While doing some testing a few weeks ago I stumbled onto what happens beneath the hood. When you type a cmdlet name and use tab completion PowerShell begins looking for matches in its list. In a way, I think of it like AJAX drop down lists and text boxes on the web. This is really evident when you type a command (in my example output) that does not resolve. For instance, I use windbg. I have this installed, but, not in a default location. When I enter it I intentionally leave off the extension to give an exaggerated effect for both paths and extensions. The key here is using the *command* wildcard in the Trace-Command -Name parameter.
Trace-Command -Name *command* -Expression { windbg } -PSHost
You can try it out on your machine to see what happens. Hopefully, youre like me and didnt properly add windbg to your %PATH% variable. (Lazy mans grin.) Later you can see the suffering youll face if you just run this. But, before I torment you with the raw output, Ill summarize the high level details so you can see the patterns to get a little glimpse of the mechanics. Ill also include line references so you can follow along below:- Line 1: The command itself.
- Line 2: The engine looks for a command: windbg.
- Line 3: It then looks up extensions for command testing in $env:PATHEXT. We will see where these come into play later: .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;.CPL
- Line 4-11: Powershell looks up paths in the $env:path (or %path% to use old school) variable. Wrapping makes this look like more.
- Line 12-45: Whats interesting here is that you see the engine attempting to append built-in values (.ps1, psm1, .psd1) first to the command name (lines 12-17), then, when those fail, it attempts to parse the extensions from the PATHEXT collection (lines 18-45). Note that it starts with %SystemRoot%system32WindowsPowerShellv1.0 ($PSHome).
- The same pattern matching search is repeated for several directories (the values stored in the $env:PATH variable) using the same, sequential order (built-in extensions then PATHEXT values):
- Line 12-45: C:Windowssystem32WindowsPowershellv1.0
- Line 46-78: C:Windowssystem32
- Line 79-111: C:Windows
- Line 112-145: C:WindowsSystem32Wbem
- Line 146-195: C:Program Files (x86)ATITechnologiesATI.ACECore-Static
- Line 196-245: C:Program Files (x86)Common FilesRoxio SharedDLLShared
- Line 246-279: C:Program FilesSlikSvn in
- Line 280-313: C:WindowsSystem32WindowsPowerShellv1.0
- Line 314-363: C:Program Files (x86)Microsoft SQL Server100ToolsBinn
- Line 364-411: C:Program FilesMicrosoft SQL Server100ToolsBinn
- Line 412-445: C:Program FilesMicrosoft SQL Server100DTSBinn
- Line 446-493: C:Program Files (x86)Microsoft SQL Server100ToolsBinnVSShellCommon7IDE
- Line 494-543: C:Program Files (x86)Microsoft SQL Server100DTSBinn
- Line 544-593: C:Program Files (x86)Microsoft Visual Studio 9.0Common7IDEPrivateAssemblies
- Lines 594-594: The host reports lookup failure, summarizing with this "All lookup paths exhausted, no more matches can be found"
- Lines 595-597: The host decided to prepend Get- to the command and rerun steps 3-5.
- Lines 598-1211: A repeat of whats listed above with Get- put before windbg (for Get-windbg). Finally, it concedes for good and begins cleaning up and notification.
- Lines 1212-1237: The host attempts a few more things (module related) before failing and returning the common error message, "ERROR: windbg is not recognized as a cmdlet, function, operable program or script file."
