Emulating the Dir Command in PowerShell

Using Windows PowerShell is a paradigm-shifting experience for Windows users who are accustomed to the Cmd.exe command shell. PowerShell is much more powerful and flexible, but most Cmd.exe commands don't have direct PowerShell equivalents. For example, PowerShell has a default dir alias that runs the Get-ChildItem cmdlet, but Get-ChildItem doesn't behave exactly the same as Cmd.exe's dir command.

Because dir is probably the command I use most frequently in Cmd.exe, I found myself missing some of dir's features when working in PowerShell—in particular, its /a (select attributes) and /o (sort order) parameters. Table 1 shows some example Cmd.exe dir commands and their PowerShell equivalents. As you can see, the PowerShell commands are all longer—and in many cases more complex—than the equivalent dir commands in Cmd.exe. To improve my productivity at the PowerShell command line, I wrote a script, D.ps1, which emulates a number of dir's most useful features.

Introducing D.ps1
You can download D.ps1 by clicking the Download the Code Here button at the top of this page. Table 2 describes D.ps1's command-line parameters. The main difference between using D.ps1 and using dir in Cmd.exe is that you use a dash (-) instead of a forward slash (/) for the parameters. All of the script's parameters are optional. If you run the script without parameters, it lists the contents of the current directory.

D.ps1 counts the number of files and directories, totals the lengths of the files, and reports them at the end of its output, as dir does. Figure 1 shows an example of D.ps1's output that displays the JScript script files in the current directory, sorted by date. Note that D.ps1's output doesn't include the displayed directory's path as Get-ChildItem and the dir command do. D.ps1 also displays each file's attributes, which Get-ChildItem does but dir doesn't do. Table 3 shows some sample D.ps1 commands and a description of each command.

By default, D.ps1 outputs formatted strings rather than file system objects. If you want to output objects or you want to list items from a location other than the file system, you must specify the -defaultoutput parameter, as noted in Table 2. If the current path isn't in the file system (e.g., HKCU:\) and you don't specify a path to list, D.ps1 outputs an error unless you use -defaultoutput.

The script is composed of a param statement, which defines the script's command-line parameters, and six functions: usage, iif, get-attributeflags, get-orderlist, get-providername, and main. The last line of the script executes the main function, which calls the other functions as needed.

The usage and iif Functions
If the -help parameter is present on the command line, the main function executes the usage function, which callout A in Listing 1 shows. The usage function simply outputs a usage message and ends the script with the exit statement.

The iif function provides a shortcut for the following frequently used syntax:

if (condition) {
  $variable = truevalue
} else {
  $variable = falsevalue
} 

By using the iif function, you can write this instead:

$variable = iif { condition } { truevalue } { falsevalue } 

Callout B in the listing shows the iif function. The function uses three script blocks as parameters. It executes the first script block ($expr); if the result is true, the script executes the second script block; otherwise, it executes the third script block.

The get-attributeflags Function
The main function uses the get-attributeflags function to convert the -attributes parameter's argument (a string containing a list of file attributes to include or exclude) into two bitmap values. If you aren't familiar with bitmap values, see the sidebar "Understanding Bitmap Values."

The get-attributeflags function, which you can see at callout C, first creates a hash table that contains the attribute characters and their associated .NET bit mask values. It then builds a string based on the hash table's keys, iterates each character in the attribute string, and uses the switch statement to decide whether to enable or disable bits in the returned values. If the attribute character isn't a dash or a valid attribute character, the function throws an error, ending the script. The last line of the get-attributeflags function returns the two bitmap values to the main function; these values are used later in the script.