Difference between revisions of "Cron"

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{{Article summary heading|Resources}}
 
{{Article summary heading|Resources}}
 
{{Article summary link|Gentoo Linux Cron Guide|http://www.gentoo.org/doc/en/cron-guide.xml}}
 
{{Article summary link|Gentoo Linux Cron Guide|http://www.gentoo.org/doc/en/cron-guide.xml}}
 +
{{Article summary heading|Related}}
 +
{{Article summary wiki|systemd/cron functionality}}
 
{{Article summary end}}
 
{{Article summary end}}
 +
{{Lowercase_title}}
  
−
Cron is a powerful job scheduler for GNU/Linux and many other operating systems. It automates recurring tasks by executing commands at a given time. It has a wide range of potential applications; most simple recurring tasks, from backups to e-mail retrieval, can be automated using cron, saving users time and headaches.
+
From [[Wikipedia:Cron|Wikipedia]]:
  
−
==Installation==
+
'''''cron''' is the time-based job scheduler in Unix-like computer operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration.''
  
−
There are multiple cron implementations available from which users may choose. {{Pkg|cronie}} is available in [core] and is installed as part of the '''base''' group, you can check it is correctly installed with:
+
== Installation ==
  
−
# pacman -S --needed cronie
+
{{Pkg|cronie}} is installed by default as part of the ''base'' group. Other cron implementations exist if preferred, Gentoo's [http://www.gentoo.org/doc/en/cron-guide.xml cron guide] offers comparisons. For example, {{Pkg|fcron}}, {{AUR|bcron}} or {{AUR|vixie-cron}} are other alternatives. {{AUR|dcron}} used to be the default cron implementation in Arch Linux until May 2011.
  
−
Until May 2011, the default cron implementation for Arch Linux was {{Pkg|dcron}} (Dillon's Cron), which anyway is still supported and can be [[pacman|installed]] from [[official repositories]].
+
== Configuration ==
  
−
Alternatively, users may wish to install {{Pkg|fcron}} from [community] or {{AUR|bcron}} or {{AUR|vixie-cron}} from the [[AUR]]; all offer a wider range of features and configuration options.
+
=== Users and autostart ===
  
−
The [http://www.gentoo.org/doc/en/cron-guide.xml Gentoo Linux Cron Guide] offers a comparison between these implementations.
+
cron should be working upon login on a new system to run root scripts. This can be check by looking at the log in {{ic|/var/log/}}. In order to use crontab application (editor for job entries), users must be members of a designated group {{ic|users}} or {{ic|root}}, of which all users should already be members. To ensure cron starts on boot, enable {{ic|cronie.service}} or {{ic|dcron.service}} with {{ic|systemctl enable ''service_name''}} depending on which cron implementation you use.
  
−
==Initial configuration==
+
=== Handling errors of jobs ===
−
{{Out of date|The following sections of this article are still based on {{Pkg|dcron}}: you are invited to check the validity of the contents below and fix what is out of date.}}
+
  
−
===Users & autostart===
+
cron registers the output from ''stdout'' and ''stderr'' and attempts to send it as email to the user's spools via the {{ic|sendmail}} command. Cronie disables mail output if {{ic|/usr/bin/sendmail}} is not found. To log these messages use the {{ic|-m}} option and write a script or install a rudimentary SMTP subsystem.
−
Cron should work "out-of-the-box" for most Arch Linux users. In order to use crontab, users must be members of a designated group, but in Arch Linux, that group is ''users'', of which all users should already be members. If for whatever reason some users are not members of this group, they can be added to it with the command:
+
  
−
# gpasswd -a ''username'' users
+
# [[Systemd#Editing_provided_unit_files|Edit]] the {{ic|cronie.service}} unit.
 +
# Install {{Pkg|esmtp}}, {{Pkg|msmtp}}, {{Pkg|opensmtpd}} or write a custom script.
  
