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Features

Comparison of ISC to
BSD 4.2 / 3 & System V



Environment Variables

These can be set on a per CronTab basis.


Syntax

SHELL = /bin/sh
PATH = .:/bin:/usr/bin
Test = 'This is a test'
Hello = "World"
Values = With Spaces

Variables

with special meanings.


$\huge\texttt{\textcolor{SkyBlue}{LOGNAME}}$

Defaults to data in the users `passwd` entry

$\huge\texttt{\textcolor{LimeGreen}{MAILTO}}$

If set, will mail the output of Cron to
the user with the specified login name


Useful if you decide to use BINMAIL in `cron.h`
as it doesn't know anything about aliasing

$\huge\texttt{\textcolor{SkyBlue}{SHELL}}$

Defaults to `/bin/sh`

$\huge\texttt{\textcolor{LimeGreen}{USER}}$

Read from the users `passwd`
entry and cannot be changed

$\huge\texttt{\textcolor{SkyBlue}{HOME}}$

Defaults to data in the users `passwd` entry

$\huge\texttt{\textcolor{LimeGreen}{PATH}}$

Contains the systems paths

$\huge\texttt{\textcolor{SkyBlue}{TZ}}$

Can be set, but ignored other
than for the command it runs


$$\definecolor{Day-A}{RGB}{169,88,78} \definecolor{Day-B}{RGB}{156,96,101} \definecolor{Day-C}{RGB}{141,103,125} \definecolor{Day-D}{RGB}{129,110,147} \definecolor{Day-E}{RGB}{113,119,174} \definecolor{Day-F}{RGB}{102,124,191} \definecolor{Day-G}{RGB}{83,134,223}$$

Weekdays

can be specified with their first  3
letters in whatever case you prefer.

  $\huge\texttt{\textcolor{Day-A}{Mon}}$       $\huge\texttt{\textcolor{Day-B}{Tue}}$       $\huge\texttt{\textcolor{Day-C}{Wen}}$       $\huge\texttt{\textcolor{Day-D}{Thu}}$       $\huge\texttt{\textcolor{Day-E}{Fri}}$       $\huge\texttt{\textcolor{Day-F}{Sat}}$       $\huge\texttt{\textcolor{Day-F}{Sun}}$  



$$\definecolor{Month-A}{RGB}{102,191,148} \definecolor{Month-B}{RGB}{118,190,136} \definecolor{Month-C}{RGB}{136,190,125} \definecolor{Month-D}{RGB}{155,189,111} \definecolor{Month-E}{RGB}{175,189,98} \definecolor{Month-F}{RGB}{197,188,83}$$

Months

can be specified with their first  3
letters in whatever case you prefer.

  $\huge\texttt{\textcolor{Month-A}{Jan}}$       $\huge\texttt{\textcolor{Month-B}{Feb}}$       $\huge\texttt{\textcolor{Month-C}{Mar}}$       $\huge\texttt{\textcolor{Month-D}{Apr}}$       $\huge\texttt{\textcolor{Month-E}{May}}$       $\huge\texttt{\textcolor{Month-F}{Jun}}$  

  $\huge\texttt{\textcolor{Month-A}{Jul}}$       $\huge\texttt{\textcolor{Month-B}{Aug}}$       $\huge\texttt{\textcolor{Month-C}{Sep}}$       $\huge\texttt{\textcolor{Month-D}{Oct}}$       $\huge\texttt{\textcolor{Month-E}{Nov}}$       $\huge\texttt{\textcolor{Month-F}{Dec}}$    



Mixing

Unlike standard Cron, ranges & lists can be mixed.

1,3-5


Stepping

Ranges can specify a value to step width.

10-16/2  ≍  10,12,14,16
10-16/3  ≍  10,13,16


Sunday

The day can be specified with both  0  and  7
which BSD and ATT apparently disagree about.



User

Every user has their own CronTab config,
as it is also done in System V's Cron flavor.


BSD 4.2

In this version of BSD, only the root
user could have a CronTab config.


BSD 4.3

  • The CronTab format was made incompatible.

  • Non-root UIDs could run the command.

  • Only root could edit the CronTab file.



System V

The  crontab  command is loosely compatible with
System V, but has been extended with more options.

Running the command without arguments
shows a summary on how to use of the it.



Comments

  • Must be on a line by themselves

  • Can have leading whitespace

  • #  starts the comment



Auto Reloading

If the  crontab  command makes any changes,
the Cron daemon will reload all tables before the
next iteration automatically.


Other Crons

  • May require you have to kill and restart the daemon.

  • May unnecessarily read & parse the config continuously.



Access

The CronTab files cannot be read nor modified other
than with the  crontab  /  cron  commands, this is
to allow for automatic reloading.

This shouldn't pose a problem however, as  crontab
allows you to adjust everything in your own CronTab.

With root you can of course access anyone's CronTab.



MailTo

Any output generated by commands to  stdout  and
stderr  will be mailed to the owner of the CronTab.

Alternatively the  MAILTO  variable is used.
⤷ Check the Environment Variable section.


Header

The email messages header will include:

  • The command that was run by Cron

  • The list of passed environment variables of
    which the following will always be present:

    SHELL    HOME    USER    LOGNAME

    LogName is only passed on System V



DOM / DOW

The situation is odd.


* * 1,15 * Sun  will run on:

1st  |  15th  |  Sundays


* * * * Sun  will only run on:

Sundays


* * 1,15 * *  will only run on:

1st  |  15th


This is why we keep  e -> dow_star  &  e -> dom_star

I didn't think up this behavior, it's how Cron
has always worked but the documentation
hasn't been very clear.

I have been told that some AT&T Crons do not
act this way and do the more reasonable thing,
which is - in my honest opinion - to  or  the
various field - matches together.

In that sense this Cron isn't completely
similar to some of the AT&T Crons .