Scriptblock doesn’t work with variables.

Friday, October 10th, 2014

Powershell is a fun experience which I wish I had started awhile ago.  It’s features seem endless and it greatly eases system management.

The recent scare over Shellshock caused us to evaluate our Cygwin installations.  We didn’t know the exact number of installations and the version of Bash in use.

The question of the number of Cygwin installations was easy to solve. A script to review the organization units and scan each computer for Cygwin.

After a list was generated; a script was written and three results files were created: Found, System Down, and Not Found.

Cygwin due to it’s open source nature does not follow Microsoft design standards.  It has it’s own way of doing things. Good for the user but not always good for system management purposes.

I didn’t find a “Quick and Dirty” way to get the version number of the Bash executable.   Nothing in the registry and file information lacked details.

Redhat (we use them for support) advised using the “–version” option to identify the version.  A rather tedious task if done through remote desktop or nagging the users to send the information.

What would be nice if there was way to run a command remote and get the information.

Powershell has a nice option called “invoke-command.” It’s a one use option for such things.

The command is rather simply to use:

Invoke-Command -scriptblock { c:\cygwin\bin\bash –version }  -computername <hostname>

This gave the following output:

GNU bash, version 4.1.15(1)-release (i686-pc-cygwin)
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

A little too much detail for a version number.  This was quickly addressed by using Select-String command.

Invoke-Command -scriptblock { c:\cygwin\bin\bash –version }  -computername <hostname> | Select-String -pattern “GNU bash, version”

This in turn gave:

GNU bash, version 4.1.15(1)-release (i686-pc-cygwin)

This was usable.  It also helped with simple errors.  If you don’t get the expected result, add an error message. This became evident when trying to run the command on Windows 2003 which lacked Powershell 2.0 and systems which did not have remote management configured.

Invoke-Command is great.  It gave me the information I needed without visiting each system.

I wrote a script and quickly found two problems:

  1. There are flavors of Cygwin and unless there was a conscious effort during install; you could end up with different directories (ie: C:\cygwin, C:\cygwin64, C:\rhcygwin).
  2. Running the command for several systems.  Invoke-Command can take up to five systems.  Great for a small number but not so great when you have hundreads or more.

Both of these issues required the use of variables and it was quickly discovered that -scriptblock option couldn’t use variables. For example:  $cygwinPath\bash.exe –version resulted as \bash.exe –version

The dynamic nature of scanning several computers was not something the command could do.  Rather then use another approach; I looked around the Net and found there was a way to use dynamic scriptblocks by using the NewScriptBlock command.

A quick change:

$scriptblock = $ExecutionContent.InvokeCommand.NewScriptBlock( “$cygwinPath\bash.exe –version”)
$versionResult = Invoke-Command -scriptblock $scriptblock -computername $server | Select-String -pattern “GNU bash, version”

This gave the needed results and we had an understanding of the work needed to be done.

If you would like to read more about Invoke-Command, Ed Wilson did a nice little writeup on his blog.

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