−
and they should then be able to edit their own crontabs.
+
==== Example with msmtp ====
  
−
To ensure cron starts on boot, add the ''crond'' daemon to the daemons array of [[rc.conf]]. See [[Daemon#Starting_on_Boot]] for details.
+
Here are two ways to obtain emails from cronie with msmtp:
  
−
===Handling errors of jobs===
+
# Install the {{Pkg|msmtp-mta}} package which effectively creates a symbolic link from {{ic|/usr/bin/sendmail}} to {{ic|/usr/bin/msmtp}}. You must then provide a way for msmtp to convert your username into an email address.
−
Errors can occur during execution of jobs. When this happens cron register '''stderr''' output of job as e-mail and try to send it by default via '''sendmail''' command.
+
#* Either add a {{ic|MAILTO}} line to your crontab, like so:{{bc|<nowiki>MAILTO=your@email.com</nowiki>}} '''or''':
−
To log this message you can use '''-M''' option of crontd and write you own script or install rudimentary SMTP subsystem ('''esmtp''' in this example):
+
#* Add this line to {{ic|/etc/msmtprc}}: {{bc|aliases /etc/aliases}}and create {{ic|/etc/aliases}}: {{bc|your_username: your@email.com<br># Optional:<br>default: your@email.com}}
 +
# [[Systemd#Editing_provided_unit_files|Edit]] the {{ic|cronie.service}} unit. For example, create {{ic|/etc/sytemd/system/cronie.service.d/msmtp.conf}}:{{bc|<nowiki>[Service]
 +
ExecStart=
 +
ExecStart=/usr/bin/crond -n -m '/usr/bin/msmtp -t'</nowiki>}}
 +
{{Note|The empty {{ic|<nowiki>ExecStart=</nowiki>}} cancels any previous {{ic|ExecStart}} commands.}}
  
−
# pacman -S esmtp procmail
+
==== Example with esmtp ====
  
−
After installation create file {{ic|/etc/esmtprc}} with this content:
+
Install {{Pkg|esmtp}} and {{Pkg|procmail}}.
  
−
identity myself@myisp.com
+
After installation configure the routing:
−
        hostname mail.myisp.com:25
+
{{hc|/etc/esmtprc|
−
        username "myself"
+
identity ''myself''@myisp.com
−
        password "secret"
+
      hostname mail.myisp.com:25
−
        starttls enabled
+
      username ''"myself"''
−
        default
+
      password ''"secret"''
−
mda "/usr/bin/procmail -d %T"
+
      starttls enabled
−
Procmail needs root privileges to work in delivery mode but it is not an issue if you are running the cronjobs as root.
+
      default
 +
mda "/usr/bin/procmail -d %T"
 +
}}
  
−
To test that everything works correctly, create a file {{ic|message.txt}} with ''"test message"'' in it.  
+
Procmail needs root privileges to work in delivery mode but it is not an issue if you are running the cronjobs as root anyway.
  
−
From the directory containing {{ic|message.txt}} run:
+
To test that everything works correctly, create a file {{ic|message.txt}} with {{ic|"test message"}} in it.
  
−
$ sendmail user_name < message.txt
+
From the same directory run:
  
−
then:
+
$ sendmail ''user_name'' < message.txt
  
−
$ cat /var/spool/mail/user_name
+
then:
  
−
You should see in the terminal, the ''"test message"'', the time and date it was sent.
+
$ cat /var/spool/mail/''user_name''
  
−
Thats all, all error output of jobs now will be redirected to {{ic|/var/spool/mail/$user_name}}.
+
You should now see the test message and the time and date it was sent.
  
−
Due to the privileged issue, it is hard to create and send the email to root. So you could ask esmtp to forward all root's email to some ordinary users. Add the following lines in esmtprc
+
The error output of all jobs will now be redirected to {{ic|/var/spool/mail/''user_name''}}.
−
force_mda="user-name"
+
  
−
'''If the above test did not work''', try creating a file {{ic|~/.esmtprc}} with the same content as {{ic|/etc/esmtprc}}, (you can just create a copy as normal user).
+
Due to the privileged issue, it is hard to create and send emails to root (e.g. {{ic|su -c ""}}). You can ask {{ic|esmtp}} to forward all root's email to an ordinary user with:
 +
{{hc|/etc/esmtprc|
 +
2=force_mda="''user-name''"
 +
}}
  
−
Run the following command to make sure it has the correct 710 permission:  
+
{{Note|If the above test didn't work, you may try creating a local configuration in {{ic|~/.esmtprc}} with the same content.
 +
 
 +
Run the following command to make sure it has the correct permission:  
  
 
  $ chmod 710 ~/.esmtprc
 
  $ chmod 710 ~/.esmtprc
  
−
Now just repeat the test with {{ic|message.txt}} exactly as before.
+
Then repeat the test with {{ic|message.txt}} exactly as before.}}
 +
 
