Bash Cron – How to Redirect Output to a File from Within Cron

bashcronio-redirection

I have a backup script which I need to run at a particular time of a day so I am using cron for this task and from within cron am also trying to redirect the output of backup script to a logfile.

crontab -e

*/1 * * * * /home/ranveer/backup.sh &>> /home/ranveer/backup.log

In the above cron entry I am redirecting both stderr and stdout to a log file.

The above cron job executes fine according to syslog and it performs the task mentioned in the backup.sh file but it doesn't write anything to the log file.

/var/log/syslog

Oct 19 20:26:01 ranveer CRON[15214]: (ranveer) CMD (/home/ranveer/backup.sh &>> /home/ranveer/backup.log)

When I run the script from cli it works as required and output is written to a log file

ranveer@ranveer:~$ ./backup.sh &>> backup.log 
ranveer@ranveer:~$ cat backup.log
Fri Oct 19 20:28:01 IST 2012
successfully copied testdir
test.txt successfully copied
-------------------------------------------------------------------------------------
ranveer@ranveer:~$

So, why the output of file is not getting redirected to the file from within cron.

Best Answer

I solved the problem. There are two ways:

Change the redirection from &>> to 2>&1. So now crontab -e looks like

*/1 * * * * /home/ranveer/vimbackup.sh >> /home/ranveer/vimbackup.log 2>&1

I believe the above works because by default cron is using sh to run the task instead of bash so &>> is not supported by sh.

Change the default shell by adding SHELL=/bin/bash in the crontab -e file.

Example

$ echo "hi" 2>&1 | logger &
[1] 26818
[1]+  Done                    echo "hi" 2>&1 | logger

$ sudo tail /var/log/messages
Apr 12 17:53:57 greeneggs saml: hi

You can use the abbreviated notation here as well, if used cautiously in a actual Bash shell (not to be confused with Dash):

$ echo "hi" |& logger &

NOTE: This is equivalent to <cmd1> 2>&1 | <cmd2>. Again only use the above when making use of an actual Bash shell interactively, would be a good way to approach it.

excerpt from ABSG

# |& was added to Bash 4 as an abbreviation for 2>&1 |.

References

Advanced Bash Scripting Guide - Chapter 20. I/O Redirection

linux cron – Email Only Occasionally Sent on Output and Errors

Upon further testing, I suspect the & is messing with your results. As you point out, &>/dev/null is bash syntax, not sh syntax. As a result, sh is creating a subshell and backgrounding it. Sure, the subshell's echo creates stderr, but my theory is that:

cron is not catching the subshell's stderr, and
the backgrounding of the subshell always completes successfully, thus bypassing your || echo ....

... causing the cron job to have no output and thus no mail. Based on my reading of the vixie-cron source, it would seem that the job's stderr and stdout would be captured by cron, but it must be getting lost by the subshell.

Test it yourself in a /bin/sh environment (assuming you do not have a file named 'bar' here):

(grep foo bar) &
echo $?

Best Answer

Related Solutions

Linux Shell – How to Redirect stdout & stderr to Logger

Example

References

linux cron – Email Only Occasionally Sent on Output and Errors

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