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最後更新: 2019-09-13
目錄
- Cron jobs 設定的格式
- Shell 與 Path
- Add cron job by echo
- crontab utc or local time
- "@" commands
- run-parts
- anacron
- 同一時間的 cron job 係 parallel 的
- 很可怕的 Options "-r"
- Run cron jobs permission
- Config file Permission(BAD FILE MODE)
- Do something on last day on month
- Troubleshoot
- cron run every 10s
- Add crontab from cli
- Doc
Cron jobs 設定的格式
Format
# 分 時 日 月 週 指令 m h d M w cmd
每一個 field 的 value
field allowed values
----- --------------
minute 0-59
hour 0-23
day of month 1-31
month 1-12 (or names, see below)
day of week 0-7 (0 or 7 is Sun, or use names)
- any *
- Lists 2,5,9,18-24
- every n */n
- steps "1-9/2" is the same as "1,3,5,7,9"
Notes(dom & dow)
If both 'day of month' and 'day of week' are restricted (i.e. 不是 "*" )
The command will be run when either field matches the current time.
"40 8 1,15 * 7" 相當於每月的 1, 15 號行. 另外每星期日也會行.
Shell 與 Path
/etc/crontab
SHELL=/bin/bash PATH=/sbin:/bin:/usr/sbin:/usr/bin MAILTO=root
Add cron job by echo
# 用 "-e" 加 remark
echo -e "# daily report \n 1 10 * * * $MyScript" | crontab -
# 在 shell 入面行
echo \" 1 * * * * wget -O - <URL> | bash; \" | crontab - ;
crontab utc or local time
* Not all versions of cron support running jobs using a time zone(TZ) other than the system's
* does not support per-user timezones.
如果 cron 支援 TZ
# This file has been deprecated: /etc/default/cron
可以在 /etc/init/cron.conf 加入
TZ="UTC"
支不支援請看:
man 5 crontab
# 如果系統不支援, 那就要靠 system 的 timezone 設定了
/etc/timezone
# Also be sure to restart cron as it won’t pick up the timezone change
/etc/init.d/cron restart
"@" commands
@
- @reboot Run once, at startup.
- @yearly Run once a year, "0 0 1 1 *".
- @annually (same as @yearly)
- @monthly Run once a month, "0 0 1 * *".
- @weekly Run once a week, "0 0 * * 0".
- @daily Run once a day, "0 0 * * *".
- @midnight (same as @daily)
- @hourly Run once an hour, "0 * * * *".
Usage
# sync time @reboot /usr/sbin/ntpdate stdtime.gov.hk > /dev/null 2>&1 3 * * * * /usr/sbin/ntpdate stdtime.gov.hk > /dev/null 2>&1
run-parts
The run-parts script loops over all executable files in this directory.
SYNOPSIS
run-parts DIRECTORY
opt:
--test # print the names of the scripts which would be run
--verbose # print the name of each script to stderr before running
--report # similar to --verbose, but only prints the name of scripts which produce output.
Centos 6:
daily, weekly, monthly 由 anacrontab 掌管了 !!
/etc/anacrontab
# period in days delay in minutes job-identifier command 1 5 cron.daily nice run-parts /etc/cron.daily 7 25 cron.weekly nice run-parts /etc/cron.weekly @monthly 45 cron.monthly nice run-parts /etc/cron.monthly # delay in minutes "0" = no delay. 它是設定 anacron 的 延遲 # (用於錯開 cron 的 jobs) # 5 + 45 min random delay RANDOM_DELAY=45 # 03:00-22:00 START_HOURS_RANGE=3-22
Centos 5:
/etc/crontab
01 * * * * root run-parts /etc/cron.hourly <==每小時 02 4 * * * root run-parts /etc/cron.daily <==每天 22 4 * * 0 root run-parts /etc/cron.weekly <==每週日 42 4 1 * * root run-parts /etc/cron.monthly <==每個月 1 號
Debian:
apt-get install anacron
/etc/crontab
# 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 )
anacron
它是由 cron 發動 /etc/cron.hourly/0anacron
It execute commands periodically, with a frequency specified in days.
After the command exits, Anacron records the date in a special timestamp file for that job,
so it can know when to execute it again.
(Unlike cron, anacron does not assume that the machine is running continuously.)
The period is specified in days (Only the date is used for the time calculations. The hour is not used.)
# Directory is used by Anacron for storing timestamp files
/var/spool/anacron
原理
cron -> /etc/cron.hourly/0anacron -> anacron -s
-s Serialize execution of jobs.
Anacron will not start a new job before the previous one finished.
