Tuesday, December 2, 2014

My First Mobile Application - With More Functionality

Next step in my fist mobile app was to actually add something meaningful to the empty application, for example the functionality to take a photo using camera or select a photo from the gallery. My goal was to add as little as possible to the app to be able to identify only the necessary steps, to create, so to speak, a "minimal viable app". Also I concentrated on the Android because I down't own an iOS device. But I show some iOS related code below in some cases anyway. There may be more needed to create a proper iOS app. First step - download jQuery mobile. Next, after unpacking, I copied the following files into my project structure, into App/www/js folder.
  • jquery.1.11.1.min.js
  • jquery.mobile-1.4.5.js
  • jquery.mobile-1.4.5.min.js
  • jquery.mobile-1.4.5.min.map
Next, changes to the application config.xml file. Most important change is, probably, the addition of a camera feature. 


  <param name="android-package" value="org.apache.cordova.camera.CameraLauncher />


 <param name="ios-package" value="CDVCamera" />
You may modify the apps details from the default ones 
Camera app

 JQuery Mobile and Camera


 Evgeny
If you use icons in your app (I guess there should be at least the one), they should be created for all screen resolutions. A good idea, of course, is to create a quality icon in high resolution and then scale it down, rather than vice versa.
Same deal with the splash screen.
Now the actual functionality. I placed all of it into the index.html, but the javaScript could be placed in the separate file. Reference the required javaScript files.
Add a div with the buttons that will provide functionality.
Add a div to test that the image can be retrieved from the camera. Let's place the image into a div on the main page. 

Name: Evgeny

Job Title: Developer
Add some javaScript. Call the onLoad from the body tag, for example. What we want is to make sure that PhoneGap is ready to handle native events. After the device is ready, we will take the photo using the camera. If that is successful, the photo should appear on the main screen of the app. If not, we should see the alert which explains the reason for failure. 

<body onload="onload()">

...

function onLoad(){
  document.addEventListener("deviceready", onDeviceReady, false);
}

function onDeviceReady(){
  console.log(navigator.camera);
  alert('Device is ready');
}

And actual utilisation of the camera

function TakePhotoUsingCamera(){
  TakePhoto(Camera.PictureSourceType.CAMERA);
}

function TakePhotoFromLibrary(){
  TakePhoto(Camera.PictureSourceType.PHOTOLIBRARY);
}

function onSuccess(imageData){
  var image = document.getElementById('myImage');
  image.src = "data:image/jpeg;base64," + imageData;
}

function onFail(message){
  alert('Failed because: ' + message);
}

function TakePhoto(sourceType){
   var camOptions = {
  quality:50,
  destinationType: Camera.DestinationType.DATA_URL,
  sourceType: sourceType,
  correctOrientation: true
   };
 navigator.camera.getPicture(onSuccess, onFail, camOptions);
}
This is probably all of the code that will be needed. Also, the API for the camera has to be added explicitly. Run the following command

phonegap plugin add org.apache.cordova.camera

You will notice that the new folder was added under plugins.

Finally, as with the "Hello world" application, build it and then run on the emulator.

phonegap build android
phonegap install android

You won't be able to use the camera on the emulator, so to test it install the apk on the Android device.
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Tuesday, November 25, 2014

My First Mobile Application

It is time to write my first mobile application. The inspiration comes from a free online course on cross platform mobile development. The course uses PhoneGap for development. This post is about configuring the environment on Windows and creating an empty "Hello World" application. Step one was to gather and install the required software. The list includes
  • AndroidSDK
  • ApacheAnt
  • Eclipse
  • JDK
  • NodeJS
It is important to download the correct version of your operating system (Windows 64 or 32 bit in my case), and also where the software is copied to or installed to, because it will be necessary to add these locations to environment variables. Now, the installation - Node.js and JDK are installed, and Eclipse and Ant are just extracted to your preferred location. I, personally, tried to make the path to this location as simple as I could, i.e. C:\Development\ant. Next, install PhoneGap. After you installed Node.js, you have Node.js command prompt in your Start menu. Run it and execute
npm install -g phonegap Now some path modifications are required. Here are brief instructions to locate where it is done:
  • Right-click My Computer and click Properties
  • Click Advanced System Settings link in the left column
  • In the System Properties window click the Environment Variables button
  • Select the variable you want to edit (i.e. PATH)
  • Select the edit button.
Here is what was necessary for me:
Create ANT_HOME (my value is C:\Development\ant)

Create JAVA_HOME (my value is C:\Program Files\Java\jdk1.8.0_25)
Add the following to the PATH variable at the end:
;C:\Development\adt\sdk\platform-tools;C:\Development\adt\sdk\tools;%ANT_HOME%\bin;%JAVA_HOME%;%JAVA_HOME%\bin.