 +
==== Example with opensmtpd ====
 +
 
 +
Install {{Pkg|opensmtpd}}.
 +
 
 +
Edit {{ic|/etc/smtpd/smtpd.conf}}. The following configuration allows for local delivery:
 +
 
 +
listen on localhost
 +
accept for local deliver to mbox
 +
 
 +
You can proceed to test it:
 +
# systemctl start smtpd
 +
$ echo test | sendmail user
 +
 
 +
''user'' can check his/her mail in with any [[:Category:Email_Client|reader]] able to handle mbox format, or just have a look at the file {{ic|/var/spool/mail/''user''}}. If everything goes as expected, you can enable opensmtpd for future boots:
 +
# systemctl enable smtpd
 +
 
 +
This approach has the advantage of not sending local cron notifications to a remote server. Not even network connection is needed. On the downside, you need a new daemon running.
 +
 
 +
{{Note|
 +
* At the moment of writing the Arch opensmtpd package does not create all needed directories under {{ic|/var/spool/smtpd}}, but the daemon will warn about that specifying the required ownerships and permissions. Just create them as suggested.
 +
* Even though the suggested configuration does not accept remote connections, it's a healthy precaution to add an additional layer of security blocking port 25 with [[iptables]] or similar.
 +
}}
 +
 
 +
==== Long cron job ====
 +
 
 +
Suppose this program is invoked by cron :
 +
 
 +
#!/bin/sh
 +
echo "I had a recoverable error!"
 +
sleep 1h
 +
 
 +
What happens is this:
 +
# cron runs the script
 +
# as soon as cron sees some output, it runs your MTA, and provides it with the headers. It leaves the pipe open, because the job hasn't finished and there might be more output.
 +
# the MTA opens the connection to postfix and leaves that connection open while it waits for the rest of the body.
 +
# postfix closes the idle connection after less than an hour and you get an error like this :
 +
smtpmsg='421 … Error: timeout exceeded' errormsg='the server did not accept the mail'
 +
 
 +
To solve this problem you can use the command chronic or sponge from {{Pkg|moreutils}}.
 +
From their respective man page:
 +
; chronic: chronic runs a command, and arranges for its standard out and standard error to only be displayed if the command fails (exits nonzero or crashes). If the command succeeds, any extraneous output will be hidden.
 +
; sponge: sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file… If no output file is specified, sponge outputs to stdout.
 +
 
 +
Even if it's not said chronic buffer the command output before opening its standard output (like sponge does).
  
−
==Crontab format==
+
== Crontab format ==
  
 
The basic format for a crontab is:
 
The basic format for a crontab is:
  
−
  <minute> <hour> <day_of_month> <month> <day_of_week> <command>
+
  ''minute'' ''hour'' ''day_of_month'' ''month'' ''day_of_week'' ''command''
  
 
* ''minute'' values can be from 0 to 59.
 
* ''minute'' values can be from 0 to 59.
Line 92: Line 150:
 
Multiple times may be specified with a comma, a range can be given with a hyphen, and the asterisk symbol is a wildcard character. Spaces are used to separate fields. For example, the line:
 
Multiple times may be specified with a comma, a range can be given with a hyphen, and the asterisk symbol is a wildcard character. Spaces are used to separate fields. For example, the line:
  
−
  *0,*5 9-16 * 1-5,9-12 1-5 /home/user/bin/i_love_cron.sh
+
  *0,*5 9-16 * 1-5,9-12 1-5 ~/bin/i_love_cron.sh
  
−
Will execute the script {{Ic|i_love_cron.sh}} at five minute intervals from 9 AM to 4:55 PM every weekday of the month except during the summer months (June, July, and August). More examples and advanced configuration techniques can be found below.
+
Will execute the script {{ic|i_love_cron.sh}} at five minute intervals from 9 AM to 4:55 PM on weekdays except during the summer months (June, July, and August). More examples and advanced configuration techniques can be found below.
  
−
==Basic commands==
+
== Basic commands ==
  
−
Crontabs should never be edited directly; instead, users should use the ''crontab'' program to work with their crontabs. To be granted access to this command, user must be a member of the users group (see gpasswd command).
+
Crontabs should never be edited directly; instead, users should use the {{ic|crontab}} program to work with their crontabs. To be granted access to this command, user must be a member of the users group (see the {{ic|gpasswd}} command).
  
