I have been asked to review and edit, if needed, some scripts. I have entered the relevant part of this one below. I believe its correct, but I cannot figure out if the period at the beginning of the second to last line is needed or just a typo. It appears to be a source operator, but I don't see why it'd be needed there.
As always, you folks here are the salt of the earth and deserve much more plaudits than you get and than I can give. Thank you so much for continuing to making me look better at this than I am.
$Assembly = 'D:\MgaLin2.dll'
."C:\Windows\Microsoft.NET\Framework\v4.0.30319\RegAsm.exe" -codebase -tlb $Assembly
Copy-Item -Path D:\Mga -Destination "C:\Program Files (x86)\Common Files\COMPANY_NAME\COMPANY_SUBFOLDER\" -Include *.*
The use of a period (.) (the dot-sourcing operator) is unusual in this case, but it works.
More typically, you'll see use of &, the call operator in this situation.
Only for PowerShell scripts (and script blocks, { ... }) does the distinction between . and & matter (. then dot-sources the code, i.e. runs it directly in the caller's scope rather than in a child scope, as & does).
For external programs, such as RegAsm.exe, you can technically use . and & interchangeably, but to avoid conceptual confusion, it is best to stick with &.
Note that the reason that & (or .) is required in this case is that the executable path is quoted; the same would apply if the path contained a variable reference (e.g., $env:ProgramFiles) or a subexpression (e.g., $($name + $ext)).
For more information about this requirement, which is a purely syntactic one, see this answer.
Related
I recently learned that you can use special characters in PowerShell variables, e.g.: ${hello world!}, but then I stumbled across this:
${^}
What does it do? I first thought it referenced the newest created variable, but it does other stuff I haven't really figured out.
The documentation says:
Contains the first token in the last line received by the session.
Examples:
dir c:\windows 🡒 dir
0..9 🡒 0
&{ dir } 🡒 &
It was likely introduced to get the last command used (dir, copy, rm, ...), but in reality that will only work for the most simplest cases, and thus it's not very useful.
The corresponding $$ returns the last token from the last line.
Note: The curly braces {} are only necessary for variable names containing characters which are not allowed in variables, except automatic variables (look here). In this case, you can omit them:
$^
It shows the first word/token in the last executed command.
I know I can dot source a file:
. .\MyFunctions.ps1
But, I would like to dot source the commands in a string variable:
. $myFuctions
I see that this is possible:
.{$x=2}
And $x equals 2 after the script block is sourced.
But... .{$myFunctions} does not work.
I tried $myFunctions | Invoke-Expression, but it doesn't keep the source function in the current scope. The closest I have been able to come up with is to write the variable to a temporary file, dot source the file, and then remove the file.
Inevitably, someone will ask: "What are you trying to do?" So here is my use case:
I want to obfuscate some functions I intend to call from another script. I don't want to obfuscate the master script, just my additional functions. I have a user base that will need to adjust the master script to their network, directory structure and other local factors, but I don't want certain functions modified. I would also like to protect the source code. So, an alternate question would be: What are some good ways to protect PowerShell script code?
I started with the idea that PowerShell will execute a Base64-encoded string, but only when passed on the command line with -EncodedCommand.
I first wanted to dot source an encoded command, but I couldn't figure that out. I then decided that it would be "obfuscated" enough for my purposes if I converted by Base64 file into a decode string and dot sourced the value of the string variable. However, without writing the decoded source to a file, I cannot figure out how to dot source it.
It would satisfy my needs if I could Import-Module -EncodedCommand .\MyEncodedFile.dat
Actually, there is a way to achieve that and you were almost there.
First, as you already stated, the source or dot operator works either by providing a path (as string) or a script block. See also: . (source or dot operator).
So, when trying to dot-source a string variable, PowerShell thinks it is a path. But, thanks to the possibility of dot-sourcing script blocks, you could do the following:
# Make sure everything is properly escaped.
$MyFunctions = "function Test-DotSourcing { Write-Host `"Worked`" }"
. { Invoke-Expression $MyFunctions }
Test-DotSourcing
And you successfully dot-sourced your functions from a string variable!
Explanation:
With Invoke-Expression the string is evaluated and run in the child scope (script block).
Then with . the evaluated expressions are added to the current scope.
See also:
Invoke-Expression
About scopes
While #dwettstein's answer is a viable approach using Invoke-Expression to handle the fact that the function is stored as a string, there are other approaches that seem to achieve the same result below.
One thing I'm not crystal clear on is the scoping itself, Invoke-Expression doesn't create a new scope so there isn't exactly a need to dot source at that point...
