从 crontab 运行的脚本会留下一个空文件

Question

I've tried to run a script from crontab on my Linux system which is leaving an empty file. When I run the script logged on the terminal, it works fine.

Cron line:

@reboot sleep 1m; /bin/bash /root/start_reader_services

The script "start_reader_services" calls a Python script as below:

/root/java/tag_output >> $TAGS_PATH/tags_$DATE_FILE.log

tag_output basically prints out a series of IDs. The same mechanics used to work when I was sending the stdout to my serial port (tag_output > /dev/ttyO0), but now, writing it to the file from cron, the file is created, but is empty.

As I mentioned, running start_reader_services or any piece of that on command line, it works as expected.

Have done: - Set bash as cron shell - Set java environments on cron

As requested:

ls -l /root/java/tag_output
-rwxr-xr-x 1 root root 1981 Aug  6 12:06 /root/java/tag_output

First line of tag output:

#!/usr/bin/python

Any help?

Answer 1

Is it possible that tag_output is writing to stderr instead of stdout ? Try redirecting stderr too:

/root/java/tag_output >> $TAGS_PATH/tags_$DATE_FILE.log 2>&1

As an aside, you might also want to quote and use braces with the shell variable expansion; that makes it easier to read and probably a little safer:

/root/java/tag_output >> "${TAGS_PATH}/tags_${DATE_FILE}.log" 2>&1

Answer 2

After initialising and updating the crontab using crontab python and before issuing

cron.write(), give the command, cron.__reduce__()[2]['lines'] = filter(None, cron.__reduce__()[2]['lines'])

Doing this will remove the empty lines being printed in Linux crontab.