 
To view their crontabs, users should issue the command:
 
To view their crontabs, users should issue the command:
Line 124: Line 182:
 
  # crontab -u ''username'' -e
 
  # crontab -u ''username'' -e
  
−
This same format (appending "-u ''username''" to a command) works for listing and deleting crontabs as well.
+
This same format (appending {{ic|-u ''username''}} to a command) works for listing and deleting crontabs as well.
  
−
To use nano rather than vi as crontab editor, add the following lines to /etc/bash.bashrc:
+
== Examples ==
−
 
+
−
export EDITOR="/usr/bin/nano"
+
−
 
+
−
And restart terminal/s
+
−
 
+
−
==Examples==
+
  
 
The entry:
 
The entry:
Line 138: Line 190:
 
  01 * * * * /bin/echo Hello, world!
 
  01 * * * * /bin/echo Hello, world!
  
−
runs the command {{Ic|/bin/echo Hello, world!}} on the first minute of every hour of every day of every month (i.e. at 12:01, 1:01, 2:01, etc.)
+
runs the command {{ic|/bin/echo Hello, world!}} on the first minute of every hour of every day of every month (i.e. at 12:01, 1:01, 2:01, etc.).
  
 
Similarly,
 
Similarly,
Line 144: Line 196:
 
  */5 * * jan mon-fri /bin/echo Hello, world!
 
  */5 * * jan mon-fri /bin/echo Hello, world!
  
−
runs the same job every five minutes on weekdays during the month of January (i.e. at 12:00, 12:05, 12:10, etc.)
+
runs the same job every five minutes on weekdays during the month of January (i.e. at 12:00, 12:05, 12:10, etc.).
  
 
As noted in the ''Crontab Format'' section, the line:
 
As noted in the ''Crontab Format'' section, the line:
Line 152: Line 204:
 
Will execute the script {{Ic|i_love_cron.sh}} at five minute intervals from 9 AM to 5 PM (excluding 5 PM itself) every weekday (Mon-Fri) of every month except during the summer (June, July, and August).
 
Will execute the script {{Ic|i_love_cron.sh}} at five minute intervals from 9 AM to 5 PM (excluding 5 PM itself) every weekday (Mon-Fri) of every month except during the summer (June, July, and August).
  
−
==More information==
+
== Default editor ==
  
−
The cron daemon parses a configuration file known as ''crontab''. Each user on the system can maintain a separate crontab file to schedule commands individually. The root user's crontab is used to schedule system-wide tasks (though users may opt to use {{ic|/etc/crontab}} or the {{ic|/etc/cron.d}} directory, depending on which cron implementation they choose).
+
To use an alternate default editor, define the {{ic|EDITOR}} environment variablee it in a shell initialization script ({{AUR|vim-default-editor}} is available for vim users). For example:
 +
 
 +
{{hc|/etc/profile.d/nano-default-editor|2=<nowiki>
 +
#!/bin/sh
 +
 
 +
export EDITOR=/usr/bin/nano
 +
</nowiki>}}
 +
 
 +
As a regular user, {{ic|su}} will need to be used instead of {{ic|sudo}} for the environment variable to be pulled correctly:
 +
 
 +
$ su -c "crontab -e"
 +
 
 +
To have an alias to this {{ic|printf}} is required to carry the arbitrary string because {{ic|su}} launches in a new shell:
 +
 
 +
alias scron="su -c $(printf "%q " "crontab -e")"
 +
 
 +
== More information ==
 +
 
 +
The cron daemon parses a configuration file known as {{ic|crontab}}. Each user on the system can maintain a separate crontab file to schedule commands individually. The root user's crontab is used to schedule system-wide tasks (though users may opt to use {{ic|/etc/crontab}} or the {{ic|/etc/cron.d}} directory, depending on which cron implementation they choose).
  
