My file a.txt contains:
delete from test_$suffix
My PowerShell is:
$a = get-content a.txt
$suffix = "tableA"
How would I manipulate $a to obtain the string delete from test_tableA ?
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3 Answers
Invoke-Expression is the equivalent.
$strExpression = "5 + 5 -eq 10"
Invoke-Expression $strExpression
True
See this blog post for more information.
5 Comments
POMATu
Thanks for that, accepted answer did not worked for me, it just echoed variable contents. Your answer works well and even shorter
Ambrose Leung
The accepted answer answers the OP's question of doing a "format string". This answers the title of the question of how to eval a string. If you just want to do "format string", the accepted answer is safer as it will not run arbitrary code (in case the input file is malicious)
mklement0
@AmbroseLeung, good contrast, but note that
$ExecutionContext.InvokeCommand.ExpandString() is not inherently safer, because arbitrary commands can be embedded in an expandable string via $(...); e.g., $ExecutionContext.InvokeCommand.ExpandString('$(Get-Date)')Ambrose Leung
@mklement0 thanks for that - you are correct. I thought that if you read $(...) from a file, powershell would escape the $, but this is not the case. Perhaps sanitizing user input (replace $ with ``$)
...ExpandString((gc myinput.txt).Replace('$','`$')) - there's probably some way to bypass this as well? Best to avoid expanding user inputmklement0
@AmbroseLeung: Given that expanding variable references in the string (e.g,
$suffix) is the very point of using $ExecutionContext.InvokeCommand.ExpandString() in the case at hand, the challenge is to selectively ignore embedded $(...) subexpressions. Doing this robustly, without getting tricked by escaping embedded in the source is somewhat tricky: $a = $a -replace '(?<!`)((?:``)*)\$\(', '$1`$$('$a=get-content a.txt
$suffix="tableA"
$ExecutionContext.InvokeCommand.ExpandString($a)
7 Comments
mjolinor
Just be aware that if $a contains a quoted string (e.g. $a = 'delete from "test_$suffix"') you're going to lose your quotes when it gets expanded.
Doug Finke
That is true. Same goes for $ signs. You can escape them with a tick `. Also, the here string technique only allows lines of recognizable PowerShell. I cannot have 'This is text' in the file (without the quotes). Enabling that is useful when doing templating type applications.
mjolinor
I don't understand this part: "Also, the here string technique only allows lines of recognizable PowerShell. I cannot have 'This is text' in the file (without the quotes). Enabling that is useful when doing templating type applications." mjolinor.wordpress.com/2011/02/02/here-string-test
Doug Finke
My apologies, the testfile.txt I tested against was saved from ISE as Unicode, I re-saved it as Ascii and it worked.
mjolinor
No problem. I've had times when I've thought I understood things better than I did, and made statements were just flat wrong. I hate when I do that, and wanted to be sure this wasn't one of those times.
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Here's one way. Variables in a double-quoted here-string get substituted automatically. Just be sure your input file conforms to the PS rules for here-strings.
function convertto-herestring {
begin {$temp_h_string = '@"' + "`n"}
process {$temp_h_string += $_ + "`n"}
end {
$temp_h_string += '"@'
iex $temp_h_string
}
}
$suffix = "tableA"
get-content testfile.txt
delete from test_$suffix
get-content testfile.txt | convertto-herestring
delete from test_tableA