#Define your function as a string
PS> $MyUselessFunction = "function Test-WriteSomething { 'It works!' }"
#Invoke-Expression would let you use the function
PS> Invoke-Expression $MyUselessFunction
PS> Test-WriteSomething
It works!
#Dot sourcing works fine if you use a script block
PS> $ScriptBlock = [ScriptBlock]::Create($MyUselessFunction)
PS> . $ScriptBlock
PS> Test-WriteSomething
It works!
#Or just create the function as a script block initially
PS> $MyUselessFunction = {function Test-WriteSomething { 'It works!' }}
PS> . $MyUselessFunction
PS> Test-WriteSomething
It works!
In other words, there are probably a myriad of ways to get something similar to what you want - some of them documented, and some of them divined from the existing documentation. If your functions are defined as strings, then Invoke-Expression might be needed, or you can convert them into script blocks and dot source them.
At this time it is not possible to dot source a string variable.
I stand corrected! . { Invoke-Expression $MyFunctions } definitely works!
I need to build a string that is actually a command-line, and then execute the contents of that command-line. I'd hoped the call operator (&) would help, but it appears not. Here is a simple contrived example. The following works as expected, it pings a website:
$command = "ping"
$website = "www.bbc.co.uk"
& $command $website
however if I change it to this:
$command = "ping"
$website = "www.bbc.co.uk"
$cmd = "$command $website"
& $cmd
I get an error:
The term 'ping www.bbc.co.uk' is not recognized as the name of a
cmdlet, function, script file, or operable program.
Is there a way to dynamically build up a command-line as a string, and then execute it?
Yes, but you need to use Invoke-Expression (which is just like eval), instead of the call operator. Note that you also need to ensure that all your quoting is correct in that case. E.g.
$cmd = "'$command' '$website'"
would work in your trivial example, unless $command or $website contained single quotes. The problem here is essentially that everything you put into the string is subject to the usual parsing rules of PowerShell.
Generally, if you can, stay as far away from Invoke-Expression as you can. There are a few problems that need it, but invoking external programs ... not so much.
A much better alternative, especially if you have an arbitrary number of arguments, is to just collect the arguments in an array and use the splat operator (note the # in the code example below):
$command = 'ping'
$arguments = '-t','www.bbc.co.uk'
&$command #arguments
This ensures that arguments are properly quoted when necessary and generally avoids a lot of headaches you're going to get with Invoke-Expression.
(Side note: Whenever you have a problem in PowerShell and think »Oh, I'm just going to use a string«, it's often time to rethink that. This includes handling file names, or command lines. PowerShell has objects, reducing it to the capabilities of earlier shells just yields the same pain you have elsewhere too, e.g. multiple levels of escaping, sometimes with different syntaxes, etc. And most of the time there are better ways of solving the problem.)
I recently started using PowerShell, and noticed that I could pass argument values using a space between the argument name and value, or using a colon, like this:
MyFunction -Parameter value
or
MyFunction -Parameter:value
I started using the colon because it differentiates the line a bit more (for readability reasons), but from what I've seen, most people do not use it.
I've read a bit also about the differences between these approaches when working with switch typed arguments, that normally do not need values. In that situation, one needs to use the colon, or else the command will not work. This is another reason why I'm leaning towards using the colon for every parameter, for consistency reasons.
Is there something else I should keep in mind?
Generally speaking, when I need to execute a function with a switch parameter set to false, I simply omit the switch. That's the design intent of a switch parameter. The only time I think I would ever use the colon in a parameter is when I need to programmatically determine the value of a switch.
For example, let's say I need to get a regular directory listing on even days, and a recursive directory listing on odd days:
Get-ChildItem -Path $Path -Recurse:$((Get-Date).Day % 2 -eq 1) | ForEach-Object {...}
Beyond that, I personally wouldn't bother with the colon unless it significantly added to the readability of a given statement. It's not a commonly used syntax, so people who read your code later are more likely to be confused by it.
In general I would leave the colon off. Only use it in the situation of setting switch a parameter (typically when you want to pass a variable to it, like -Verbose:$someVariable.
Basically, I think you should be consistent with the more accepted style, which is what I've described.
If you really want to set all parameters in an internally consistent way which allows for variables for switch parameters, and is an accepted (though less known) way of passing parameters, consider splatting like so:
$params = #{
'Param1' = $value1
'Param2' = 5
'WhatIf' = $true
}
Some-Cmdlet #params
This is a weird one. Normally when I execute an external command from powershell I use the & operator like this:
& somecommand.exe -p somearguments
However, today I came across the . operator used like this:
.$env:systemdrive\chocolatey\chocolateyinstall\chocolatey.cmd install notepadplusplus
What purpose does the period serve in this scenario? I don't get it.