 
There are slight differences between the crontab formats of the different cron daemons. The default root crontab for dcron looks like this:
 
There are slight differences between the crontab formats of the different cron daemons. The default root crontab for dcron looks like this:
Line 160: Line 230:
 
{{hc|/var/spool/cron/root
 
{{hc|/var/spool/cron/root
 
|2=<nowiki>
 
|2=<nowiki>
−
# root crontab
+
# Run command at a scheduled time
−
# DO NOT EDIT THIS FILE MANUALLY! USE crontab -e INSTEAD
+
# Edit this 'crontab -e' for error checking, man 1 crontab for acceptable format
  
−
# man 1 crontab for acceptable formats:
+
# <@freq>                       <tags and command>
−
#    <minute> <hour> <day> <month> <dow> <tags and command>
+
−
#    <@freq> <tags and command>
+
−
 
+
−
# SYSTEM DAILY/WEEKLY/... FOLDERS
+
 
@hourly        ID=sys-hourly  /usr/sbin/run-cron /etc/cron.hourly
 
@hourly        ID=sys-hourly  /usr/sbin/run-cron /etc/cron.hourly
 
@daily          ID=sys-daily    /usr/sbin/run-cron /etc/cron.daily
 
@daily          ID=sys-daily    /usr/sbin/run-cron /etc/cron.daily
 
@weekly        ID=sys-weekly  /usr/sbin/run-cron /etc/cron.weekly
 
@weekly        ID=sys-weekly  /usr/sbin/run-cron /etc/cron.weekly
 
@monthly        ID=sys-monthly  /usr/sbin/run-cron /etc/cron.monthly
 
@monthly        ID=sys-monthly  /usr/sbin/run-cron /etc/cron.monthly
 +
 +
# mm  hh  DD  MM  W /path/command (or tags) # W = week: 0-6, Sun=0
 +
  21  01  *  *  * /usr/bin/systemctl suspend
 
</nowiki>}}
 
</nowiki>}}
  
Line 193: Line 262:
 
See the crontab [[man page]] for further information and configuration examples.
 
See the crontab [[man page]] for further information and configuration examples.
  
−
==run-parts issue==
+
== run-parts issue ==
−
cronie use run-parts to carry out script in cron.daily/cron.week.y/cron.montly. Be carefull that the script name in these folders should not have any '.', like backup.sh. Since run-parts without options will ignore them. Detail information see man run-parts.
+
  
−
==Running X apps==
+
cronie uses {{ic|run-parts}} to carry out script in {{ic|cron.daily}}/{{ic|cron.weekly}}/{{ic|cron.monthly}}. Be careful that the script name in these won't include a dot (.), e.g. {{ic|backup.sh}}, since {{ic|run-parts}} without options will ignore them (see: {{ic|man run-parts}}).
  
−
If you find that you cannot run X apps from cron jobs then put this before the
+
== Running X.org server-based applications ==
−
command:
+
  
−
export DISPLAY=:0.0 ;
+
Cron does not run under the X.org server therefore it cannot know the environmental variable necessary to be able to start an X.org server application so they will have to be defined. One can use a program like {{AUR|xuserrun}} to do it:
  
−
That sets the DISPLAY variable to the first display; which is usually right
+
17 02 * ... /usr/bin/xuserrun /usr/bin/xclock
−
unless you like to run multiple xservers on your machine.
+
  
−
If it still does not work then you need to use xhost to give your user control
+
Or then can be defined manually ({{ic|echo $DISPLAY}} will give the current DISPLAY value):
−
over X11:
+
  
−
  # xhost +si:localuser:$(whoami)
+
  17 02 * ... env DISPLAY=:0 /usr/bin/xclock
  
−
I put it in my gnome `Startup Applications' like this:
+
If done through say SSH, permission will need be given:
  
−
  bash -c "xhost +si:localuser:$(whoami)"
+
  # xhost +si:localuser:$(whoami)
  
−
==Asynchronous job processing==
+
== Asynchronous job processing ==
  
 
If you regularly turn off your computer but do not want to miss jobs, there are some solutions available (easiest to hardest):
 
If you regularly turn off your computer but do not want to miss jobs, there are some solutions available (easiest to hardest):
  
−
; Dcron : Vanilla dcron supports asynchronous job processing. Just put it with @hourly, @daily, @weekly or @monthly with a jobname, like this:
+
=== Dcron ===
 +
 
 +
Vanilla {{AUR|dcron}} supports asynchronous job processing. Just put it with @hourly, @daily, @weekly or @monthly with a jobname, like this:
  
 
  @hourly        ID=greatest_ever_job      echo This job is very useful.
 
  @hourly        ID=greatest_ever_job      echo This job is very useful.
  
−
; Cronwhip ([https://aur.archlinux.org/packages.php?ID=21079 AUR], [https://bbs.archlinux.org/viewtopic.php?id=57973 forum thread]): Script to automatically run missed cron jobs; works with the default cron implementation, dcron.
+
=== Cronwhip ===
−
; Anacron ([https://aur.archlinux.org/packages.php?ID=5196 AUR]): Full replacement for dcron, processes jobs asynchronously.
+
 
 +
{{AUR|cronwhip}} is a script to automatically run missed cron jobs; it works with the former default cron implementation, ''dcron''.
 +
See also the [https://bbs.archlinux.org/viewtopic.php?id=57973 forum thread].
 +
 
 +
=== Anacron ===
 +
 
 +
{{AUR|anacron}} is a full replacement for ''dcron'' which processes jobs asynchronously.
 +
 
 +
=== Fcron ===
 +
 
 +
Like ''anacron'', {{Pkg|fcron}} assumes the computer is not always running and, unlike ''anacron'', it can schedule events at intervals shorter than a single day. Like cronwhip, it can run jobs that should have been run during the computer's downtime.
 +
See also the [https://bbs.archlinux.org/viewtopic.php?id=140497 forum thread]
 +
 
 +
== Ensuring exclusivity ==
 +
 
 +
If you run potentially long-running jobs (e.g., a backup might all of a sudden run for a long time, because of many changes or a particular slow network connection), then {{AUR|lockrun}} can ensure that the cron job won't start a second time.
 +
 
 +
  5,35 * * * * /usr/bin/lockrun -n /tmp/lock.backup /root/make-backup.sh
 +
 
 +
== See Also ==
  
−
==See also==
+
* [http://gotux.net/arch-linux/crontab-usage/ CronTab Usage Tutorial]
−
[http://wiki.gotux.net/config:crontab CronTab Config and Examples]
+

Revision as of 07:39, 30 August 2013

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From Wikipedia:

cron is the time-based job scheduler in Unix-like computer operating systems. cron enables users to schedule jobs (commands or shell scripts) to run periodically at certain times or dates. It is commonly used to automate system maintenance or administration.

Installation

cronie is installed by default as part of the base group. Other cron implementations exist if preferred, Gentoo's cron guide offers comparisons. For example, fcron, bcronAUR or vixie-cronAUR are other alternatives. dcronAUR used to be the default cron implementation in Arch Linux until May 2011.

Configuration

Users and autostart

cron should be working upon login on a new system to run root scripts. This can be check by looking at the log in /var/log/. In order to use crontab application (editor for job entries), users must be members of a designated group users or root, of which all users should already be members. To ensure cron starts on boot, enable cronie.service or dcron.service with systemctl enable service_name depending on which cron implementation you use.

Handling errors of jobs

cron registers the output from stdout and stderr and attempts to send it as email to the user's spools via the sendmail command. Cronie disables mail output if /usr/bin/sendmail is not found. To log these messages use the -m option and write a script or install a rudimentary SMTP subsystem.

  1. Edit the cronie.service unit.
  2. Install esmtp, msmtp, opensmtpd or write a custom script.

Example with msmtp

Here are two ways to obtain emails from cronie with msmtp:

  1. Install the msmtp-mta package which effectively creates a symbolic link from /usr/bin/sendmail to /usr/bin/msmtp. You must then provide a way for msmtp to convert your username into an email address.
    • Either add a MAILTO line to your crontab, like so:
      MAILTO=your@email.com
      or:
    • Add this line to /etc/msmtprc: 
      aliases /etc/aliases
      and create /etc/aliases: 
      your_username: your@email.com
      # Optional:
      default: your@email.com
  2. Edit the cronie.service unit. For example, create /etc/sytemd/system/cronie.service.d/msmtp.conf:
    [Service]
ExecStart=
ExecStart=/usr/bin/crond -n -m '/usr/bin/msmtp -t'
Note: The empty ExecStart= cancels any previous ExecStart commands.

Example with esmtp

Install esmtp and procmail.

After installation configure the routing: 

/etc/esmtprc
identity myself@myisp.com
       hostname mail.myisp.com:25
       username "myself"
       password "secret"
       starttls enabled
       default
mda "/usr/bin/procmail -d %T"

Procmail needs root privileges to work in delivery mode but it is not an issue if you are running the cronjobs as root anyway.

To test that everything works correctly, create a file message.txt with "test message" in it.

From the same directory run: 

$ sendmail user_name < message.txt

then: 

$ cat /var/spool/mail/user_name

You should now see the test message and the time and date it was sent.