As you can see, that includes Java, Ant and ADT. And to check that everything is set up properly, you need to run the following commands one by one from the command prompt:
  • java
  • javac
  • android
  • adb
  • ant

If any of those return the error '<name>' is not recognized as an internal or external command, operable program or batch file, there is probably an issue with the PATH - something is not included, or points to the wrong location. Note: when you run "ant", you will likely get the following message:

Buildfile: build.xml does not exist!
Build failed

This is expected and no need to worry about. Now the prerequisites are installed. Time to create the actual PhoneGap project. In Node.js command prompt, navigate to the place where you would like your project to be created and execute
phonegap create . "com.yourname.app" "yourappname"

The project should be created. Just to make it a little different from the empty Hello World application, I went into the www folder and modified the text in the header in index.html. Index.html can be viewed in the browser at this point.

Now the command to build the project.
phonegap install android
At this point I got the following error

As you can see, the Android-19 target is missing. The solution is to launch Eclipse, start Android SDK manager, and, to put it simple, install everything that has 19 in it. In my case it looks like this:

This will take some time. When it is done, run the command again.
phonegap install android
And in my case I received the following error:
No emulator images (avds) found ...
I solved it by running
android avd
This started the visual wizard where I could choose the desired parameters for the Android emulator and create it.

And finally, again
phonegap install android
This time it started the emulator, on which the application ran.

So this sounds fairly straightforward at the moment, but I had instructions when I started and still I spent probably about 4-6 hours on two PCs over the course of two days to figure out how to get to this point of running an empty application. In my defence, I can say that I had exactly zero prior experience in mobile development.
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Friday, March 14, 2014

Global Alignment Problem

The solution to the Manhattan problem described here only returned us the maximum score, but did not give instructions on how exactly we should walk through Manhattan to achieve this score. To lay out the path (and to use the solution in bioinformatics to align aminoacid sequences) we need an additional step: keep the history of our walk so that we could "backtrack" from the last intersection all the way back to the start. Essentially, it is simple: when we arrive to a new node, we always come from somewhere. This is the node we need to remember.

We should also consider that, unlike Manhattan graph, where we only had streets going "west" or "south", we now have "diagonal" streets if we want to adapt the problem to align aminoacid sequences. Consider the image below:

Alignment graph matches

The red diagonal arrows show the places where one sequence matches the other. We can give these edges a higher "score" so the path that goes through most of such edges is the highest scored. In this case we have three ways to reach any node s[i,j]: we can come from s[i - 1,j] by moving down, from s[i,j - 1] by moving right, or from s[i - 1, j - 1] by moving diagonally. In the simplest case, we'll calculate anything other than a match as a 0, and a match as 1. Then the score for any s[i,j] node will be calculated using the following formula:

Global alignment formula

In more complex scenarios, we may use scoring matrices, where each combination of two aminoacids is given a certain score, depending on how biologically reasonable is this combination.

Therefore, the following algorithm is used to calculate the score matrix s, and at the same time fill the backtracking matrix backtrack.

AlignmentMatrix(v, w)
 for i ← 0 to |v|
  si, 0 ← 0
 for j ← 0 to |w| 
  s0, j ← 0
 for i ← 1 to |v|
  for j ← 1 to |w|
   si, j = max{si-1, j, si,j-1, si-1, j-1 + 1 (if vi = wj)}
   backtracki, j = ↓ if si, j = si-1, j ; → if si, j = si, j-1 ; ↘ if si, j = si-1, j-1 + 1
 return s|v|, |w| , backtrack

After both the scoring matrices are filled, we will use the following algorithm to backtrack from the s[i,j] all the way back to the starting node. Along the way we will output two string. In places where there is a match between the strings, a symbol will be placed, and in other places we will fill in a dash. Following is the backtracking algorithm. We are filling two strings, v and w. If we came to the point i,j by a diagonal edge, we have a match, and output the same symbol into both strings. If we came by a vertical or horisontal edge, one of the strings receives a symbol, and the other a "mismatch" - a "-".

