Understanding Bitmap Values

The D.ps1 script described in "Emulating the Dir Command in PowerShell" relies on bitmap values for passing information between functions. A bitmap value is a concise way of storing a list of Boolean (i.e., true/false) values. You can think of a bitmap value as an array of bits rather than a discrete number. Each bit in the number is set to either 1 (enabled) or 0 (disabled). You can combine a bitmap value with a mask by using bitwise operators (in Windows PowerShell, these are -band, -bor, -bnot, and -bxor) so that you can read, set, clear, and toggle bits in the bitmap value. A mask is a number that represents the bit or bits you're interested in. Mask values are always powers of two.

File attributes are a good example of how the OS uses bitmap values. The following PowerShell command lists the .NET System.IO.FileAttributes enum and the associated mask value for each attribute:

[Enum]::GetValues([System.IO.FileAttributes]) |  select-object @{"Name" = "Name"; "Expression" = {$_}},  @{"Name" = "Value"; "Expression" = {[Int] $_}} |  format-table -auto 

Figure A shows this command and its output.

From Figure A, we can see that a mask of 1 represents the read-only attribute, 2 (10 in binary) represents the hidden attribute, 4 (100 in binary) represents the system attribute, 16 (1000 in binary) represents the directory attribute, and so forth. You can combine multiple values with the -bor operator to create a multibit mask. For example, the mask 2 -bor 32 combines the hidden (2) and archive (32) attributes.

You can get a file's attributes by reading its Attributes property, which is really the .NET type System.IO.FileAttributes. If you cast the property as an integer by using [Int], you get the Attributes property's bitmap value. Consider the following example:

PS > $attrs = (get-item c:test.txt).AttributesPS > $attrsReadOnly, ArchivePS > [Int] $attrs33 

The first command retrieves c:test.txt's attributes; the second command displays the $attrs variable, which PowerShell outputs as a list of attributes; and the third command displays the $attrs variable as an integer (i.e., its corresponding bitmap value).

PowerShell automatically converts the .NET values to integers in comparisons, which lets you use code such as the following:

PS > [System.IO.FileAttributes]::ReadOnly -eq 1TruePS > [System.IO.FileAttributes]::Hidden -eq 2True 

The following formulas describe how to set and change the bits in a bitmap value:

Using the $attrs variable from the previous example, we can use code such as the following:

($attrs -band 1) -ne 0 # True because the read-only bit is set($attrs -band 2) -ne 0 # False because the hidden bit is not set($attrs -band 32) -ne 0 # True because the archive attribute is set($attrs -band (1 -bor 32)) -eq $attrs # True because both the  read-only and archive bits are set 

The D.ps1 script described in "Emulating the Dir Command in PowerShell" uses bitmap values and masks in its get-attributeflags and main functions, which you can see in the listing with that article.