Tag Archives: Linux

Using cron to alert to known error conditions in log file

Recently I’ve been having issues with a VirtualBox machine crashing at random. The only common thread is a line of log file, and so far, google has not been helpful in determining the root cause. This machine is important enough that I need to know when it crashes, but not important enough to actually take the time to fix properly, so I researched and created a command to toss into cron to check the log file for the error line and email me when it finds it.

The command itself is simple. Search the log file with grep, if the search generated results send an email. I tried several iterations of this before finding one that did not email when the search was empty and also did not generate silent errors when empty as well. I run this via cron at 5 minute intervals

To use this command on ubuntu 14.04 you will need to install mailutils and moreutils

Software and versions used or mentioned:
mailutils
mailx
moreutils
ifne

Resources:
Forum post with grep | mailx solution
ServerFault post with ifne solution

Complications of not running SSH on port 22

Traditionally ssh runs on port 22, but that does not mean it has to. Where I work each machine that runs ssh runs it on a different port. I do not know why, but it causes quite a bit of fun when trying to use common ssh based commands that all expect ssh to run on the default port. I have had to figure out more then a few of the workarounds and will share them because I had trouble finding a few of them.

SSH

SSH is easy to connect to a non-standard port, simply use the -p flag followed by the port number. Following is an example:

ssh-copy-id

For those who are not aware, you can create an identity file and copy it to the remote machine to allow you to log in without the need of a password (Link to the guide that taught me). ssh-copy-id assumes that the remote machine is running on port 22 and you have to trick it to change the port. We use the same -p flag as with ssh, but we need to put the whole of the arguments into single quotes for it to work. See the example below:

rsync

rsync has a -e flag for setting which remote shell to use. after the -e flag you put the ssh options like is you were using ssh normally. Be sure to put them inside double quotes or they will get confused with rsync options. See the following example:

scp

SCP is very easy, simply use the -P flag to set the port number, like in the following example: