Set up `cron` job and mailto on error, on Arch

Question

I use postfix linked to an commercial smtp server for cron jobs to send their owners an email in case of error/warning. This is a one box Arch setup.

postfix works. So does cronie. But although the simplest of all cron jobs does fire, I get an email from the cron daemon at every execution, which reads:

/bin/sh: warning: command substitution: ignored null byte in input.

The cron rule, to be run every minute as a test, is:

MAILTO=MYUSERNAME@LOCALHOSTNAME

* * * * * eval "export $(grep -Ez DBUS_SESSION_BUS_ADDRESS /proc/$(pgrep -u $LOGNAME gnome-session)/environ)";/home/USERNAME/test

and the executable ~/test is basically:

#!/bin/bash
/usr/bin/notify-send 'Hello world!' --icon=dialog-information

I understand that grep -Ez [...] introduces a null byte, as is made necessary by its output with some unusual characters.

My toy example would function properly, if it weren't for that specific warning being emailed to USERNAME at every execution. How do I get rid of that ?

Answer 1

After some research.... it turns out that grep -Ez [...] is not the right way of doing this in a cron job.

/proc/[pid]/environ in Linux OS has the quirk that the null byte is the record separator, so that its content when output to stdout "looks" like a single line, i.e. with no EOL character:

$ cat -v /proc/$(pgrep -u $LOGNAME gnome-session)/environ
LC_MEASUREMENT=en_IE.UTF-8^@LC_PAPER=en_IE.UTF-8^@LC_MONETARY=en_IE.UTF-8^@LANG=en_US.UTF-8^@GDM_LANG=en_US.UTF-8^@DISPLAY=:1^@USERNAME=USERNAME^@MOZ_PLUGIN_PATH=/usr/lib/mozilla/plugins^@XDG_VTNR=2^@XDG_SESSION_ID=2^@USER=USERNAME^@DESKTOP_SESSION=gnome^@PWD=/home/USERNAME^@HOME=/home/USERNAME^@XDG_SESSION_TYPE=x11^@XDG_SESSION_DESKTOP=gnome^@LC_NUMERIC=en_IE.UTF-8^@MAIL=/var/spool/mail/USERNAME^@WINDOWPATH=2^@SHELL=/bin/bash^@XDG_CURRENT_DESKTOP=GNOME^@XDG_SEAT=seat0^@SHLVL=0^@GDMSESSION=gnome^@LOGNAME=USERNAME^@DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus^@XDG_RUNTIME_DIR=/run/user/1000^@XAUTHORITY=/run/user/1000/gdm/Xauthority^@PATH=/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl^@LC_TIME=en_IE.UTF-8^@

In the above, I used cat with the -v flag to reveal nul bytes.

So the solution is to look for the pattern "DBUS_SESSION_BUS_ADDRESS" while recognizing null bytes. awk and gawk both do that although it is not portable, by changing the record separator to the hexadecimal code for the nul byte: \x00

$ awk -F 'BEGIN {RS="\x00"} /DBUS_SESSION_BUS_ADDRESS/' /proc/$(pgrep -u $LOGNAME gnome-session)/environ
DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/1000/bus

That actually produces a clean output with no nul bytes and is well liked by cron. No warning is produced and no email is sent.

Answer 2

Replace \0 with \n:

eval "export $(tr -s \\0 \\n </proc/$(pgrep -u $LOGNAME -x gnome-session)/environ|grep DBUS_SESSION_BUS_ADDRESS)";/usr/bin/notify-send 'Hello world!'