Rsyslog to clean up /var/log/messages

Situation: Not all applications and utilities have been developed to log their messages intelligently or to their own location.

Complication: This can lead to logging bloat, especially for messages that are purely informational and noisy. /var/log/messages can be overwhelmed by this and can make it harder to figure out actual problems.

Question: How can you redirect messages to clean up /var/log/messages?

Answer: rsyslog to the rescue!


For the purposes of this post I will be analyzing the program amazon-ssm-agentThis is an Amazon proprietary program necessary to run AWS Run Command. This is a good example because it was developed to have it’s own log file, but also still fills /var/log/messages. We will:

  1. Go through the workflow of syslog, systemd and how messages get into logs
  2. Look at messages in /var/log/messages that need to be filtered
  3. Configure rsyslog to send all application logs to it’s own file
  4. Use logrotate to create a good policy for how long logs stay around
  5. Celebrate clean logs



Linux tools to parse through files

Situation: You have a lot of results in a file that you need to move through and single out and add or subtract to fields, compare entries, or many other needs

Complication: There are a TON of Linux tools to do this

Question: What can do what?

Answer: I will add to this post with specific examples consistently


Example 1: Search through a file list and return only the filename, not the full path

Let’s say you want to just look for missing files in a non-similar directory structure between multiple file lists. One way is to look through each file without the file path and get just the file name. Let’s say filelist1 has this list:


Let’s now say filelist2 has this list:


In this case you want to just figure out that you’re missing file 4, so that you could sync just that file and not all the contents of directoryone. So to do this you could use cat to show the file contents, and pipe it to cut to get just the filename

less filelist1 | cut -d/ -f4

The -d/ option is saying to use the as the delimiter, and the -f4 is saying to return the 4th entry

This would return


You could then send it to a new file that had just the filenames with >>

less filelist1 | cut -d/ -f4 >> filelist1_clean
less filelist2 | cut -d/ -f4 >> filelist2_clean

You can now easily find just the files missing by running

diff filelist1_clean filelist2_clean

You would then get


Now you can make your sync off of this

Nagios acknowledge through email


Nagios is a widely used alerting system


Sometimes you’re out to dinner and get an alert that is not immediately actionable until you finish desert


Can you ack the alert without having to patch in and ack through the nagios UI?


Yes! You can ack alerts with a simple email reply with the words “ACK”

Avleen Vig wrote a great python script to poll the nagios inbox, parse the alert info and acknowledge the problem if ACK is in the message


  1. Install Nagios
  2. The base nagios install does not include a home directory and login for the nagios user, so create it manually
    mkdir /home/nagios
  3. Create IMAP inbox for nagios to use (for both sending and receiving). This can be done through Gmail or any other IMAP server you have access to
  4. Copy Avleen’s script from Github to /home/nagios/
  5. chmod the script to be executable
    chmod 760
  6. Edit to match the nagios CMD file that is in your environment
    40 CMD_FILE = '/usr/local/nagios/var/rw/nagios.cmd'


    40 CMD_FILE=/etc/nagios/var/rw/nagios.cmd'
  7. Put in your IMAP information into the script
     42 # IMAP server, username and password
     43 IMAP_SERVER = ''
     44 IMAP_USER = ''
     45 IMAP_PASS = "imap_password"
  8. If your host names in Nagios are longer than ~15 characters, then Gmail (and potentially others) will automatically make a new line to account for that, even though the Subject line is 1 line. Get around this by adding the ability to handle new lines within the script with \n at the end of ACK
    152 if alert_class == 'Host':
    153 msg = '[%s] ACKNOWLEDGE_HOST_PROBLEM;%s;1;1;1;%s;ACK\n' % \
    154 (now, server, fromaddr)
    155 elif alert_class == 'Service':
    156 msg = '[%s] ACKNOWLEDGE_SVC_PROBLEM;%s;%s;1;1;1;%s;ACK\n' % \
    157 (now, server, service, fromaddr)
    158 open(CMD_FILE, 'w').write(msg)
    159'ACKed alert: From: %s, Host: %s, Service: %s\n' % \
    160 (fromaddr, server, service))
  9. Cron the script to run every minute to search for new acknowledgements
    crontab -e
    * * * * * /usr/bin/python $HOME/ >> /var/log/nagios/email_ack.log 2>&1
  10. Test by purposefully getting nagios to alert, and then respond with an email with just the contents “ACK”. Look in /var/log/nagios/email_ack.log. Make sure the information is getting parsed correctly. You should see something like this:
    Service,, hostname, disk_usage, ack