OUTPUTALIGNMENT(backtrack, v, w, i, j)
 if i = 0 or j = 0
  return
 if backtracki, j = ↓
  output vi = "-"
  outpuv wi
  OUTPUTLCS(backtrack, v, i - 1, j)
 else if backtracki, j = →
  output vi
  output wi = "-"
  OUTPUTLCS(backtrack, v, i, j - 1)
 else
  OUTPUTLCS(backtrack, v, i - 1, j - 1)
  output vi
  output wi

This is the C# implementation of the algorithms.

public static void GlobalAlignment(string s1, string s2)
{
 int s1l = s1.Length;
 int s2l = s2.Length;

 int[,] matrix = new int[s1l + 1, s2l + 1];
 int[,] backtrack = new int[s1l + 1, s2l + 1];

 for (int i = 0; i <= s1l; i++) for (int j = 0; j <= s2l; j++) matrix[i, j] = 0;
 for (int i = 0; i <= s1l; i++) for (int j = 0; j <= s2l; j++) backtrack[i, j] = 0;

 for (int i = 0; i <= s1l; i++) matrix[i, 0] = 0;
 for (int j = 0; j <= s2l; j++) matrix[0, j] = 0;

 for (int i = 1; i <= s1l; i++)
 {
  for (int j = 1; j <= s2l; j++)
  {
   matrix[i, j] = Math.Max(Math.Max(matrix[i - 1, j], matrix[i, j - 1]), matrix[i - 1, j - 1] + 1);

   if (matrix[i, j] == matrix[i - 1, j]) backtrack[i, j] = -1;
   else if (matrix[i, j] == matrix[i, j - 1]) backtrack[i, j] = -2;
   else if (matrix[i, j] == matrix[i - 1, j - 1] + 1) backtrack[i, j] = 1;
  }
 }

 OUTPUTLCSALIGN(backtrack, s2, s1, s1l, s2l);

 string aligned1 = ReverseString(Result);
 string aligned2 = ReverseString(Result2);
}

public static void OUTPUTLCSALIGN(int[,] backtrack, string v, string w, int i, int j)
{
 if (i == 0 || j == 0)
  return;
 if (backtrack[i, j] == -1)
 {
  Result = Result + "-";
  Result2 = Result2 + w[i - 1];
  OUTPUTLCSALIGN(backtrack, v, w, i - 1, j);
 }
 else if (backtrack[i, j] == -2)
 {
  Result = Result + v[j - 1];
  Result2 = Result2 + "-";
  OUTPUTLCSALIGN(backtrack, v, w, i, j - 1);
 }
 else
 {
  Result = Result + v[j - 1];
  Result2 = Result2 + w[i - 1];
  OUTPUTLCSALIGN(backtrack, v, w, i - 1, j - 1);
 }
}

public static string ReverseString(string input)
{
 char[] charArray = input.ToCharArray();
 Array.Reverse(charArray);
 return new string(charArray);
}

A slightly modified version of the function takes into account a penalty for an insertion / deletion (-5 in this case) and uses a BLOSUM62 scoring matrix 

public static void GlobalAlignment(string s1, string s2)
{
 int indelPenalty = -5;

 int s1l = s1.Length;
 int s2l = s2.Length;

 int[,] matrix = new int[s1l + 1, s2l + 1];
 int[,] backtrack = new int[s1l + 1, s2l + 1];

 for (int i = 0; i <= s1l; i++) for (int j = 0; j <= s2l; j++) matrix[i, j] = 0;
 for (int i = 0; i <= s1l; i++) for (int j = 0; j <= s2l; j++) backtrack[i, j] = 0;

 for (int i = 0; i <= s1l; i++) matrix[i, 0] = indelPenalty * i;
 for (int j = 0; j <= s2l; j++) matrix[0, j] = indelPenalty * j;

 for (int i = 1; i <= s1l; i++)
 {
  for (int j = 1; j <= s2l; j++)
  {
   //deletion
   int delCoef = matrix[i - 1, j] + indelPenalty;
   //insertion
   int insCoef = matrix[i, j - 1] + indelPenalty;
   //match / mismatch
   int mCoef = matrix[i - 1, j - 1] + BlosumCoeff(s1[i - 1], s2[j - 1]);

   matrix[i, j] = Math.Max(Math.Max(delCoef, insCoef), mCoef);

   if (matrix[i, j] == delCoef) backtrack[i, j] = -1; //
   else if (matrix[i, j] == insCoef) backtrack[i, j] = -2; //
   else if (matrix[i, j] == mCoef) backtrack[i, j] = 1;
  }
 }

 OUTPUTLCSALIGN(backtrack, s2, s1, s1l, s2l);

 string aligned1 = ReverseString(Result);
 string aligned2 = ReverseString(Result2);
}

Given the following two strings

PLEASANTLY
MEANLY

The following alignment will be returned

PLEASANTLY
-MEA--N-LY

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Tuesday, March 11, 2014

Manhattan Tourist problem

An introductory exercise to aligning amino acid sequences is the Manhattan tourist problem. Suppose a tourist starts his route in the top left corner of the map and wants to visit as many attractions on the way as possible, and finish in the down right corner. There is a restriction however - he can only move either to the right or down.

