Thursday, November 21, 2013

Powershell one-liner to check log files

Today I want to show you how Powershell can be used to search log files for specific strings of text and wait for updates (similarly to tail -f in the old unix world). Before the arrival of Powershell this was a pretty tedious activity involving lot of log file opening and closing. With the arrival of Powershell this task has become both easier and quicker than ever.

Let's suppose (this was my case today) that you have just joined a computer to a WSUS server and that you have some issue downloading the required patches. What you would do is to look for the Windows Update log file inside your Windows folder on your client computer.

In Powershell that's accomplished in a basic one-liner:
Get-ChildItem -Path C:\Windows *update*.log -Recurse | Get-Content -Wait | Select-String 'warning'
As you can see, I don't even know the exact name of the log file, nor the subfolder it resides in, but, with a little guessing, I can tell Powershell to look inside any subdirectory of c:\Windows for any log file contaning the word 'update' in its name, then pipe the result to Get-Content, which keeps the stream open and passes any update to the Select-String cmdlets. Select-string retrieves any line containing the 'warning' word and output it to the screen.

The result is stunning:
2013-11-20    16:01:10:783     832    160    AU      # WARNING: Failed to find updates with error code 800B0001
2013-11-20    16:01:12:346     832    8b4    Misc    WARNING: Digital Signatures on file C:\Windows\SoftwareDistribution\SelfUpdate\ are not trusted: Error 0x800b0001
2013-11-20    16:01:12:346     832    8b4    Setup    WARNING: SelfUpdate check failed to download package information, error = 0x800B0001
2013-11-20    16:01:12:346     832    8b4    Agent      * WARNING: Skipping scan, self-update check returned 0x800B0001
2013-11-20    16:01:12:346     832    8b4    Agent      * WARNING: Exit code = 0x800B0001
There I have my error! Of course, if you're less lucky, you may need to try with different log file names, or with different words in the content, but the fact is that it's anyway easier than ever before.

Note that you can replace 'warning' with a regular expression if you want to do complicated inclusion or exclusion of log messages.

For instance, to search for the keyword 'error' or 'warning' use the following regex:
Get-ChildItem -Path C:\Windows *update*.log -Recurse | Get-Content -Wait | Select-String "warning|error"
On Powershell 2.0 (which is the Powershell version of the computer I had a problem with) this can be shortened to:
gci C:\Windows *update*.log -rec | gc -Wai | Select-String "warning|error"
Not bad, isn't it? Stay tuned for more fun with Powershell.


  1. Hi,

    I'm very impress with how you can used powershell to monitor log file. On top of that, i wonder, how to further extend the capability, such as, we will continue monitor the log file, upon reading the new log entry, it will be further process and send out via blat or logger with filter information ? for example

    The original log is like this
    2013-11-20 16:01:12:346 832 8b4 Agent * WARNING: Exit code = 0x800B0001

    so we sill send out a message only consist of date time and message, either in blat or via logger
    2013-11-20 16:01:12:346 * WARNING: Exit code = 0x800B0001

    appreciate for some guidance

  2. How can you exit the watch when a condition occurs? Such as when the ScheduledTask completes and the log is no longer being updated.


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