you accept the pipe's SIGPIPE/backpressure risks.
6. Apply: `systemctl daemon-reload && systemctl restart myapp`; confirm the fd: `ls -l /proc/$(systemctl show -p MainPID --value myapp)/fd | grep log`.