A Simplified Map of Manhattan

To describe the problem in numbers and data structures, the map is converted to the directed grid shown below. A tourist can move along the edges of the grid and the score of 1 is added if he passes an attraction, which are assigned to the bold edges. Of all possible paths we are interested in the one with the highest score.

Map of Manhattan as a Graph

The solution to this problem is equivalent to a more general problem. This time every "street", or the edge of the graph, is assigned a score. The goal is to find a path which gains the highest score.

Generic Manhattan Problem

The problem can be brute forced, of course, by calculating the score for all possible paths. This is not practical for the larger graphs of course. A better approach is to calculate the highest possible score at every intersection, starting from the top left corner. Here is how the logic goes: If the tourist goes only towards the left in the grid, or only down, there are no choices for him. So it is easy to calculate the highest scores for the top and left row of intersections. Now we have these scores calculated:

Generic Manhattan Problem - First Row and Column Solved

Now when that's done, we can proceed to the next row. How can the tourist get to intersection (1, 1)? He can either go to (0, 1) first, and gain 3 + 0 = 3 score, or go to (1, 0) and gain 1 + 3 = 4 score. Therefore, the maximum score he can gain at (1, 1) is 4.

Generic Manhattan Problem - Cell (1,1) Solved

The generic formula to use is quite intuitive: To reach an intersection, you have to arrive from one of two possible previous locations, and traverse a corresponding edge. You choose the path where the sum of the score of a previous location, plus the score of the edge, is higher.

Generic Manhattan Formula

We continue calculations along the column all the way down. After that, we can move on to the next row.

Generic Manhattan Problem - Second Column Solved

Following this logic, it is easy to compute the maximum score for each intersection and find out the maximum score that can be gained while traversing Manhattan.

Generic Manhattan Problem - Fully Solved

Along the way the algorithm also becomes clear: the maximum score at any intersection is the maximum of the following two values:

MANHATTANTOURIST(n, m, down, right)
 s0, 0 ← 0
 for i ← 1 to n
  si, 0 ← si-1, 0 + downi, 0
 for j ← 1 to m
  s0, j ← s0, j−1 + right0, j
 for i ← 1 to n
  for j ← 1 to m
   si, j ← max{si - 1, j + downi, j, si, j - 1 + righti, j}
 return sn, m

The following C# function implements the algorithm and returns the highest possible score, assuming we input all the scores for the edges towards the right and all the edges pointing down in the form of two-dimension arrays.

public static int ManhattanProblem(int[,] RightMatrix, int[,] DownMatrix)
{
 int n = RightMatrix.GetLength(0) + 1;
 int m = DownMatrix.GetLength(1) + 1;
 int[,] ManhattanMatrix = new int[n, m];

 ManhattanMatrix[0, 0] = 0;

 for (int i = 1; i <= n; i++)
 {
  ManhattanMatrix[i, 0] = ManhattanMatrix[i - 1, 0] + DownMatrix[i - 1, 0];
 }

 for (int j = 1; j <= m; j++)
 {
  ManhattanMatrix[0, j] = ManhattanMatrix[0, j - 1] + RightMatrix[0, j - 1];
 }

 for (int i = 1; i <= n; i++)
 {
  for (int j = 1; j <= m; j++)
  {
   ManhattanMatrix[i, j] =
    Math.Max(ManhattanMatrix[i - 1, j] + DownMatrix[i - 1, j], 
    ManhattanMatrix[i, j - 1] + RightMatrix[i, j - 1]);
  }
 }

 return ManhattanMatrix.Cast<int>().Max();
}

Understanding this problem turns out to be important to understanding sequence alignment in bioinformatics.

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Thursday, December 26, 2013

Minimal change problem

A popular problem to introduce dynamic programming is the minimal change problem. Suppose a cashier needs to give me a certain amount of change and wants to do it with the minimal amount of coins possible. The input is a set of denominations and the amount, and the output is the set of coins.

For example, I may need to give 45 cents change and available coins are 1, 5 and 20. A solution intuitively is 20 + 20 + 5, but what is the best way to achieve it?

A recursive solution may be the first to try. A minimal collection of coins definitely belongs to the following set:

  • minimal collection of coins totalling 44 cents, plus 1 cent coin
  • minimal collection of coins totalling 40 cents, plus 5 cent coin
  • minimal collection of coins totalling 25 cents, plus 20 cent coin

So on the next step we will apply the same logic to our new change amounts: 44, 40 and 25. This looks like a classic recursion problem, which can be illustrated by the following image

Solving Minimum Change with Recursion

and described by the following algorithm / pseudocode (where |coins| is the number of denominations available)

RECURSIVECHANGE(money, coins)
	if money = 0
		return 0
	MinNumCoins ← ∞
	for i ← 1 to |coins|
		if money ≥ coini
			NumCoins ← RECURSIVECHANGE(money − coini, coins)
			if NumCoins + 1 < MinNumCoins
				MinNumCoins** ← NumCoins + 1
	 output MinNumCoins*

This should work, but is there something wrong with this approach? Well, one can see that the recursive algorithm will calculate the full solution for 19 cents 6 times on the third step only, and it will only get worse on the following steps. If the input value is large enough, the memory and time required to compute the solution will be huge. So, this is a classic example of the benefits of dynamic programming. I came across dynamic programming a few times before, but just couldn't properly figure it out.

Finally I found a good explanation. It came as a part of a free course in Bioinformatics Algorithms. Here's how it goes:

The key to dynamic programming is to take a step that may seem counterintuitive. Instead of computing MinNumCoins(m) for every value of m from 45 downward toward m = 1 via recursive calls, we will invert our thinking and compute MinNumCoins(m) from m = 1 upward toward 45, storing all these values in an array so that we only need to compute MinNumCoins(m) once for each value of m. MinNumCoins(m) is still computed via the same recurrence relation.

MinNumCoins(m) = min{MinNumCoins(m − 20) + 1, MinNumCoins(m - 5) + 1, MinNumCoins(m - 1) + 1}

For example, assuming that we have already computed MinNumCoins(m) for m < 6, MinNumCoins(6) is equal to one more than the minimum of MinNumCoins(6 - 5) = 1 and MinNumCoins(6 - 1) = 5. Thus, MinNumCoins(6) is equal to 1 + 1 = 2.

This translates into the following algorithm / pseudocode

DPCHANGE(money, coins)
 MinNumCoins(0) ← 0
 for m ← 1 to money
        MinNumCoins(m) ← ∞
        for i ← 1 to |coins|
            if m ≥ coini
                if MinNumCoins(m - coini) + 1 < MinNumCoins(m)
                    MinNumCoins(m) ← MinNumCoins(m - coini) + 1
    output MinNumCoins(money)

And a further C# implementation, which takes a comma-separated string of denominations available, and the target amount.

public static void DPCHANGE(int val, string denoms)
{
	int[] idenoms = Array.ConvertAll(denoms.Split(','), int.Parse);
	Array.Sort(idenoms);
	int[] minNumCoins = new int[val + 1];

	minNumCoins[0] = 0;
	for (int m = 1; m <= val; m++)
	{
		minNumCoins[m] = Int32.MaxValue - 1;
		for (int i = 1; i <= idenoms.Count() - 1; i++)
		{
			if (m >= idenoms[i])
			{
				if (minNumCoins[m - idenoms[i]] + 1 < minNumCoins[m])
				{
					minNumCoins[m] = minNumCoins[m - idenoms[i]] + 1;
				}
			}
		}
	}
}

References

Bioinformatics Algorithms
An Introduction to Dynamic Programming: The Change Problem
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Monday, August 12, 2013

Using Active Setup to Update Anything in HKEY_CURRENT_USER

Following my last post, I had next to make sure that every user's entry in the registry was updated, and that change had to be scripted. This turned out to be a non-trivial task and took some research. First of all, the entry is located in HKEY_CURRENT_USER registy hive. Therefore, being logged in as Admin I cannot directly set an entry for Bob because Bob is not the current user at the moment. Then what can I do? The HKEY_CURRENT_USER is a kind of shortcut to HKEY_USERS. Under HKEY_USERS I can see the following structure

HKEY_USERS\.DEFAULT
HKEY_USERS\S-1-5-18
HKEY_USERS\S-1-5-19
HKEY_USERS\S-1-5-20
HKEY_USERS\S-1-5-21-0123456789-012345678-0123456789-1004
HKEY_USERS\S-1-5-21-0123456789-012345678-0123456789-1004_Classes

The first 4 entries correspond to built-in system accounts, and the rest are real user accounts on a PC. So, one way to make the change I need is to loop through all users and make the changes as requested. Someone even wrote a VB script which does exactly that. My case is a bit different, though. I only have a small handful of users, but the change I'm making in the registry key depends on the user. So, maybe I can map a username to the registry key.

