我有一台计算机，它似乎有时会运行 cron 作业。当我监控系统日志时，我可以看到提到的作业，但看不到命令的结果。我该如何解决这个问题？
此盒子的版本：
username:/etc$ uname -rvp
4.13.0-32-generic #35~16.04.1-Ubuntu SMP Thu Jan 25 10:13:25 UTC 2018 i686
文件/etc/crontab内容（附有我的注释）：
username:/etc$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
# m = minute
# h = hour (24-hour)
# dom = day of month
# mon = month (numeric)
# dow = day of week (sunday is 0, monday is 1, saturday is 6)
# user = user
# command = yep
# * = blank or null entry or wildcard
# m h dom mon dow user command
17 * * * * root cd / && run-parts --report /etc/cron.hourly
#25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
#47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
#52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )
01 18 * * 1 root /sbin/shutdown -h now
00 20 * * 1 root /sbin/shutdown -h now
01 18 * * 2 root /sbin/shutdown -h now
00 20 * * 2 root /sbin/shutdown -h now
01 18 * * 3 root /sbin/shutdown -h now
00 20 * * 3 root /sbin/shutdown -h now
01 18 * * 4 root /sbin/shutdown -h now
00 20 * * 4 root /sbin/shutdown -h now
01 18 * * 5 root /sbin/shutdown -h now
00 20 * * 5 root /sbin/shutdown -h now
01 16 * * 6 root /sbin/shutdown -h now
00 20 * * 6 root /sbin/shutdown -h now
## This is a test entry.
45 10 * * * root /usr/bin/notify-send 'Cronjob Test Message' 'This is a cronjob test message.'
46 10 * * 1 root /usr/bin/notify-send 'Cronjob Monday Test Message' ' This is Mondays cronjob test message.'
20 11 * * * root /usr/bin/notify-send 'Test' 'Message goes here.'
#51 11 * * 4 root shutdown -h now
# This line is the last line, be sure to terminated this line with a new line.
请注意，我正在进行各种测试。我意识到这很令人困惑，所以我将简化文件/etc/crontab以便于故障排除。这是我的新/etc/crontab故障排除文件：
username:/etc$ cat /etc/crontab
# /etc/crontab: system-wide crontab
# Unlike any other crontab you don't have to run the `crontab'
# command to install the new version when you edit this file
# and files in /etc/cron.d. These files also have username fields,
# that none of the other crontabs do.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
56 11 * * * root /usr/bin/notify-send 'Test' 'Message goes here.'
现在，我查看我的系统日志：
username:/etc$ /usr/bin/sudo /usr/bin/tail /var/log/syslog -f
Jan 29 10:58:31 hostname dbus[730]: [system] Successfully activated service 'org.freedesktop.timedate1'
Jan 29 10:58:31 hostname systemd[1]: Started Time & Date Service.
Jan 29 11:10:01 hostname cron[759]: (*system*) RELOAD (/etc/crontab)
Jan 29 11:10:01 hostname CRON[3240]: (root) CMD (/usr/bin/notify-send 'Test' 'Message goes here.')
Jan 29 11:17:01 hostname CRON[3283]: (root) CMD ( cd / && run-parts --report /etc/cron.hourly)
Jan 29 11:19:01 hostname cron[759]: (*system*) RELOAD (/etc/crontab)
Jan 29 11:20:01 hostname CRON[3314]: (root) CMD (/usr/bin/notify-send 'Test' 'Message goes here.')
Jan 29 11:34:35 hostname crontab[3386]: (root) BEGIN EDIT (root)
Jan 29 11:35:46 hostname crontab[3386]: (root) END EDIT (root)
Jan 29 11:55:01 hostname cron[759]: (*system*) RELOAD (/etc/crontab)
Jan 29 11:56:01 hostname cron[759]: (*system*) RELOAD (/etc/crontab)
Jan 29 11:56:01 hostname CRON[3537]: (root) CMD (/usr/bin/notify-send 'Test' 'Message goes here.')
最后一行说在 11:56:01 运行了命令/usr/bin/notify-send但没有显示弹出消息。
如果我从终端运行该精确（复制和粘贴）命令，/usr/bin/sudo则会出现弹出窗口。
username:/etc$ /usr/bin/sudo /usr/bin/notify-send 'Test' 'Message goes here.'
如果我以普通用户登录的身份从终端运行该精确（复制和粘贴）命令，则会出现弹出窗口。
username:/etc$ /usr/bin/notify-send 'Test' 'Message goes here.'
是什么赋予了？