Summary of some reasons why crontab scheduled tasks are not executed

Update time: January 9, 2019 09:34:57 Author: Hope on the Field

This article mainly summarizes and introduces to you some reasons why crontab scheduled tasks are not executed. It also provides solutions to every possible inducement. It has certain reference and learning value for colleagues who encounter this problem. Students who need it, please follow the editor to learn together

Preface

Recently, I encountered some problems at work. The crontab scheduled tasks were not executed. Later, when I searched online, I found that the Internet mainly mentioned these 5 incentives:

1crond service is not started

crontab is not a function of the Linux kernel, but relies on a crond service. This service can be started or stopped. If it stops, it will be difficult to execute any scheduled tasks. The solution is to open it:

crond

or

service crond start

If it prompts that the crond command does not exist, it may have been deleted. You can reinstall it under CentOS through this command:

yum -y install crontabs

2Permission issue

For example: the script does not have x execution permission, solution:

Reduce execution permissions, or use bashabc.sh to execute

It is also possible that the user to which the crontab task belongs does not have write permissions for a certain directory and it will fail.

3 Path problem

Some commands execute normally in the shell, but always fail when executed in crontab. It may be that the sh used by crontab does not recognize the path correctly. For example: after logging in to the shell as root and executing a /root/test.sh, just execute

./test.sh

That’s it. And this script will not be found in crontab, for example, write it completely:

/root/test.sh

4 Time difference issue

Due to the time difference between the server and the client, the crontab time is based on the server time.

The issue of jet lag is really annoying. I have experienced this myself. The phenomenon is as follows:

(1) I set up a scheduled script and used the date command to observe the server time when it reached the script execution time and found that it was not executed

(2)And I set the script to execute once every minute, which is OK

Damn it, is the server time correct? Do I need to add which time zones? So I tried reducing the script time by 10, 12, or 8 hours, but it didn't work.

然而很顯著是時間不一致引起的不執(zhí)行。

最后用如下兩行解決了問題：

cp /usr/share/zoneinfo/Asia/Shanghai /etc/localtime
service crond restart

參考這篇文章：

5變量問題

有時侯命令中富含變量，但crontab執(zhí)行時卻沒有，也會導(dǎo)致執(zhí)行失敗。

驗證后，我的定時腳本test.sh不執(zhí)行不是上述任何一種緣由，雖然我的腳本就一句話：

#!/bin/bash
echo 123 >> testFile

我希望通過這些方法來測試我設(shè)置的定時腳本起作用了，于是我設(shè)置了該腳本每分鐘執(zhí)行一次，而且死活在腳本所在目錄看不到這個文件linux 計劃任務(wù)沒執(zhí)行，我自動執(zhí)行

# sh test.sh

卻能看見在腳本所在目錄能看見這個文件

我懷疑是crontab根本沒有執(zhí)行，于是我在crontab中直接添加了

*/1 * * * * echo 123 >> /home/denglinjie/testFile

testFile文件生成了，說明crontab是執(zhí)行了的，那看來是我腳本自身存在問題

最后發(fā)覺，原先是testFile這兒必須寫完整的路徑，我天真的以為testFile會生成在腳本所在的目錄，所以改成了如下方式

#!/bin/bash
echo 123 >> /data/denglinjie/testFile

之后就可以了。

雖然路徑是個十分容易出問題的地方，假定在/home/denglinjie目錄下有一個腳本文件test1.sh，之后在該目錄下還有一個腳本文件test2.sh

在test1.sh中執(zhí)行了test2.sh，并且用的是相對路徑，即相對test1.sh所在的路徑。

若果在crontab-e中編輯的時侯，執(zhí)行的方法是

sh/home/denglinjie/test1.sh，當(dāng)執(zhí)行到調(diào)用shtest2.sh的時侯，系統(tǒng)會覺得是從crontab文件所在的目錄去找test2.sharm linux，而且顯然是找不到的，導(dǎo)致執(zhí)行失敗

最開始我想的方式是，我要將我寫的待執(zhí)行的腳本文件以及被調(diào)用的其他的腳本和crontab文件放在一個地方，這樣就可以拉，并且失敗了，可能是由于權(quán)限問題，我進不去/var/spool/cron目錄。

所以另外一個解決方式就是在執(zhí)行腳本之前先通過cd/home/denglinjie命令步入到腳本所在目錄

------------------------------------------------------------------

近來又發(fā)覺一種新的導(dǎo)致crontab不執(zhí)行的誘因

這兒我要執(zhí)行的是python腳本，我python腳本的目錄為：

/data/denglinjie/work/UpdateModuleSwitch

一開始我的定時任務(wù)是這樣寫的：

0 * * * * cd /data/denglinjie/work/UpdateModuleSwitch;python update_switch.py

發(fā)覺到了時間點竟然沒有執(zhí)行,其中update_switch.py的部份內(nèi)容如下：

import pymongo

就是我的腳本中引入了自己安裝的pymongo，注意，這個pymongo是安裝到了指定的python版本上的

不執(zhí)行緣由：crontab定時任務(wù)執(zhí)行的時侯，使用的python不是我的那種python，使用的這個python沒有安裝pymongo，致使import失敗

解決辦法，改成如下方式：

0 * * * * cd /data/denglinjie/work/UpdateModuleSwitch;/data/zhoumi/install_evn/bin/python update_switch.py

指定運行使用的python，這個python早已安裝綁定了pymongo，或則用如下方式：

0 * * * * export PATH=/data/zhoumi/install_evn/bin/:$PATH;cd /data/denglinjie/work/UpdateModuleSwitch;python update_switch.py

由于我的這個python是安裝在我自己的用戶目錄下linux 計劃任務(wù)沒執(zhí)行，所以系統(tǒng)找不到這個python，所以只要將我的python也加入到系統(tǒng)PATH環(huán)境變量中就可以了

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