If I run the following from the command line wmic useraccount get name,sid, I will see a table similar to the following

Name            SID
Administrator   S-1-5-21-1180699209-877415012-3182924384-500
Guest           S-1-5-21-1180699209-877415012-3182924384-501
Tim             S-1-5-21-1180699209-877415012-3182924384-1004

Great. Now I can script my change and run it. However - it does not work. It appears that user hives are usually only loaded for currently logged in users. That complicates things.

Fortunately, I came across the alternative solution - use Active Setup. It's original use is, likely, to check if a specific version of the software is installed to help installers to install, uninstall and repair software. It can, however, be used to write pretty much anything in the HKCU of the user who logs on. Here's how it works:

When the user logs on, the following registry key is checked: HKCU\Software\Microsoft\Active Setup\Installed Components\

If the HKCU key is not found in the registry then the contents of the string value StubPath is executed. This is essentially all that's important, so here is my example.

reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\MountDrive" /v "Version" /d "1" /t REG_SZ /f

reg add "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\MountDrive" /v "StubPath" /d "reg add HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run /v "MountDrive" /d "C:\map.cmd" /t REG_SZ /f" /f

Or, translating the reg add commands into PowerShell script

$mapcmd = "C:\map.cmd"
$regKey = "HKLM:\SOFTWARE\Microsoft\Active Setup\Installed Components\MountDrive"
New-Item -path $regKey | Out-Null
$regKey = "HKLM:\SOFTWARE\Microsoft\Active Setup\Installed Components\MountDrive"
$regName = "Version"
$value = "1"
New-ItemProperty -path $regKey -name $regName -value $value | Out-Null
$regName = "StubPath"
$value ="reg add HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run /v MountDrive /d $mapcmd /f"
New-ItemProperty -path $regKey -name $regName -value $value | Out-Null

Here's what happens when the user logs on:

  • HKCU\Software\Microsoft\Active Setup\Installed Components\MountDrive is checked. There is nothing there.
  • string value in StubPath is executed. The value is "reg add" command and it creates a MountDrive string under Run key, with a value "C:\map.cmd". Therefore, this cmd script will run on user logon.
  • Also, a Version entry is created in HKCU\Software\Microsoft\Active Setup\Installed Components\MountDrive with a value of 1.
  • Next time the user logs on, step 1 find the Version entry, thefore no actions will be performed.

Seems a little complicated, but after running once and observing the changes as they are made in the registry, it becomes clear.

References:

Update a registry key for ALL users on a system
HKEY_USERS
Security Identifier
How To Find a User's Security Identifier (SID) in Windows
Adding Registry Settings
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Saturday, July 13, 2013

A Little Strangeness in the Way A Script is Run by Registry

An observation from today.

Currently net.exe is executed when the user logs in and maps a network drive (may also use credentials based on the user, etc.). This is achieved as follows:

Scenario 1. Normal conditions

Under the registry key HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Run. Create a string value name MountDrive and data net.exe use b: /persistent:no \\localhost\mydrive

Behaviour: Command runs, drive is mapped.

Now suppose command fails occasionally. To understand why, I would like to redirect the output of net.exe into the text file. That should be easy to do.

Scenario 2. Modified Value in Registry

I modified the registry entry to

net.exe use b: /persistent:no \\localhost\mydrive >>C:\netlog.txt

Behaviour: Command runs, drive is mapped, but no output file. If I run the same command from command line, the drive is mapped and output file is created (as expected).

Next, I came up with a workaround

Scenario 3. Run a cmd file from Registry.

  • create map.cmd with the contents net.exe use b: /persistent:no \\localhost\mydrive >>C:\netlog.txt
  • place it on C: drive
  • modify the registry entry to C:\map.cmd

Behaviour: Command runs, drive is mapped, output file is created with the contents The command completed successfully. Why does it behave like that - I don't know.

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