PS > $PSVersionTable
Name Value
---- -----
WSManStackVersion 3.0
PSCompatibleVersions {1.0, 2.0, 3.0}
SerializationVersion 1.1.0.1
BuildVersion 6.2.8158.0
PSVersion 3.0
CLRVersion 4.0.30319.239
PSRemotingProtocolVersion 2.103
And, now, for some really, really, really long output (all 1237 lines of it). I will just put it up for effect specifically to demonstrate whats going on at the engine while you sit there staring at the screen waiting for some sign of life:
0001: PS C:Usersowner> Trace-Command -Name *command* -Expression { windbg } -PSHost
0002: DEBUG: CommandDiscovery Information: 0 : Looking up command: windbg
0003: DEBUG: CommandDiscovery Information: 0 : PATHEXT: .COM;.EXE;.BAT;.CMD;.VBS;.VBE;.JS;.JSE;.WSF;.WSH;.MSC;.CPL
0004: DEBUG: CommandDiscovery Information: 0 : PATH:
0005: %SystemRoot%system32WindowsPowerShellv1.0;C:Windowssystem32;C:Windows;C:WindowsSystem32Wbem;C:Program Files
0006: (x86)ATI TechnologiesATI.ACECore-Static;C:Program Files (x86)Common FilesRoxio SharedDLLShared;C:Program
0007: FilesSlikSvn in;C:WindowsSystem32WindowsPowerShellv1.0;C:Program Files (x86)Microsoft SQL
0008: Server100ToolsBinn;C:Program FilesMicrosoft SQL Server100ToolsBinn;C:Program FilesMicrosoft SQL
0009: Server100DTSBinn;C:Program Files (x86)Microsoft SQL Server100ToolsBinnVSShellCommon7IDE;C:Program Files
0010: (x86)Microsoft SQL Server100DTSBinn;C:Program Files (x86)Microsoft Visual Studio
0011: 9.0Common7IDEPrivateAssemblies
0012: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.ps1 in %SystemRoot%system32WindowsPowerShellv1.0
0013: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0014: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psm1 in %SystemRoot%system32WindowsPowerShellv1.0
0015: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0016: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psd1 in %SystemRoot%system32WindowsPowerShellv1.0
0017: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0018: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.COM in %SystemRoot%system32WindowsPowerShellv1.0
0019: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0020: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.EXE in %SystemRoot%system32WindowsPowerShellv1.0
0021: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0022: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.BAT in %SystemRoot%system32WindowsPowerShellv1.0
0023: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0024: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CMD in %SystemRoot%system32WindowsPowerShellv1.0
0025: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0026: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBS in %SystemRoot%system32WindowsPowerShellv1.0
0027: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0028: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBE in %SystemRoot%system32WindowsPowerShellv1.0
0029: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0030: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JS in %SystemRoot%system32WindowsPowerShellv1.0
0031: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0032: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JSE in %SystemRoot%system32WindowsPowerShellv1.0
0033: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0034: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSF in %SystemRoot%system32WindowsPowerShellv1.0
0035: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0036: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSH in %SystemRoot%system32WindowsPowerShellv1.0
0037: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0038: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.MSC in %SystemRoot%system32WindowsPowerShellv1.0
0039: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0040: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CPL in %SystemRoot%system32WindowsPowerShellv1.0
0041: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0042: DEBUG: CommandDiscovery Information: 0 : Looking for windbg in %SystemRoot%system32WindowsPowerShellv1.0
0043: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0044: DEBUG: CommandSearch Information: 0 : WriteLine Current patterns exhausted in current directory:
0045: %SystemRoot%system32WindowsPowerShellv1.0
0046: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.ps1 in C:Windowssystem32
0047: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0048: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psm1 in C:Windowssystem32
0049: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0050: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psd1 in C:Windowssystem32
0051: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0052: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.COM in C:Windowssystem32
0053: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0054: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.EXE in C:Windowssystem32
0055: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0056: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.BAT in C:Windowssystem32
0057: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0058: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CMD in C:Windowssystem32
0059: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0060: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBS in C:Windowssystem32
0061: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0062: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBE in C:Windowssystem32
0063: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0064: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JS in C:Windowssystem32
0065: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0066: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JSE in C:Windowssystem32
0067: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0068: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSF in C:Windowssystem32
0069: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0070: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSH in C:Windowssystem32
0071: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0072: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.MSC in C:Windowssystem32
0073: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0074: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CPL in C:Windowssystem32
0075: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0076: DEBUG: CommandDiscovery Information: 0 : Looking for windbg in C:Windowssystem32
0077: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0078: DEBUG: CommandSearch Information: 0 : WriteLine Current patterns exhausted in current directory: C:Windowssystem32
0079: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.ps1 in C:Windows
0080: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0081: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psm1 in C:Windows
0082: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0083: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psd1 in C:Windows
