Ubuntu系统定时作业无效果的排错
因一个数据库日志文件无限扩展，最终填满磁盘空间而导致数据库Hang住，为监控日志文件增长情况，创建一个定时作业以执行如下SQL命令来查询数据文件大小：
select f.name, file_id fileid, physical_name filename, g.name filegroup
, iif(size < 128, cast(size * 8 as varchar) + ' KB'
, iif(size / 128 < 1024, cast(size / 128 as varchar) + ' MB'
, cast(size / 128 / 1024 as varchar) + ' GB')) size
, iif(max_size=-1, 'Unlimited'
, iif(max_size<128, convert(varchar(20), cast(max_size as bigint) * 8) + ' KB'
, iif(max_size / 128 < 1024, cast(max_size / 128 as varchar) + ' MB'
, cast(max_size / 128 / 1024 as varchar) + ' GB'))) maxsize
, iif(is_percent_growth=1, cast(growth as varchar(3)) + '%'
, iif(growth < 128, convert(varchar(20), growth * 8) + ' KB'
, convert(varchar(20), growth / 128) + ' MB')) growth
, f.type_desc usage
from sys.database_files f left join sys.filegroups g on f.data_space_id=g.data_space_id
创建作业：
csyy@server02:~$ crontab -l
25 * * * * sqlcmd -S . -U sa -P 'Password' -i MonitEmrFileSize.sql -o checkfilesize/`date +%Y%m%d.%H%M.txt`
csyy@server02:~$ cat MonitEmrFileSize.sql
use emr22
go
select cast(f.name as varchar(20)) name, cast(file_id as varchar(6)) fileid
, cast(physical_name as varchar(40)) filename, cast(g.name as varchar(10)) filegroup
, cast(iif(size < 128, cast(size * 8 as varchar) + ' KB'
, iif(size / 128 < 1024, cast(size / 128 as varchar) + ' MB'
, cast(size / 128 / 1024 as varchar) + ' GB')) as varchar(10)) size
, cast(iif(max_size=-1, 'Unlimited'
, iif(max_size<128, convert(varchar(20), cast(max_size as bigint) * 8) + ' KB'
, iif(max_size / 128 < 1024, cast(max_size / 128 as varchar) + ' MB'
, cast(max_size / 128 / 1024 as varchar) + ' GB'))) as varchar(10)) maxsize
, cast(iif(is_percent_growth=1, cast(growth as varchar(3)) + '%'
, iif(growth < 128, convert(varchar(20), growth * 8) + ' KB'
, convert(varchar(20), growth / 128) + ' MB')) as varchar(10)) growth
, cast(f.type_desc as varchar(10)) usage
from sys.database_files f left join sys.filegroups g on f.data_space_id=g.data_space_id
go
exit
过了定时作业执行时间，未生成结果文件。
1 、一开始怀疑是普通用户没有执行定时作业的权限，
将作业任务改为root用户来执行，但还是没有效果；
2 、检查cron服务状态，提示"未安装MTA，放弃输出"，；
root@server02:/home/csyy# service cron status
● cron.service - Regular background program processing daemon
Loaded: loaded (/lib/systemd/system/cron.service; enabled; vendor preset: enabled)
Active: active (running) since Tue 2019-11-26 22:38:26 CST; 2 days ago
Docs: man:cron(8)
Main PID: 3690 (cron)
Tasks: 1
Memory: 119.8M
CPU: 12.711s
CGroup: /system.slice/cron.service
└─3690 /usr/sbin/cron -f
Nov 29 15:55:01 server02 CRON[171781]: (root) CMD (MonitEmrFileSize.sh > checksize/`date +)
Nov 29 15:55:01 server02 CRON[171769]: pam_unix(cron:session): session closed for user root
Nov 29 15:55:01 server02 CRON[171770]: (CRON) info (No MTA installed, discarding output)
Nov 29 15:55:01 server02 CRON[171770]: pam_unix(cron:session): session closed for user root
Nov 29 15:55:01 server02 CRON[171773]: (CRON) info (No MTA installed, discarding output)
Nov 29 15:55:01 server02 CRON[171773]: pam_unix(cron:session): session closed for user csyy
Nov 29 15:55:01 server02 CRON[171772]: (CRON) info (No MTA installed, discarding output)
Nov 29 15:55:01 server02 CRON[171772]: pam_unix(cron:session): session closed for user csyy
Nov 29 15:55:01 server02 CRON[171771]: (CRON) info (No MTA installed, discarding output)
Nov 29 15:55:01 server02 CRON[171771]: pam_unix(cron:session): session closed for user root
原因：系统未安装MTA，所以在Job作业的命令中不能有标准输出和标准错误输出，否则作业会将输出重定向到邮件，导致触发"未安装MTA"的错误。
3 、将CRON中的命令最后加上" >/dev/null 2>&1"后，任务还是未有结果；
csyy@server02:~$ crontab -l
25 * * * * sqlcmd -S . -U sa -P 'Password' -i MonitEmrFileSize.sql -o checkfilesize/`date +%Y%m%d.%H%M.txt`>/dev/null 2>&1
4 、将CRON中的命令作为一个shell命令，在CRON中执行此shell命令，并在最后加上" >/dev/null 2>&1"后，定时任务终于有了结果；
csyy@server02:~$ crontab -l
25 * * * * ShellCmd.sh >/dev/null 2>&1
csyy@server02:~$ cat ShellCmd.sh
sqlcmd -S . -U sa -P 'Password' -i MonitEmrFileSize.sql -o checkfilesize/`date +%Y%m%d.%H%M.txt`
解释：第3步后，虽然从Shell命令的角度看，标准输出和标准错误输出已经全部进行了重定向，命令已经没有了输出，但在命令中还嵌套了一个Shell命令date，标准输出和标准错误输出未进行重定向，导致触发"未安装MTA"的错误。