One-liner for largest files/folders

Do you ever find yourself wanting to know what the deuce is taking up all your space? Here’s a simple one-liner to figure out the top 20 folders eating your disk:

du -ha <folder_location*> | sort -h | tail -n20

du is the disk usage utility that teamed with sort and tail can be very valuable. Usually you would want to make the <folder_location> be at the root of the volume (/) but maybe you have areas you don’t want searched. Just include the exclude!

du -ha /* --exclude "/home/" | sort -h | tail -n20

This will work for Linux, but if you’re on OSX you will likely not have the sort command. First you’ll need to install brew, which is a package manager for OSX:

/usr/bin/ruby -e "$(curl -fsSL"
Here’s the official Brew site with context on the install
Once you’ve got Brew, you can install the GNU core utilities
brew install coreutils

You can now use gsort -g on OSX in place of sort -h on Linux with the same results

ADCLI on CentOS 6

If you are trying to bind a Linux machine to Active Directory, a very simple tool to use is ADCLI. You can use it to join servers, query AD, and also add/delete objects. It is not currently available through the yum repos, however. Here’s how to get past that:

  1. Download
  2. You’ll need to rebuild the source rpm like this:
    1. rpmbuild –rebuild adcli-0.8.1-1.el6.src.rpm
  3. You may end up with needing other dependencies such as openldap-devel and xmlto (and potentially others). You can install these through yum:
    1. yum install -y openldap-devel xmlto
  4. To bind a new server:
    1. adcli join -U <ADadminUser> domain
    2. Enter password
  5. Verify the bind was a success by querying AD:
    1. adcli info <domain>

You should now be able to login with any AD user on that machine and do application level Active Directory integration

AWS Run Command

Amazon Web Services recently came out with a new feature called “Run Command”. If you have instances in AWS it allows you to send a set of commands to a subset (or all) of your instances, with the ability for extended logging of the output sent to an S3 bucket, if you wish.

People will sometimes use tools such as Puppet to send a new system configuration that may only be a single command, such as a systemctl enable command. But the AWS Run Command will let you do this without having to create a Puppet module for a small set of commands.


  • amazon-ssm-agent must be running.
  • Security groups must be configured to allow this agent

Security Groups

AWS Identity and Access Management has a default SSM Policy which you can apply to your instances if you just search for it in IAM:

Screen Shot 2016-04-20 at 12.34.13 AM.png

After you have applied the AmazonEC2RoleforSSM you need to get the amazon-ssm-agent service running. You can bake this into whichever AMI baking tool you are using, or configuration management (Chef, Puppet etc). But to test you can set it up like this for Linux instances upon creation or in a script run by your configuration management software:

cd /tmp 
curl https://amazon-ssm-<your-region> -o amazon-ssm-agent.rpm
yum -y install amazon-ssm-agent.rpm

Start the service for CentOS 7.x using:

sudo systemctl start amazon-ssm-agent

For CentOS 6.x use:

service start amazon-ssm-agent

Once amazon-ssm-agent is running, then you can issue commands to your instances. Here’s the basics on how to do it:

In AWS Console –> EC2 –> Commands

Screen Shot 2016-04-19 at 11.35.52 AM

Then select the type of Command you wish to run, for example a simple Shell Script:

Screen Shot 2016-04-19 at 11.28.30 AM

You can then choose to send the full output of your commands to an S3 bucket:

Screen Shot 2016-04-19 at 11.45.14 AM

Once the command is run you can view the output by clicking “View result”:

Screen Shot 2016-04-19 at 11.42.00 AM


The AWS Run Command is extremely powerful for one-off commands to be run without the overhead and delay of a configuration management tool. The amazon-ssm-agent takes a little extra time to set up, but once you have it started you can utilize the Run Command at no cost other than minimal cross-region traffic costs.