The error output of all jobs will now be redirected to /var/spool/mail/user_name.

Due to the privileged issue, it is hard to create and send emails to root (e.g. su -c ""). You can ask esmtp to forward all root's email to an ordinary user with: 

/etc/esmtprc
force_mda="user-name"
Note: If the above test didn't work, you may try creating a local configuration in ~/.esmtprc with the same content.

Run the following command to make sure it has the correct permission: 

$ chmod 710 ~/.esmtprc
Then repeat the test with message.txt exactly as before.

Example with opensmtpd

Install opensmtpd.

Edit /etc/smtpd/smtpd.conf. The following configuration allows for local delivery: 

listen on localhost
accept for local deliver to mbox

You can proceed to test it: 

# systemctl start smtpd
$ echo test | sendmail user

user can check his/her mail in with any reader able to handle mbox format, or just have a look at the file /var/spool/mail/user. If everything goes as expected, you can enable opensmtpd for future boots: 

# systemctl enable smtpd

This approach has the advantage of not sending local cron notifications to a remote server. Not even network connection is needed. On the downside, you need a new daemon running.

Note:
  • At the moment of writing the Arch opensmtpd package does not create all needed directories under /var/spool/smtpd, but the daemon will warn about that specifying the required ownerships and permissions. Just create them as suggested.
  • Even though the suggested configuration does not accept remote connections, it's a healthy precaution to add an additional layer of security blocking port 25 with iptables or similar.

Long cron job

Suppose this program is invoked by cron : 

#!/bin/sh
echo "I had a recoverable error!"
sleep 1h

What happens is this:

  1. cron runs the script
  2. as soon as cron sees some output, it runs your MTA, and provides it with the headers. It leaves the pipe open, because the job hasn't finished and there might be more output.
  3. the MTA opens the connection to postfix and leaves that connection open while it waits for the rest of the body.
  4. postfix closes the idle connection after less than an hour and you get an error like this :
smtpmsg='421 … Error: timeout exceeded' errormsg='the server did not accept the mail'

To solve this problem you can use the command chronic or sponge from moreutils. From their respective man page:

chronic
chronic runs a command, and arranges for its standard out and standard error to only be displayed if the command fails (exits nonzero or crashes). If the command succeeds, any extraneous output will be hidden.
sponge
sponge reads standard input and writes it out to the specified file. Unlike a shell redirect, sponge soaks up all its input before opening the output file… If no output file is specified, sponge outputs to stdout.

Even if it's not said chronic buffer the command output before opening its standard output (like sponge does).

Crontab format

The basic format for a crontab is: 

minute hour day_of_month month day_of_week command
  • minute values can be from 0 to 59.
  • hour values can be from 0 to 23.
  • day_of_month values can be from 1 to 31.
  • month values can be from 1 to 12.
  • day_of_week values can be from 0 to 6, with 0 denoting Sunday.

Multiple times may be specified with a comma, a range can be given with a hyphen, and the asterisk symbol is a wildcard character. Spaces are used to separate fields. For example, the line: 

*0,*5 9-16 * 1-5,9-12 1-5 ~/bin/i_love_cron.sh

Will execute the script i_love_cron.sh at five minute intervals from 9 AM to 4:55 PM on weekdays except during the summer months (June, July, and August). More examples and advanced configuration techniques can be found below.

Basic commands

Crontabs should never be edited directly; instead, users should use the crontab program to work with their crontabs. To be granted access to this command, user must be a member of the users group (see the gpasswd command).

To view their crontabs, users should issue the command: 

$ crontab -l

To edit their crontabs, they may use: 

$ crontab -e

To remove their crontabs, they should use: 

$ crontab -r

If a user has a saved crontab and would like to completely overwrite their old crontab, he or she should use: 

$ crontab saved_crontab_filename

To overwrite a crontab from the command line (Wikipedia:stdin), use 

$ crontab -

To edit somebody else's crontab, issue the following command as root: 

# crontab -u username -e

This same format (appending -u username to a command) works for listing and deleting crontabs as well.

Examples

The entry: 

01 * * * * /bin/echo Hello, world!

runs the command /bin/echo Hello, world! on the first minute of every hour of every day of every month (i.e. at 12:01, 1:01, 2:01, etc.).

Similarly, 

*/5 * * jan mon-fri /bin/echo Hello, world!

runs the same job every five minutes on weekdays during the month of January (i.e. at 12:00, 12:05, 12:10, etc.).

