我有一个运行 Ubuntu 的虚拟机，它具有以下 cron 设置，每天运行四个作业：
10 6 * * * cd $HOME/...
30 4 * * * mongodump ...
30 4 * * * . .bashrc && ...
10 4 * * * cd $HOME/...
的 authlog/var/log/auth.log显示这些内容：
Apr 10 18:17:01 ip-172-26-9-252 CRON[24761]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 10 18:17:01 ip-172-26-9-252 CRON[24761]: pam_unix(cron:session): session closed for user root
Apr 10 18:40:01 ip-172-26-9-252 CRON[24888]: pam_unix(cron:session): session opened for user bitnami by (uid=0)
Apr 10 18:40:01 ip-172-26-9-252 CRON[24888]: pam_unix(cron:session): session closed for user bitnami
Apr 10 19:17:01 ip-172-26-9-252 CRON[25146]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 10 19:17:01 ip-172-26-9-252 CRON[25146]: pam_unix(cron:session): session closed for user root
Apr 10 19:40:01 ip-172-26-9-252 CRON[25261]: pam_unix(cron:session): session opened for user bitnami by (uid=0)
Apr 10 19:40:01 ip-172-26-9-252 CRON[25261]: pam_unix(cron:session): session closed for user bitnami
Apr 10 20:17:01 ip-172-26-9-252 CRON[25523]: pam_unix(cron:session): session opened for user root by (uid=0)
Apr 10 20:17:01 ip-172-26-9-252 CRON[25523]: pam_unix(cron:session): session closed for user root
Apr 10 20:40:01 ip-172-26-9-252 CRON[25642]: pam_unix(cron:session): session opened for user bitnami by (uid=0)
Apr 10 20:40:02 ip-172-26-9-252 CRON[25642]: pam_unix(cron:session): session closed for user bitnami
为什么cron每 30 分钟登录和退出一次会话？