Error in crontab entry

Question

I have been trying to make a cron entry for a shell script:

50 */4 * * * /path/script-file.sh > /dev/null 2>&1

aimed to run the script at HH:50 at a frequency of 4 hours. But this errors out with the message:

crontab: error on previous line; unexpected character found in line.

crontab: errors detected in input, no crontab file generated.

I removed the "/4" and the error vanished, but I know that cron does allow this format. Does anybody know what the issue could be?

Thank you very much for any help.

Answer 1

Some cron implementations don't support steps (e.g. */4) - check man 5 crontab on your particular system.

You can use the list 0,4,8,12,16,20 instead.

Off-topic: If you are using bash, you could probably replace > /dev/null 2>&1 with the shorter &>/dev/null or just close stdout and stderr with 1>&- 2>&-. (see @Keith Thompson's comment below)