subogero linux blog

While setting up the ssh-agent on Cygwin, I run into a small problem. After reboot, the ssh-agent is not running yet, but the /tmp/.ssh* files are still there from the previous session. When starting a Cygwin shell, the start of the ssh-agent fails, if these files have not been manually removed before.

Task: delete these files automatically at boot time. I googled it, and the Windows way of doing it seemed extremely complicated. As usual, as it springs to the lips of smug Linux geeks. Not me. Then I ran across the “cron” scheduling daemon somehow, and there it was: insert the line below into the “/etc/crontab” file.

@reboot SYSTEM rm -f /tmp/.ssh*

The rest of this post is a distilled version of my adventures with setting up cron on Cygwin 1.7.5.

cron is the Unix-equivalent of “Scheduled Tasks” in Windows, just better. For instance, you can schedule something to run at boot time (I may have mentioned that before) and, the scheduled commands don’t throw up a scary cmd-window on your desktop. So in the meantime I’ve moved all my scheduled tasks to cron.

Start the cygwin-setup and add the “cron” and “cygrunsrv” packages from the “Admin” category.

We’ll run cron as a service by user SYSTEM. Poor SYSTEM therefore needs a home directory and a shell. The “/etc/passwd” file will define them.

$ mkdir /root
$ chown SYSTEM:root /root
$ mcedit /etc/passwd
SYSTEM:*:......:/root:/bin/bash

The start the service:

$ cron-config
Do you want to remove or reinstall it (yes/no) yes
Do you want to install the cron daemon as a service? (yes/no) yes
Enter the value of CYGWIN for the daemon: [ ] ntsec
Do you want the cron daemon to run as yourself? (yes/no) no
Do you want to start the cron daemon as a service now? (yes/no) yes

Local users can now define their scheduled tasks like this (crontab will start your favourite editor):

$ crontab -e  # edit your user specific cron-table
HOME=/home/foo
PATH=/usr/local/bin:/usr/bin:/bin:$PATH
# testing
* * * * *   touch ~/cron
@reboot     ~/foo.sh
45 11 * * * ~/lunch_message_to_mates.sh

Domain users: it does not work. Poor cron is unable to run scheduled tasks on behalf of domain users on the machine. But there is another way: cron also runs stuff found in the system level cron table in “/etc/crontab”. So insert your suff there, so that SYSTEM does it on its own behalf:

$ touch /etc/crontab
$ chown SYSTEM /etc/crontab
$ mcedit /etc/crontab
HOME=/root
PATH=/usr/local/bin:/usr/bin:/bin:$PATH
* * * * *   SYSTEM touch ~/cron
@reboot     SYSTEM rm -f /tmp/.ssh*

Finally a few words about crontab entries. They are either environment settings or scheduled commands. As seen above, on Cygwin it’s best to  create a usable PATH. Home dir and shell are normally taken from “/etc/passwd”.

As to the columns of scheduled commands see the manual page.

If certain crontab entries do not run, the best diagnostic tool is this:

$ cronevents

It will print out details on all successful and failed commands.