Summary of some reasons why crontab scheduled tasks are not executed

Preface

I recently encountered some problems at work. The crontab scheduled tasks did not execute. Later, when I searched online, I found that the main reasons were:

1 The crond service is not started

Crontab is not a function of the Linux kernel, but relies on a crond service, which can be started and stopped. If it is stopped, no scheduled tasks can be executed. The solution is to turn it on:

crond

or

service crond start

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

yum -y install crontabs

2. Permission issues

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

Add execution permissions, or use bash abc.sh to execute

It is also possible that the user to whom the crontab task belongs does not have write permission to a certain directory, which will also cause it to fail.

3 Path Problem

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

./test.sh

That's it. But in crontab, you won't find this script, for example, write the complete:

/root/test.sh

4. Time difference problem

Because of the time difference between the server and the client, the crontab time is based on the server time.

The time difference is really annoying. I have experienced it myself. The phenomenon is as follows:

(1) I set up a scheduled script and used the date command to observe the server time. When the script was executed, it was not executed.

(2) But I set the script to execute once every minute, and it works OK

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

But it is obvious that the non-execution is caused by time inconsistency.

Finally, the problem was solved with the following two lines:

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

Refer to this article: https://www.jb51.net/article/154296.htm

5. Variable Problems

Sometimes the command contains variables, but crontab does not have them when it is executed, which can also cause execution failure.

After verification, my scheduled script test.sh is not executed for any of the above reasons. In fact, my script is just one sentence:

#!/bin/bash
echo 123 >> testFile

I hope to test the timing script I set up in this way. So I set the script to execute once a minute, but I can't find the file in the directory where the script is located. I manually execute

# sh test.sh

But you can see this file in the directory where the script is located

I suspected that crontab was not executed at all, so I added it directly in crontab

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

The testFile file is generated, indicating that crontab is executed, so it seems that there is a problem with my script itself.

Finally, I found that the full path must be written here for testFile. I naively thought that testFile would be generated in the directory where the script is located, so I changed it to the following form

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

And then it works.

In fact, the path is a very easy place to go wrong. Suppose there is a script file test1.sh in the /home/denglinjie directory, and then there is a script file test2.sh in the same directory.

test2.sh is executed in test1.sh, and a relative path is used, that is, the path relative to test1.sh.

If you edit it in crontab -e, the execution method is

sh /home/denglinjie/test1.sh. When calling sh test2.sh, the system will think that it is looking for test2.sh in the directory where the crontab file is located, but it cannot find it, causing the execution to fail.

At first, I thought that I would put the script files I wrote to be executed and other called scripts and crontab files in one place, so that I could pull them, but it failed, probably because of permission issues, I couldn't enter the /var/spool/cron directory.

So another solution is to enter the directory where the script is located through the cd /home/denglinjie command before executing the script

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

Recently, a new reason for crontab not to execute was discovered

What I want to execute here is the python script. The directory of my python script is:

/data/denglinjie/work/UpdateModuleSwitch

At the beginning, my scheduled task was written like this:

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

It was found that it was not executed at the time point. Part of the content of update_switch.py ​​is as follows:

import pymongo

That is, I introduced the pymongo I installed in my script. Note that this pymongo is installed on the specified python version.

Reason for non-execution: When the crontab scheduled task is executed, the python used is not my python, and the python used does not have pymongo installed, resulting in import failure

The solution is to change it to the following form:

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

Specify the Python program to run. This Python program has pymongo installed and bound to it, or use the following format:

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

Because my python is installed in my own user directory, the system cannot find this python, so I just need to add my python to the system PATH environment variable.

Summarize

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