# /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 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 )
解決：
# ll /bin/sh
lrwxrwxrwx 1 root root 4 Jun 25 2017 /bin/sh -> dash*
# rm -f sh
# ln /bin/bash /bin/sh -s
# ll /bin/sh
lrwxrwxrwx 1 root root 9 Nov 22 13:54 /bin/sh -> /bin/bash*
或改/etc/crontab 文件內的"SHELL=/bin/sh"爲"SHELL=/bin/bash"
關於bash和dash區別，自行百度。