I want to take a step back and talk about hashtables. I use them all the time now. I was teaching someone about them after our user group meeting last night and I realized I had the same confusion about them as he had. Hashtables are really important in PowerShell so it's good to have a solid understanding of them.
The original version of this article appeared on the blog written by KevinMarquette. The PowerShell team thanks Kevin for sharing this content with us.
Please check out his blog at PowerShellExplained. I want you to first see a Hashtable as a collection in the traditional definition of a hashtable. This definition gives you a fundamental understanding of how they work when they get used for more advanced stuff later. Skipping this understanding is often a source of confusion.
Before I jump into what a Hashtable is, I need to mention arrays first. For the purpose of this discussion, an array is a list or collection of values or objects. Once you have your items into an array, you can either use foreach to iterate over the list or use an index to access individual elements in the array.
I just scratched the surface on arrays but that should put them into the right context as I move onto hashtables. I'm going to start with a basic technical description of what hashtables are, in the general sense, before I shift into the other ways PowerShell uses them.
A hashtable is a data structure, much like an array, except you store each value object using a key. First, we create an empty hashtable.
Notice that braces, instead of parentheses, are used to define a hashtable. Then we add an item using a key like this:.
Once you add your values to the hashtable, you can pull them back out using that same key instead of using a numeric index like you would have for an array. When I want Kevin's age, I use his name to access it. We can use this approach to add or update values into the hashtable too. This is just like using the add function above. There's another syntax you can use for accessing and updating values that I'll cover in a later section. If you're coming to PowerShell from another language, these examples should fit in with how you may have used hashtables before.
So far I've created an empty hashtable for these examples. You can pre-populate the keys and values when you create them. The real value of this type of a hashtable is that you can use them as a lookup table. Here is a simple example. This gets even better when you dynamically build the lookup table to use it later. So think about using this approach when you need to cross reference something.
I think we would see this even more if PowerShell wasn't so good at filtering on the pipe with Where-Object. If you're ever in a situation where performance matters, this approach needs to be considered.
I won't say that it's faster, but it does fit into the rule of If performance matters, test it. PowerShell allows you to provide an array of keys to get multiple values. In this example, I use the same lookup hashtable from above and provide three different array styles to get the matches. This is a hidden gem in PowerShell that most people aren't aware of.
The first thing to notice is that if you pipe your hashtable, the pipe treats it like one object.In Windows 10 and Windows 8, it is possible to get Hash values for a file without using third party tools. Here is how it can be done. The ability to calculate the file hash is a part of the Windows cryptograpic API. The user interface of the operating system has no option to calculate or show the hash value for files.
Here is how to use it. Open PowerShell and type the command above to test it. It calculates the SHA hash value for the given file and produces the output as follows.
To calculate the hash value other than SHA, use the switch -Algorithm.
For example, to get the MD5 hash value, execute the following command:. Another useful switch you need to know is -LiteralPath. It specifies the path to a file. Unlike the default path parameter, the value of the LiteralPath parameter is used exactly as it is typed. No characters are interpreted as wildcard characters. If the path includes escape characters, enclose the path in single quotation marks.
Single quotation marks instruct Windows PowerShell to not interpret characters as escape sequences. Using the Get-FileHash cmdlet, you can get hash values for a file natively.
It is very useful, especially when you are working in a secure environment where third-party software is not allowed. The PowerShell console is bundled with Windows 10 and is accessible in every installed instance, so you can use the native Get-FileHash cmdlet everywhere. Winaero greatly relies on your support. You can help the site keep bringing you interesting and useful content and software by using these options:.H ash value, or hash checksum, for a file is commonly used to verify the integrity of the file, especially on large files downloaded over the Internet where the downloads are corrupted or may not be completed properly and fully.
Thus, hash checksum provides a cryptographically-secure way to verify that the contents of a file have not been changed. Any changes to the content, even though just a single character, changes the hash value of the file, even though the file name or extension stays the same. By comparing the hash value of the file against the officially published hash value, one can determine if the file is exactly the same and identical with the original, intact, genuine, unmodified, untouched, not corrupt, and usable.
There are plenty of utilities available that can calculate the hash value or hash checksum for files. The PowerShell cmdlet to computer the hash value by using a specified hash algorithm is Get-FileHashwith the following syntax:. For example. Any hash algorithm that is supported by the operating system can be used. In the command above, The output is piped to the Format-List cmdlet to format the output as a list.
For security reasons, MD5 and SHA1, which are no longer considered secure, should only be used for simple change validation, and should not be used to generate hash values for files that require protection from attack or tampering.
Go to Top.Hey, Scripting Guy! I have a folder and I would like to detect if files within it have changed. I do not want to write a script to parse file sizes and dates modified because that seems to be a lot of work. Is there a way I can use an MD 5 hash to do this? Oh, by the way, I do have a reference folder that I can use. Hello RS. Microsoft Scripting Guy, Ed Wilson, is here.
