Difference between revisions of "Cron"

From ArchWiki
Jump to: navigation, search
m (Default editor: added '.sh' to nano-default-editor as for me this suggestion was ineffective without that file suffix.)
(Default editor: merge request)
Line 223: Line 223:
== Default editor ==
== Default editor ==
{{Merge|Environment variables|The {{ic|EDITOR}} configuration part should be merged, just link here will be enough}}
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:
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:

Revision as of 20:46, 16 March 2014


Related articles

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.


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.


Activation and autostart

cron provided implementation cronie is not enabled by default in new Arch installs. This can be checked by looking at the log in /var/log/ or by issuing:

$ systemctl is-enabled cronie

Therefore, cronie systemd service must be started and enabled via systemctl prior or after setting the first cron job:

# systemctl start cronie
# systemctl enable cronie

Adapt these commands depending on your chosen implementation, e.g.:

# systemctl start dcron
# systemctl enable dcron
Note: Three daily jobs are present by default in /etc/cron.daily for being part of the files installed by the base group: logrotate, man-db and shadow. There is also the 0anacron hourly job provided by default by cronie, which allows for delayed runs of other jobs e.g. if the computer was switched off at the moment of standard execution. Activating cron service will trigger all of them.

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. Restart cronie to make sure it detects the new sendmail command. 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:
    • 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/systemd/system/cronie.service.d/msmtp.conf:
    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:

identity myself@myisp.com
       hostname mail.myisp.com:25
       username "myself"
       password "secret"
       starttls enabled
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 


$ 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:

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.

  • 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 :

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 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 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.


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.).


*/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.).

The line (as noted in "man 5 crontab"):

*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).

Periodical settings can also be entered as in this crontab template:

# Chronological table of program loadings                                       
# Edit with "crontab" for proper functionality, "man 5 crontab" for formatting
# User: johndoe

# mm  hh  DD  MM  W /path/progam [--option]...  ( W = weekday: 0-6 [Sun=0] )
  21  01  *   *   * /usr/bin/systemctl hibernate
  @weekly           $HOME/.local/bin/trash-empty

Default editor

Merge-arrows-2.pngThis article or section is a candidate for merging with Environment variables.Merge-arrows-2.png

Notes: The EDITOR configuration part should be merged, just link here will be enough (Discuss in Talk:Cron#)

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:


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")"

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):


Cronie comes with anacron included.


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.


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


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


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


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).

# 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.

See also