cron syntax

gw1500se · 06-12-2020, 09:23 AM

I'm trying to run a script on the first Monday of the month. I set up this:

0 1 * * mon [ $(date +%d) -le 07 ] && /usr/local/bin/pi_backup full

Apparently there is something wrong with my syntax because the job does not run and all I get in the cron.log is:

CMD ([ $(date +)

I'm guessing I need to escape the % but I'm not sure why that is necessary.

Ser Olmy · 06-12-2020, 09:58 AM

Rather than invoking [ and date to check that it's the first week of the month, why not specify a date range as a crontab parameter:

Code:

0 1 1-7 * mon /usr/local/bin/pi_backup full

(But it would be interesting to know if the "%" sign needs some form of escaping when used in a crontab, and if so, why.)

michaelk · 06-12-2020, 10:00 AM

Because % is a cron special character.

Quote:

man (5) crontab:

Percent-signs (%) in the command, unless escaped with backslash (\),
will be changed into newline characters, and all data after the
first % will be sent to the command as standard input.

The reason why 0 1 1-7 * mon does not work.

Quote:

Commands are executed by cron(8) when the minute, hour,
and month of year fields match the current time, and when at least one of
the two day fields (day of month, or day of week) matches the current

gw1500se · 06-12-2020, 10:12 AM

Got it. Thanks.

Ser Olmy · 06-12-2020, 10:32 AM

Quote:

Originally Posted by michaelk

Because % is a cron special character.

Quote:

man (5) crontab:

Percent-signs (%) in the command, unless escaped with backslash (\),
will be changed into newline characters, and all data after the
first % will be sent to the command as standard input.

Interesting. It seems there are significant differences between the various cron implementations.

For instance, on a system that uses "Dillon's lightweight cron daemon", the page displayed by man crontab does not contain the text above. Which cron implementation was the quote taken from?

Quote:

Originally Posted by michaelk

The reason why 0 1 1-7 * mon does not work.

Quote:

Commands are executed by cron(8) when the minute, hour,
and month of year fields match the current time, and when at least one of
the two day fields (day-of-month and or day-of-week) matches the current

That seems to indicate that the day-of-month and day-of-week fields are evaluated together as an "or" statement.

This, too, appears implementation-specific. From the man crontab page for Dillon's cron:

Quote:

If you specify both a day in the month and a day of week, it will be interpreted as the Nth such day in the month.

TB0ne · 06-12-2020, 10:36 AM

Quote:

Originally Posted by gw1500se

Got it. Thanks.

To further clarify/complicate/muddy things (depending on how you look at it):
I typically paste this into the tops of all my crontab files.

Code:

# Format is as follows. Six fields, with the sixth being a command to run. The first five are                                                                                                                                                                                                                                     
# date/time related. An asterisk means run matching ANY condition                                                                                                                                                                                                                                                          
#                                                                                                                                                                                                                                                                                                                          
# *     *     *   *    *        command to be executed                                                                                                                                                                                                                                                                     
# -     -     -   -    -                                                                                                                                                                                                                                                                                                   
# |     |     |   |    |                                                                                                                                                                                                                                                                                                   
# |     |     |   |    +----- day of week (0 - 6) (Sunday=0)                                                                                                                                                                                                                                                               
# |     |     |   +------- month (1 - 12)                                                                                                                                                                                                                                                                                  
# |     |     +--------- day of month (1 - 31)                                                                                                                                                                                                                                                                      
# |     +----------- hour (0 - 23)                                                                                                                                                                                                                                                                                         
# +------------- min (0 - 59)

...just for reference purposes. Crontabs can take fairly complicated strings, as well as regex'es, such as:
Running a script on the last Friday of the month:

Code:

1 * * * 5 [ $(date +"\%m") -ne $(date -d 7days +"\%m") ] && /path/to/your/script

The $(date +"\%m") -ne $(date -d 7days +"\%m") statement checks to see if the month today is not equal to month next week (7days from now - same day). If they are equal then current day (Friday in our case) is not the last weekday (we specified Friday) of the month. If not, it is the last Friday, so it runs your script.

Running a script on the last DAY of the month:

Code:

05 10 * * * [ `date -d tomorrow +\%d` -eq '01' ] && /path/to/script

No matter what date is the end of the month, the next day is always "01". The time code will kick it off, but the script will only run if tomorrows date is "01".

You can test things on the command line with:

Code:

echo $(date +"%m") -ne $(date -d 7days +"%m")

...to play with output until you get what you want.

michaelk · 06-12-2020, 11:41 AM

vixie-cron

Your correct there are multiple versions i.e fcron, dcron (Dillon's), vixie/cronie and bcron. Without knowing what distribution/version you can not tell but Mandriva was based on Redhat which did/does use vixie/cronie. And the fact the cron log indicated a new line...

ehartman · 06-12-2020, 12:32 PM

Quote:

Originally Posted by gw1500se

$(date +%d) -le 07 ]

This is a bash expression, NOT a crontab date field.
You can only specify a (fixed) date range in that field, not an expression.

michaelk · 06-12-2020, 12:42 PM

cron is run via sh and using date is a legal command in the 6th field.

Ser Olmy · 06-12-2020, 01:15 PM

Or to be more specific, any command is legal in the 6th field. Including (in this case) [, which may or may not be a builtin, depending on the shell being used (vixie/cronie allows a user-specified shell).