How to redirect output to a file from within cron?
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.
I solved the problem. There are two ways:
M1
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.
M2
Change the default shell by adding SHELL=/bin/bash in the crontab -e file.
disclaimer [1].
I would like to add a footnote or addendum to @RanRag’s answer.
Make sure your shell redirection syntax conforms to /bin/sh. If you try to use shell redirection syntax that is not valid with /bin/sh then your command will fail and your cron job will not ever run.
In your /etc/cron.d/example1 config files if you specify a user other than root and that user’s login shell is not /bin/bash… you must still use /bin/sh syntax in /etc/cron.d/example1 command.
For example
If your user has shell csh or zsh or ksh set for his login shell. In your /etc/cron.d/example1 config file, the command must use /bin/sh syntax. Specifically any shell redirection must be /bin/sh syntax.
If you try to use for example csh shell redirect syntax in your /etc/cron.d/example1, then your cron job will never run. The logfile for crond located at /var/log/cron shall say that the command is run but the command will error out with a syntax error before your command gets run.
Where does crond emit error messages for a syntax error?
The error is not ever reported in /var/log/cron. crond instead by default emits any error messages using mail. So you must check /var/spool/mail/${USER} to see what is the error.
[1]
Disclaimer
- This answer assumes a
sysvsystem systemdinformation may differ- Specifically this information was learned for
centos-6distro and may not apply to differentsysvdistros- I mention
centos-6specifically, because different distros may have a differentcrondimplementation that differs fromcentos-6
- I mention
You can direct output information to a log file with 2 greater-than signs like this example:
* * * * * /usr/bin/python /path/to/script.py >> /path/to/logfile/output.log
Please see this convient crontab-line generator in this link: https://crontab-generator.org/