0084: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0085: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.COM in C:Windows
0086: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0087: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.EXE in C:Windows
0088: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0089: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.BAT in C:Windows
0090: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0091: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CMD in C:Windows
0092: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0093: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBS in C:Windows
0094: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0095: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBE in C:Windows
0096: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0097: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JS in C:Windows
0098: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0099: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JSE in C:Windows
0100: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0101: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSF in C:Windows
0102: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0103: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSH in C:Windows
0104: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0105: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.MSC in C:Windows
0106: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0107: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CPL in C:Windows
0108: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0109: DEBUG: CommandDiscovery Information: 0 : Looking for windbg in C:Windows
0110: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0111: DEBUG: CommandSearch Information: 0 : WriteLine Current patterns exhausted in current directory: C:Windows
0112: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.ps1 in C:WindowsSystem32Wbem
0113: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0114: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psm1 in C:WindowsSystem32Wbem
0115: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0116: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psd1 in C:WindowsSystem32Wbem
0117: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0118: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.COM in C:WindowsSystem32Wbem
0119: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0120: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.EXE in C:WindowsSystem32Wbem
0121: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0122: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.BAT in C:WindowsSystem32Wbem
0123: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0124: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CMD in C:WindowsSystem32Wbem
0125: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0126: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBS in C:WindowsSystem32Wbem
0127: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0128: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBE in C:WindowsSystem32Wbem
0129: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0130: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JS in C:WindowsSystem32Wbem
0131: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0132: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JSE in C:WindowsSystem32Wbem
0133: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0134: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSF in C:WindowsSystem32Wbem
0135: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0136: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSH in C:WindowsSystem32Wbem
0137: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0138: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.MSC in C:WindowsSystem32Wbem
0139: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0140: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CPL in C:WindowsSystem32Wbem
0141: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0142: DEBUG: CommandDiscovery Information: 0 : Looking for windbg in C:WindowsSystem32Wbem
0143: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0144: DEBUG: CommandSearch Information: 0 : WriteLine Current patterns exhausted in current directory:
0145: C:WindowsSystem32Wbem
0146: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.ps1 in C:Program Files (x86)ATI
0147: TechnologiesATI.ACECore-Static
0148: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0149: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psm1 in C:Program Files (x86)ATI
0150: TechnologiesATI.ACECore-Static
0151: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0152: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.psd1 in C:Program Files (x86)ATI
0153: TechnologiesATI.ACECore-Static
0154: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0155: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.COM in C:Program Files (x86)ATI
0156: TechnologiesATI.ACECore-Static
0157: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0158: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.EXE in C:Program Files (x86)ATI
0159: TechnologiesATI.ACECore-Static
0160: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0161: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.BAT in C:Program Files (x86)ATI
0162: TechnologiesATI.ACECore-Static
0163: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0164: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.CMD in C:Program Files (x86)ATI
0165: TechnologiesATI.ACECore-Static
0166: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0167: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBS in C:Program Files (x86)ATI
0168: TechnologiesATI.ACECore-Static
0169: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0170: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.VBE in C:Program Files (x86)ATI
0171: TechnologiesATI.ACECore-Static
0172: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0173: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JS in C:Program Files (x86)ATI
0174: TechnologiesATI.ACECore-Static
0175: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0176: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.JSE in C:Program Files (x86)ATI
0177: TechnologiesATI.ACECore-Static
0178: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0179: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSF in C:Program Files (x86)ATI
0180: TechnologiesATI.ACECore-Static
0181: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0182: DEBUG: CommandDiscovery Information: 0 : Looking for windbg.WSH in C:Program Files (x86)ATI
0183: TechnologiesATI.ACECore-Static
0184: DEBUG: CommandSearch Information: 0 : WriteLine Current directory results are invalid
0185: DEBUG: Com
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