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Where does the user's cron output go to by default on Ubuntu?

bd flag
Flimm

If I run crontab -e, Ubuntu shows me this default cron job for the current user:

# Edit this file to introduce tasks to be run by cron.
# 
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
# 
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
# 
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
# 
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
# 
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
# 
# For more information see the manual pages of crontab(5) and cron(8)
# 
# m h  dom mon dow   command

Notice the part that says this:

Output of the crontab jobs (including errors) is sent through email to the user the crontab file belongs to (unless redirected).

How do I view this output? What email is associated with my Linux user?

If I look at grep CRON /var/log/syslog , I see lines like this:

Feb 10 17:27:01 desktop-ubuntu CRON[36079]: (CRON) info (No MTA installed, discarding output)

I assume MTA means Mail Transfer Agent. It seems like the output is thrown away.

Personally, I would like output to be saved somewhere without going to a real email address on the Internet. But my question is more about understanding the default behaviour.

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Artur Meinild avatar Ubuntu
vn flag
Artur Meinild

In order for cron to send emails, you need to have postfix or another MTA (mail transfer agent) installed.

If no specific email address is set, mail will be delivered to the internal user email addresses at /var/mail/<username>.

You can also specify an external email address that cron sends to with the MAILTO variable. So in your crontab, set:

MAILTO=your@email-address.tld

Then cron will send emails to that address.

If you want to disable that cron sends email, in crontab set:

MAILTO=""

This would be how you control the default behavior.

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bd flag
Flimm
What if `MAILTO` is not set? According to the comment, "# Output of the crontab jobs (including errors) is sent through email to the user the crontab file belongs to (unless redirected).` What does "sent through email to the user the crontab file belongs to" mean? I vaguely recall that Linux users have mailboxes, is that what it is talking about?
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Artur Meinild avatar
vn flag
Artur Meinild
I added this to the answer..
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bd flag
Flimm
I installed `postfix` using `sudo apt install postfix`, and I was prompted to choose a configuration. I chose "Local only". Now my user's crontab is sending emails by appending the email contents to `/var/mail/<username>` , which is viewable in a text editor. I presume that is what you meant.
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bd flag
Flimm
Also, if I'm not misunderstanding, it's not cron that chose the location `/var/mail/<username>`, but rather postfix
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Artur Meinild avatar
vn flag
Artur Meinild
Sounds right...
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avatar Ubuntu
bd flag
Flimm

I liked @Artur Meinild's answer, and I wanted to dig a bit deeper, because I really wanted to understand how this works. I had a look at the source code of cron as shipped by Ubuntu, by enabling the source code repositories and running apt-get source cron. Here's what I learned:

cron just uses the command sendmail

To send an email, cron simply runs the command sendmail, like this:

/usr/sbin/sendmail -FCronDaemon -i -B8BITMIME -oem "$MAILTO"

(the exact command-line arguments might differ, but that's the gist of it)

MAILTO is an environment variable that can be set in the crontab. If it is not set, then LOGNAME environment variable is used instead. Cron sets LOGNAME to be the username of the Linux user. From man 5 crontab:

Several environment variables are set up automatically by the cron(8) daemon. SHELL is set to /bin/sh, and LOGNAME and HOME are set from the /etc/passwd line of the crontab's owner. PATH is inherited from the environment. HOME, SHELL, and PATH may be overridden by settings in the crontab; LOGNAME is the user that the job is running from, and may not be changed.

In addition to LOGNAME, HOME, and SHELL, cron(8) will look at MAILTO and MAILFROM if it has any reason to send mail as a result of running commands in this crontab.

If MAILTO is defined (and non-empty), mail is sent to the user so named. MAILTO may also be used to direct mail to multiple recipients by separating recipient users with a comma. If MAILTO is defined but empty (MAILTO=""), no mail will be sent. Otherwise mail is sent to the owner of the crontab.

If MAILFROM is defined, the sender email address is set to MAILFROM. Otherwise mail is sent as "root (Cron Daemon)".

By default, Ubuntu does not ship with the command sendmail installed. In that case, you will notice in the logs this error:

Feb 10 17:27:01 desktop-ubuntu CRON[36079]: (CRON) info (No MTA installed, discarding output)

Several different packages provide a sendmail command on Ubuntu. One that is often recommended is postfix:

$ sudo apt install postfix

(When prompted in the installation, I chose the "Local only" option, since I wasn't interested in sending email over the Internet.)

Postfix installs a command sendmail in order to be compatible with other packages like cron that expect to a sendmail command to be found. Other packages that also install sendmail are:

$ apt-file search -F /usr/sbin/sendmail
courier-mta: /usr/sbin/sendmail           
dma: /usr/sbin/sendmail
esmtp-run: /usr/sbin/sendmail
exim4-daemon-heavy: /usr/sbin/sendmail
exim4-daemon-light: /usr/sbin/sendmail
lsb-invalid-mta: /usr/sbin/sendmail
masqmail: /usr/sbin/sendmail
msmtp-mta: /usr/sbin/sendmail
nullmailer: /usr/sbin/sendmail
opensmtpd: /usr/sbin/sendmail
postfix: /usr/sbin/sendmail
ssmtp: /usr/sbin/sendmail

It seems that if postfix is installed with the "local only" configuration, then mail sent to a Linux gets stored in this location /var/mail/<username>. Here is some example content of this file once a cron job running echo hi has run:

$ less /var/mail/$USER
From flimm@desktop-ubuntu  Tue Feb 14 08:54:01 2023
Return-Path: <flimm@desktop-ubuntu>
X-Original-To: flimm
Delivered-To: flimm@desktop-ubuntu
Received: by desktop-ubuntu (Postfix, from userid 1000)
        id 9C7137E12E4; Tue, 14 Feb 2023 08:54:01 +0100 (CET)
From: root@desktop-ubuntu (Cron Daemon)
To: flimm@desktop-ubuntu
Subject: Cron <flimm@desktop-ubuntu> echo hi
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
X-Cron-Env: <SHELL=/bin/sh>
X-Cron-Env: <HOME=/home/flimm>
X-Cron-Env: <LOGNAME=flimm>
Message-Id: <20230214075401.9C7137E12E4@desktop-ubuntu>
Date: Tue, 14 Feb 2023 08:54:01 +0100 (CET)

hi

If you prefer, you can use a mail client like mutt (on the command-line) or Thunderbird (with a GUI) to view these emails stored in this location.

References:

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