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Old 01-30-2009, 12:15 PM   #1
jlinkels
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Use a pipe in a cron command?


I have set up a cron job in /etc/crontab.

The line looks like this:
Code:
02 * * * *      root    /bin/echo "update process_control set log_level='INFO' where name ='TransmitterController TH1'" | /usr/bin/mysql -uphp -pmypasswd event_reports_dev
What I see in /var/log/syslog is this:
Code:
/bin/echo "update process_control set log_level='INFO' where name ='TransmitterController TH1'" | /usr/bin/mysql -uphp -pmypasswd event_reports_dev
That is the command line exactly as I want it.

However, the requested setting is not performed with mysql. When I enter the command on the command line, it is set, so my syntax should be correct.

I know that piping creates a different process, different subshell, but I don't see an apparent reason why I should not be allowed to do this in a cron job. I create a string, echo it, pipe it, another shell gets started, mysql is called and the string is piped from one shell to the other.

Anyway, I put the whole command line including pipe in a script file, and call the script file from cron. It works perfectly.

Somehow I don't seem to understand what it going on in cron.

jlinkels
 
Old 01-30-2009, 12:59 PM   #2
paulsm4
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Hi -

It's probably permissions - the script is running in the context of a user who's authorized to use mysql; the failing cron job isn't.

'Hope that helps .. PSM
 
Old 01-30-2009, 01:44 PM   #3
Matir
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Most implementations of cron do not start a shell with the process in it, and a shell is what would parse out the pipe and run the two processes separately. Echo is seeing the pipe and mysql command as part of its arguments, nothing more. You should be able to do what you'd like by placing the commands in a bash script and calling it from cron, or by using the the -c option to bash, but be careful of your quoting then!
 
Old 01-30-2009, 01:57 PM   #4
jlinkels
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Quote:
Originally Posted by paulsm4 View Post
It's probably permissions - the script is running in the context of a user who's authorized to use mysql;
Not likely, I am running the command as root, and I give MySQL username and password on the command line.

Quote:
Originally Posted by matir
Most implementations of cron do not start a shell with the process in it, and a shell is what would parse out the pipe and run the two processes separately.
That must be it. I knew pipes were dangerous, and this is a good explanation.

Quote:
Originally Posted by matir
You should be able to do what you'd like by placing the commands in a bash script and calling it from cron
Quote:
Originally Posted by jlinkels
Anyway, I put the whole command line including pipe in a script file, and call the script file from cron. It works perfectly.
That is what I did, and it works, I just couldn't understand why I didn't work with the pipe. I hope someone else finds the post

jlinkels
 
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