To ensure that the most imperative programs remain online as much as possible (even after a server crash or reboot), one can create a short bash script to check if the program is running, and if it is not, to launch it. By using cron to schedule the script to be executed on a regular basis, we can make sure that program relaunches whenever it goes down.
The first step in this process is to create the script itself. There are a variety of programs such as upstart, supervisor, and monit, that have the capability to start and monitor applications on a virtual private server in a very nuanced way— this bash script will simply provide an on switch.
Below is a sample script that starts apache if it finds it off.
nano launch.sh
#!/bin/sh ps auxw | grep apache2 | grep -v grep > /dev/null if [ $? != 0 ] then /etc/init.d/apache2 start > /dev/null fi
Once you have saved the script, you must give it executable permissions in order to be able to run it:
chmod +x launch.sh
Apache can be replaced with any required application. Should you want to set up the script for a variety of applications, you can create a new script for each one, placing it on its own line in the cron file.
With the script in hand, we need to set up the schedule on which it will run. The cron utility allows us to schedule at what intervals the script should execute. Start by opening up the cron file:
crontab -e
Cron has a detailed explanation of how the timing system works at the beginning.
Once you know how often you want the script to run, you can write in the corresponding line.
The most often that the script can run in cron is every minute. Should you want to set up such a small increment, you can use this template:
* * * * * ~/launch.sh
Every five minutes would be set up like this:
*/5 * * * * ~/launch.sh
Setting up this simple script will keep the program starting up after it shuts down for any reason. This is convenient as it will ensure that the longest time that a program will be down is for the interval of time that you specified in the cron configuration.
Should you need a program that is even slightly more subtle, you can set up the details of your startup with one of the several server monitoring programs (Supervisor, Upstart, or Monit).
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Don’t you need to make sure that the script is executable by doing something like u+x ~/launch.sh?
Yes, you’re correct! Before you can run a script from the command line or through a cron job, it needs to have the appropriate executable permissions. You can do this with the
chmod
command. Here’s how to ensure your script is executable:Replace
/path/to/your/script/relaunch.sh
with the actual path to your script.The output should show something like
-rwxr-xr-x
, indicating that the script is executable.Regards
@Marc: That is correct, I’ve updated the article. Thanks!
I’m using this script to restart mysql with no luck. Log says
The error message you’re encountering (
unexpected operator
) typically arises from using[
for conditional tests in a shell script where the syntax is not correctly followed. In this case, it might be due to the way you’re checking the exit status of the previous command. In shell scripts, the correct syntax for string comparison is=
or!=
rather than using==
as you might in other programming languages.Here’s a revised version of your script that includes proper syntax and adds some logging for troubleshooting:
If the script does not work as expected, check the log file at
/var/log/mysql-check.log
for any output that may indicate what went wrong.Regards
@robert: Can you pastebin the contents of
/root/mysql-check.sh
?I’m trying to utilise this script and crontab to keep an eye on MySQL.
I couldn’t get the script to work.
I’m very inexperienced with this stuff but reviewing the above shouldn’t it be:
I’m not really sure the significance of the $? in the above though I would assume it’s a print of the output from the previous command.
I’m probably wrong here but if MySQL is down and thus not printed in ps auxw and we’re removing the grep process too then isn’t an output of nothing what should be triggering the process restart?
EDIT: No that still doesn’t solve the problem. If I use = 0 the service reboots at every run of the script. If I use != 0 then the service never reboots. Confused.
It sounds like you’re on the right track with your script, but let’s clarify how the logic should work and how the exit status
$?
is used.Exit Status
$?
: This special variable captures the exit status of the last command executed. In your case:ps auxw | grep mysql | grep -v grep
finds a running MySQL process, it will exit with status0
.1
(or another non-zero value).Condition Logic:
!= 0
).0
, it means MySQL is running, and you should not restart it.1
, it means MySQL is not running, and you want to restart it.Here’s the script with comments explaining each part:
ps auxw | grep mysql | grep -v grep
: This command lists all running processes and checks for “mysql”, filtering out the grep process itself.> /dev/null
: Suppresses output.if [ $? != 0 ]
: This checks if the exit status is not0
, indicating MySQL is not running./var/log/mysql-check.log
so you can track when it checks the status and any actions taken.To set this script to run periodically (e.g., every minute), you can add it to your crontab:
Your original logic is correct. Keep the
!= 0
condition to restart MySQL when it’s not running. The added logging will help you diagnose what the script is doing when it runs via cron. If you still face issues, check the log file for entries and errors to help pinpoint the problem.Regards
Anyone able to help on this?
@savage above had some good stuff and I modified it for systemd based Debian and to my liking as follows - this works, and no, I also could not get the original syntax to work.
#!/bin/sh
#functions
RESTART=“/bin/systemctl restart apache2.service” SERVICE=“apache2.service” LOGFILE=“/home/username/Desktop/apache.log”
#check for the word dead in the service output from systemctl
if
then
else echo “User, apache2 was running as of $(date)” >> $LOGFILE
fi
Thanks for the tutorial of how to make a bash script. However for this case I would prefer something like “service apache2 status || service apache2 start” directly on the crontab line. I saw that trick here https://www.digitalocean.com/community/questions/how-do-you-check-mysql-status-via-cron?answer=18288
You can simplify your cron job by using a one-liner directly in the crontab. This approach checks the status of the Apache service and starts it if it’s not running, all in one command.
Regards
hello i add the commande and when i try to add cron tab i got this ? */1 * * * * ~/launch.sh -bash: */1: No such file or directory
The error you’re encountering is due to how you’re trying to add the cron job. When you add a cron job, the timing syntax should be specified without any leading characters like
*/1
. Also,~
(the home directory shortcut) may not resolve properly in the cron environment.Here’s how to properly set up your cron job:
Specify the Full Path: Instead of using
~/launch.sh
, specify the full path to your script. For example, if your script is located in/home/yourusername/launch.sh
, you would use that full path.Cron Timing Syntax: If you want to run the script every minute, you can simply use
* * * * *
instead of*/1 * * * *
. They both serve the same purpose, but the former is more commonly used.Regards
Sorry but it does not work… It doesn’t restart my programs…
You can double check the syntax in your commands and also examine the command output and also service logs to see if there is anything wrong with the services itself before automating the process to restart them.
Regards
Some problem here too.
My script works just fine. It is in /etc/apache2/relaunch.sh, file permissions are
here’s the script
I try to run it from root’s cron. Here’s the cron entry.
Strangely, the script fires every minute and writes an entry into my log file without a problem, but won’t work correctly when it is run from Cron. Is it some problem with environment variable? Maybe I need to set that in the script too?
If I stop Apache, the script still writes the the log file but doesn’t restart Apache.
The issue you’re encountering is likely due to the environment in which the cron job runs, which is more limited than your normal shell environment. When cron executes a script, it doesn’t inherit the same environment variables, which may affect how commands like
/etc/init.d/apache2 restart
work.Here are a few things to check and modify to improve the functionality of your script when run from cron:
Modifications to the Script
apache2
, it’s good practice to specify the full path forps
andgrep
as well.#!/bin/sh
, you might want to use#!/bin/bash
for better compatibility, especially if you’re using features specific to bash.Explanation of Changes
ps
andgrep
are specified to ensure that cron can find them.Cron Entry
Ensure the cron job is set correctly:
Permissions
Make sure the script is executable:
Testing
After making these changes, monitor the log file to see if it behaves correctly when Apache is stopped. If it still doesn’t restart Apache, check the log file for any errors reported by the restart command. If issues persist, you can also check
/var/log/syslog
for cron-specific logs that may provide additional insights.