With Powershell 2, the Start-Job command was added, which allows statements to be executed in the background. There are a few oddities regarding script blocks that need to be understood first though.
- Powershell doesn’t support closures
- The current working directory isn’t preserved
I confronted these issues first hand today while working with a Rails app. I wanted to execute ruby script\server in a job, but it wasn’t working.
PS C:\Users\Brian\workspace\myapp> start-job { ruby script\server }
Id Name State HasMoreData Location Command
-- ---- ----- ----------- -------- -------
3 Job3 Running True localhost ruby script\server
PS C:\Users\Brian\workspace\myapp> receive-job 3
C:\Ruby19\bin\ruby.exe: No such file or directory -- script/server (LoadError)
+ CategoryInfo : NotSpecified: (C:\Ruby19\bin\r...ver (LoadError):String) [], RemoteException
+ FullyQualifiedErrorId : NativeCommandError
For some reason, when executing the job, it is executing in a different working directory. It is pretty easy to confirm.
PS C:\Users\Brian\workspace\myapp> start-job { get-location }
Id Name State HasMoreData Location Command
-- ---- ----- ----------- -------- -------
3 Job3 Running True localhost get-location
PS C:\Users\Brian\workspace\myapp> receive-job 3
Path
----
C:\Users\Brian\Documents
Knowing that the working directory was not preserved, I figured it would be easy enough to write my own function that added this functionality. This was my first stab at it.
function start-jobhere([scriptblock]$block){ start-job -argumentlist (get-location),$block { set-location $args[0]; . $args[1] } }
But, it didn’t work.
PS C:\Users\Brian\workspace\myapp> start-jobhere { ruby script\server }
Id Name State HasMoreData Location Command
-- ---- ----- ----------- -------- -------
1 Job1 Running True localhost set-location $args[0]...
PS C:\Users\Brian\workspace\myapp> receive-job 1
The term ' ruby script\server ' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
+ CategoryInfo : ObjectNotFound: ( ruby script\server :String) [], CommandNotFoundException
+ FullyQualifiedErrorId : CommandNotFoundException
I had a sneaking suspicsion that Powershell was serializing the scriptblock to a string, which I confirmed.
PS C:\Users\Brian\workspace\myapp> $sb = { "hello world "}
PS C:\Users\Brian\workspace\myapp> start-job -ArgumentList $sb { $args[0] | gm }
Id Name State HasMoreData Location Command
-- ---- ----- ----------- -------- -------
15 Job15 Running True localhost $args[0] | gm
PS C:\Users\Brian\workspace\myapp> receive-job 15
TypeName: System.String
Name MemberType Definition
---- ---------- ----------
Clone Method System.Object Clone()
CompareTo Method int CompareTo(System.Object value), int CompareTo(string strB)
Contains Method bool Contains(string value)
CopyTo Method System.Void CopyTo(int sourceIndex, char[] destination, int destinationIndex,...
EndsWith Method bool EndsWith(string value), bool EndsWith(string value, System.StringCompari...
Equals Method bool Equals(System.Object obj), bool Equals(string value), bool Equals(string...
GetEnumerator Method System.CharEnumerator GetEnumerator()
GetHashCode Method int GetHashCode()
GetType Method type GetType()
GetTypeCode Method System.TypeCode GetTypeCode()
IndexOf Method int IndexOf(char value), int IndexOf(char value, int startIndex), int IndexOf...
IndexOfAny Method int IndexOfAny(char[] anyOf), int IndexOfAny(char[] anyOf, int startIndex), i...
Insert Method string Insert(int startIndex, string value)
IsNormalized Method bool IsNormalized(), bool IsNormalized(System.Text.NormalizationForm normaliz...
LastIndexOf Method int LastIndexOf(char value), int LastIndexOf(char value, int startIndex), int...
LastIndexOfAny Method int LastIndexOfAny(char[] anyOf), int LastIndexOfAny(char[] anyOf, int startI...
Normalize Method string Normalize(), string Normalize(System.Text.NormalizationForm normalizat...
PadLeft Method string PadLeft(int totalWidth), string PadLeft(int totalWidth, char paddingChar)
PadRight Method string PadRight(int totalWidth), string PadRight(int totalWidth, char padding...
Remove Method string Remove(int startIndex, int count), string Remove(int startIndex)
Replace Method string Replace(char oldChar, char newChar), string Replace(string oldValue, s...
Split Method string[] Split(Params char[] separator), string[] Split(char[] separator, int...
StartsWith Method bool StartsWith(string value), bool StartsWith(string value, System.StringCom...
Substring Method string Substring(int startIndex), string Substring(int startIndex, int length)
ToCharArray Method char[] ToCharArray(), char[] ToCharArray(int startIndex, int length)
ToLower Method string ToLower(), string ToLower(System.Globalization.CultureInfo culture)
ToLowerInvariant Method string ToLowerInvariant()
ToString Method string ToString(), string ToString(System.IFormatProvider provider)
ToUpper Method string ToUpper(), string ToUpper(System.Globalization.CultureInfo culture)
ToUpperInvariant Method string ToUpperInvariant()
Trim Method string Trim(Params char[] trimChars), string Trim()
TrimEnd Method string TrimEnd(Params char[] trimChars)
TrimStart Method string TrimStart(Params char[] trimChars)
Chars ParameterizedProperty char Chars(int index) {get;}
Length Property System.Int32 Length {get;}
So, instead of invoking the scriptblock like your normally would, you need to call Invoke-Expression instead. My function ended up looking like the this.
function start-jobhere([scriptblock]$block){ start-job -argumentlist (get-location),$block { set-location $args[0]; invoke-expression $args[1] } }
Now it works like a charm!
