Ubuntu가 정지됩니다. 이 ^@ 기호는 왜 그리고 무엇을 의미합니까?

Ubuntu가 정지됩니다. 이 ^@ 기호는 왜 그리고 무엇을 의미합니까?
Jul 3 15:58:01 nusik-ryzen CRON[8641]: (root) CMD (pgrep xmr-stak> /dev/null || cd /xmr-stak && ./xmr-stak) 
Jul 3 15:58:01 nusik-ryzen CRON[8642]: (root) CMD (pgrep xmrig> /dev/null || /xmrig/build/xmrig) 
Jul 3 15:58:01 nusik-ryzen CRON[8639]: (CRON) info (No MTA installed, discarding output) ^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@^@
Jul 3 16:09:23 nusik-ryzen rsyslogd: [origin software="rsyslogd" swVersion="8.16.0" x-pid="969" x-info="http://www.rsyslog.com"] start 
Jul 3 16:09:23 nusik-ryzen rsyslogd-2222: command 'KLogPermitNonKernelFacility' is currently not permitted - did you already set it via a RainerScript command (v6+ config)? [v8.16.0 try http://www.rsyslog.com/e/2222 ]

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