As noted in the Crontab Format section, the line: 

*0,*5 9-16 * 1-5,9-12 1-5 /home/user/bin/i_love_cron.sh

Will execute the script i_love_cron.sh at five minute intervals from 9 AM to 5 PM (excluding 5 PM itself) every weekday (Mon-Fri) of every month except during the summer (June, July, and August).

Default editor

To use an alternate default editor, define the EDITOR environment variablee it in a shell initialization script (vim-default-editorAUR is available for vim users). For example: 

/etc/profile.d/nano-default-editor
#!/bin/sh

export EDITOR=/usr/bin/nano

As a regular user, su will need to be used instead of sudo for the environment variable to be pulled correctly: 

$ su -c "crontab -e"

To have an alias to this printf is required to carry the arbitrary string because su launches in a new shell: 

alias scron="su -c $(printf "%q " "crontab -e")"

More information

The cron daemon parses a configuration file known as crontab. Each user on the system can maintain a separate crontab file to schedule commands individually. The root user's crontab is used to schedule system-wide tasks (though users may opt to use /etc/crontab or the /etc/cron.d directory, depending on which cron implementation they choose).

There are slight differences between the crontab formats of the different cron daemons. The default root crontab for dcron looks like this: 

/var/spool/cron/root

# Run command at a scheduled time
# Edit this 'crontab -e' for error checking, man 1 crontab for acceptable format

# <@freq>                       <tags and command>
@hourly         ID=sys-hourly   /usr/sbin/run-cron /etc/cron.hourly
@daily          ID=sys-daily    /usr/sbin/run-cron /etc/cron.daily
@weekly         ID=sys-weekly   /usr/sbin/run-cron /etc/cron.weekly
@monthly        ID=sys-monthly  /usr/sbin/run-cron /etc/cron.monthly

# mm  hh  DD  MM  W /path/command (or tags) # W = week: 0-6, Sun=0
  21  01  *   *   * /usr/bin/systemctl suspend

These lines exemplify one of the formats that crontab entries can have, namely whitespace-separated fields specifying:

  1. @period
  2. ID=jobname (this tag is specific to dcron)
  3. command

The other standard format for crontab entries is:

  1. minute
  2. hour
  3. day
  4. month
  5. day of week
  6. command

The crontab files themselves are usually stored as /var/spool/cron/username. For example, root's crontab is found at /var/spool/cron/root

See the crontab man page for further information and configuration examples.

run-parts issue

cronie uses run-parts to carry out script in cron.daily/cron.weekly/cron.monthly. Be careful that the script name in these won't include a dot (.), e.g. backup.sh, since run-parts without options will ignore them (see: man run-parts).

Running X.org server-based applications

Cron does not run under the X.org server therefore it cannot know the environmental variable necessary to be able to start an X.org server application so they will have to be defined. One can use a program like xuserrunAUR to do it: 

17 02 * ... /usr/bin/xuserrun /usr/bin/xclock

Or then can be defined manually (echo $DISPLAY will give the current DISPLAY value): 

17 02 * ... env DISPLAY=:0 /usr/bin/xclock

If done through say SSH, permission will need be given: 

# xhost +si:localuser:$(whoami)

Asynchronous job processing

If you regularly turn off your computer but do not want to miss jobs, there are some solutions available (easiest to hardest):

Dcron

Vanilla dcronAUR supports asynchronous job processing. Just put it with @hourly, @daily, @weekly or @monthly with a jobname, like this: 

@hourly         ID=greatest_ever_job      echo This job is very useful.

Cronwhip

cronwhipAUR is a script to automatically run missed cron jobs; it works with the former default cron implementation, dcron. See also the forum thread.

Anacron

anacronAUR is a full replacement for dcron which processes jobs asynchronously.

Fcron

Like anacron, fcron assumes the computer is not always running and, unlike anacron, it can schedule events at intervals shorter than a single day. Like cronwhip, it can run jobs that should have been run during the computer's downtime. See also the forum thread

Ensuring exclusivity

If you run potentially long-running jobs (e.g., a backup might all of a sudden run for a long time, because of many changes or a particular slow network connection), then lockrunAUR can ensure that the cron job won't start a second time. 

  5,35 * * * * /usr/bin/lockrun -n /tmp/lock.backup /root/make-backup.sh

See Also

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