The "." dot sourcing operator will send AND receive variables from other scripts you have called. The "&" call operator will ONLY send variables.
For instance, considering the following:
Script 1 (call-operator.ps1):
clear
$funny = "laughing"
$scriptpath = split-path -parent $MyInvocation.MyCommand.Definition
$filename = "laughing.ps1"
"Example 1:" # Call another script. Variables are passed only forward.
& $scriptpath\$filename
"Example 2:" # Call another script. Variables are passed backwards and forwards.
. $scriptpath\$filename
$variableDefinedInOtherScript
Script 2 (laughing.ps1):
# This is to test the passing of variables from call-operator.ps1
"I am $funny so hard. Passing variables is so hilarious."
$variableDefinedInOtherScript = "Hello World!"
Create both scripts and ONLY run the first one. You'll see that the "." dot sourcing operator sends and receives variables.
Both have their uses, so be creative. For instance, the "&" call operator would be useful if you wanted to modify the value(s) of variables in another script while preserving the original value(s) in the your current script. Kinda a safeguard. ;)
The Short:
It is a Special Operator used to achieve what regular operators cannot achieve. This particular operator . actually has two distinctively different Special Operator use cases.
The Long:
As with any other language, scripting or otherwise, PowerShell script also supports many different types of Operators to help manipulate values. These regular operators include:
Arithmetic
Assignment
Comparison
Logical
Redirection
List item
Split and Join
Type
Unary
However, PowerShell also supports whats known as Special Operators which are used to perform tasks that cannot be performed by the other types of operators.
These Special Operators Include:
#() Array subexpression operator
& Call operator
[ ] Cast operator
, Comma operator
. Dot sourcing operator
-f Format operator
[ ] Index operator
| Pipeline operator
. Property dereference operator
.. Range operator
:: Static member operator
$( ) Subexpression operator
. Dot sourcing operator: is used in this context to allow a script to run in the current scope essentially allowing any functions, aliases, and variables which has been created by the script to be added to the current script.
Example:
. c:\scripts.sample.ps1
NoteThat this application of the . Special Operator is followed by a space to distinguish it from the (.) symbol that represents the current directory
Example:
. .\sample.ps1
. Property dereference operator: Allows access to the properties and methods of of an object which follows the . by indicating that the expression on the left side of the . character is an object and the expression on the right side of the is an object member (a property or method).
Example:
$myProcess.peakWorkingSet
(get-process PowerShell).kill()
Disclaimer & Sources:
I had the same question while looking at a PowerShell script that I was trying to expand on its feature sets and landed here when doing my research for the answer. However I managed to find my answer using this magnificent write up on the Microsoft Development Network supplemented with this further expansion of the same ideas from IT Pro.
Cheers.
The dot is a call operator:
$a = "Get-ChildItem"
. $a # (executes Get-ChildItem in the current scope)
In your case, however, I don't see what it does.
.Period or .full stop for an objects properties; like
$CompSys.TotalPhysicalMemory
See here: http://www.computerperformance.co.uk/powershell/powershell_syntax.htm#Operators_
This answer is to expand slightly upon those already provided by David Brabant and his commenters. While those remarks are all true and pertinent, there is something that has been missed.
The OPs use of & when invoking external commands is unnecessary. Omitting the & would have no effect (on the example of his usage). The purpose of & is to allow the invocation of commands whose names are the values of a (string) expression. By using the & above, powershell then (essentially) treats the subsequent arguments as strings, the first of which is the command name that & duly invokes. If the & were omitted, powershell would take the first item on the line as the command to execute.
However, the . in the second example is necessary (although, as noted by others, & would work just as well in this case). Without it, the command line would begin with a variable access ($env:systemdrive) and so powershell would be expecting an expression of some form. However, immediately following the variable reference is a bare file path which is not a valid expression and will generate an error. By using the . (or &) at the beginning of the line, it is now treated as a command (because the beginning doesn't look like a valid expression) and the arguments are processed as expandable strings (" "). Thus, the command line is treated as
. "$env:systemdrive\chocolatey\chocolateyinstall\chocolatey.cmd" "install" "notepadplusplus"
The first argument has $env:systemdrive substituted into it and then . invokes the program thus named.
Note: the full description of how powershell processes command line arguments is way more complicated than that given here. This version was cut down to just the essential bits needed to answer the question. Take a look at about_Parsing for a comprehensive description. It is not complete but should cover most normal usage. There are other posts on stackoverflow and github (where powershell now resides) that cover some of the seemingly quirky behaviour not listed in the official documentation. Another useful resource is about_Operators though again this isn't quite complete. An example being the equivalence of . and & when invoking something other than a powershell script/cmdlet getting no mention at all.