Configuration File
/etc/anacrontab
SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin HOME=/root LOGNAME=root # the maximal random delay added to the base delay of the jobs RANDOM_DELAY=10 # 只有 03:00 ~ 22:00 才行 anacrontab 的 cron jobs START_HOURS_RANGE=3-22 # These replace cron's entries #period in days delay in minutes job-identifier command 1 5 cron.daily run-parts --report /etc/cron.daily 7 10 cron.weekly run-parts --report /etc/cron.weekly @monthly 15 cron.monthly run-parts --report /etc/cron.monthly
RANDOM_DELAY
Add a randomly delay between 0 ~ 10 minutes to the user defined delay.
1 5 cron.daily run-parts --report /etc/cron.daily
delay will be 5 minutes + RANDOM_DELAY for cron.daily
START_HOURS_RANGE
當 miss 在 START_HOURS_RANGE 時間執行(ie. 如那段時間關機了),
那就會略過那次執行 (不論是 daily, weekly, monthly 的 jobs)
i.e.
grep zfs-auto-snap /var/log/messages
Mar 26 03:39:02 tim zfs-auto-snap: ... Mar 27 03:31:02 tim zfs-auto-snap: ... Mar 28 03:33:02 tim zfs-auto-snap: ... Mar 29 03:19:02 tim zfs-auto-snap: ... Mar 30 03:33:02 tim zfs-auto-snap: ...
job-identifier
name for the job’s timestamp file
ls -l /var/spool/anacron
total 12 -rw-------. 1 root root 9 Feb 9 03:13 cron.daily -rw-------. 1 root root 9 Feb 9 03:07 cron.monthly -rw-------. 1 root root 9 Feb 9 03:33 cron.weekly
cat /var/spool/anacron/cron.daily
20220209
所有在 cron job 內行的 cmd 的 stdout 及 stderr 都會送到指定郵箱.
# If MAILTO is defined but empty (MAILTO=""), no mail will be sent.
# Otherwise mail is sent to the owner of the crontab.
MAILTO=""MAILTO
strings /usr/sbin/cron | grep mail
mailed %d byte%s of output but got status 0x%04x from MTA
bad mailto
/usr/sbin/sendmailCentos 6
- MAILTO
- MAILFROM
同一時間的 cron job 係 parallel 的
The tasks listed in cron will run in parallel,
2 4 * * * /path/to/command1 2 4 * * * /path/to/command2
如果想行完 command1 再行 command2 就要寫成
# Use a semicolon to run command2 after command1 has completed 2 4 * * * /path/to/command1; /path/to/command2
很可怕的 Options "-r"
current crontab to be removed
Run cron jobs permission
cron.allow 及 cron.deny 係用來 control 某一個 user 能否用 "crontab"
亦即是說設定好的 jobs 會繼續行的 !!
- /etc/cron.allow
If the file cron.allow exists, only users listed in it are allowed to use cron
- /etc/cron.deny
* If cron.allow does not exist, users listed in cron.deny are not allowed to use cron.
* If both files exist then cron.allow takes precedence
* In the case where neither file exists, the default on current Ubuntu is to allow all users to run jobs with crontab.
(but not some other Linux and UNIX systems)
測試
crontab -u www-data -l
The user www-data cannot use this program (crontab)
per user cron jobs file
/var/spool/cron/crontabs/username
Remark
為了令系統更安全, 行以下 cmd 會不錯
touch /etc/cron.allow /etc/cron.deny
Other User run cron jobs
行 cron jobs 的那個 user 要有 home directory
Dec 4 15:34:01 assp1 CROND[14606]: (CRON) ERROR chdir failed (/home/assp): No such file or directory
Config file Permission(BAD FILE MODE)
在 Centos6 上
Dec 27 15:46:01 mail crond[23299]: (root) BAD FILE MODE (/etc/cron.d/response)
一定要 permission 0644
ie.
chmod root: /etc/cron.d /etc/cron.d/* chmod 755 /etc/cron.d chmod 644 /etc/cron.d/*
Do something on last day on month
#!/bin/bash
root-path=/home/vhosts/datahunter.org
nday=`date --date='1 day' +%d`
if [[ $nday == 01 ]];
then
date
/usr/bin/php $root-path/cron.php >> $root-path/cron.log 2>&1
fi
Troubleshoot
[1] Debug $PATH Variable
system 's crontab
/etc/crontab
* * * * * root /usr/bin/env &> /tmp/env_dump.txt
daily 's crontab
/etc/cron.daily/env_dump # chmod 750 /etc/cron.daily/env_dump
/usr/bin/env &> /tmp/env_dump.txt
* cron.daily 係用 /etc/anacrontab 發動
user 's crontab
$PATH in user crontab(crontab -l) is different from system crontab (/etc/crontab)
PATH=/sbin:/bin:/usr/sbin:/usr/bin * * * * * env &> /tmp/env_dump.txt
Add crontab from cli
應用了 "crontab [-u user] file"
e.g.
crontab -l > mycron cat >> mycron << EOF # fix ftp permission * * * * * /root/scripts/fix-ftp-permission.sh EOF crontab mycron rm -f mycron
Doc
詳見 man 5 crontab