Things are certainly beginning to get crazy. It will be a great day with one entire track devoted to Windows PowerShell. In fact, our Florida road trip begins with the monthly meeting of the Charlotte Windows PowerShell User Group we actually leave for our trip from the group meeting. If you find all this a bit confusing, I do too.
That is why I am glad we have the Scripting Community pageso I can keep track of everything. The intent of the series is not to teach security forensics, but rather to illustrate how Windows PowerShell could be utilized to assist in such an inquiry. The first blog discussed using Windows PowerShell to capture and to analyze process and service information. The third blog talked about computing MD5 hashes for files in a folder.
It is extremely easy to spot a changed file in a folder by making a simple addition to the technique discussed yesterday.
In fact, it does not require writing a script. The trick is to use the Compare-Object cmdlet. In the image that follows, two folders reside beside one another. The Ref folder contains all original files and folders. The Changed folder contains the same content, with a minor addition made to the a. The chief difference here is the addition of the Compare-Object cmdlet. The command a single logical command is shown here.
The command works because the Compare-Object cmdlet knows how to compare objects, and because the two Get-Hash commands return objects. The arrows indicate which object contains the changed objects. The first one exists only in the Difference object, and the second one only exists in the Reference object. Using the information from the previous command, I create a simple filter to return more information about the changed file.
The easy way to do this is to highlight the hash, and place it in a Where-Object command the? The command is shown here. I use essentially the same commands to find the differences between the two files. First, I make sure that I know the reference file that changed. Here is the command that I use for that:. When I have ensured that it is, in fact, the a.
Here is the command I use to compare the two files:. RS, that is all there is to using finding modifications to files in folders when you have a reference folder.I'm familiar with using Hashcalc which works very well with single files I have written a small console program that compute the hash of the content of a directory. It will explore it recursively and it uses the lexicographical order on names for sorting its content before performing the hash computation.
It can be called as follows : DirHash. I hope this will help. If you have saved the code above into Microsoft. Many thanks - I've had a very quick look at your utility and it seems very simple that's a good thing in execution. I'll do a little further testing but I think it does exactly what I want.
I just tested the script on Windows 7 x86 and whilst it seemed to run OK there was no output generated I'm not sure what I should be expecting as in does it output the hash to the console?
Forgive me my ignorance regarding PowerShell! I tried running this as you said by copying and pasting into the PowerShell prompt, I also tried by saving the text to a file called Get-FolderHash. I really do apologise for being a PowerShell novice Save notepad document.
Everything you wanted to know about hashtables
Re-open PowerShell console. Each time you want to calculate the hash for a folder you need to type only the following command:. When you saved the code into PS1 file and executed you just loaded a function.
To get an output, you need to call this function. Could you please provide source for this tool as well? I would like to integrate this approach in one of my projects. Is there any particular problem when the folder size exceeds 2GB? Is your question about performance? Anyway, with my tool DirHash there is no issue with big folders: on a Core i7 K, it takes 12 seconds to hash 2.
AddRange [System. I would like to use DirHash, but it has two small disadvantages for me: - 0-bytes files be ignored for hashcode - renaming file no change with content and order is the same, no change in hashcode. The shell script posted above is not practical because it reads all the content of the files into memory before doing the hash.
This can't work in real life. I have modified my tool DirHash by adding a new switch -hashnames that will activate the use of names of files and directories in the hash computation.The detailed, in depth, article by Jeff Hicks over at Petri. The script makes two lists of hashes, one for each folder, and compares the two. If the two folders are the same i. For example. Wow, awesome weblog layout! How long have you been blogging for?
Using PowerShell to check a folder copied
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Use PowerShell to Compute MD5 Hashes and Find Changed Files
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This is certainly preferable since it avoids the problems the first solution offers as identified in the comments uses a stream, closes it, and supports large files. Thanks to davor above for the suggestion to use Open instead of ReadAllBytes and to jpmc26 for the suggestion to use a finally block.
Another built-in command that's long been installed in Windows by default dating back to is Certutilwhich of course can be invoked from PowerShell, too. There are a lot of examples online using ComputeHash. My testing showed this was very slow when running over a network connection.
The snippet below runs much faster for me, however your mileage may vary:. It uses the. NET framework to instantiate an instance of the MD5 hash algorithm to calculate the hash. As stated in the accepted answer, Get-FileHash is easy to use with files, but it is also possible to use it with strings:. The example is from the official documentation of PowerShell 5.
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The documentation has more examples. Here is a pretty print example attempting to verify the SHA fingerprint. I downloaded gpg4win v3. Here is a one-line-command example with both computing the proper checksum of the filelike you just downloaded, and comparing it with the published checksum of the original. For instance, I wrote an example for downloadings from the Apache JMeter project.