How to concatenate strings from a text file and execute them in Powershell

Question

I have a text file which a list of JSON files.

Using Powershell, I am able to extract these out of the file and then print the filenames out.

However I want to write a command then execute that in a foreach loop, but the concatenation is proving tricky.

This is my code so far:

foreach($line in [System.IO.File]::ReadLines("C:\Temp\DbAdmin\ImportDERReturn\ImportDERReturn.txt"))
{
  Write-Host 'aws s3 rm  s3://Derwent-storage-internal/staging/returns/' + $line.Substring(0,16) + '  --profile DER'
}

However im getting the following output:

aws s3 rm s3://Derwent-storage-internal/staging/returns/ + RET02022203.json +   --profile DER

But what I want is the following:

aws s3 rm  s3://Derwent-storage-internal/staging/returns/RET02022203.json --profile DER

How do I get this output? How do I get my Powershell script to execute this line?

Answer 1

• Don't use Write-Host to output data; it is meant for to-display output, and bypasses PowerShell's success output stream (see this answer for more information).

• To output data, you don't need an explicit output command at all, so the following by itself is enough:

  'aws s3 rm s3://Derwent-storage-internal/staging/returns/' + $line.Substring(0,16) + ' --profile DER'
  

  • If you do want to use an explicit output command to emit data (which is rarely necessary), use Write-Output or its built-in alias, echo; in that case you need to enclose your string-concatenation expression (using the + operator), in (...), the grouping operator:

    # Note the use of (...) around the concatenation expression.
    echo ('aws s3 rm s3://Derwent-storage-internal/staging/returns/' + $line.Substring(0,16) + ' --profile DER')
    

• As for executing your lines: just invoke aws directly and use either the (...) technique or, more simply string interpolation via a expandable (double-quoted) string ("...") (line breaks with ` for line continuations are purely for readability):

  aws s3 rm `
   "s3://Derwent-storage-internal/staging/returns/$($line.Substring(0,16))" `
   --profile DER
  

  • If you first want to echo the command, use a script block (note that here ` is used to escape the embedded " inside "..."):

    $sb = [scriptblock]::Create("s3 rm `"s3://Derwent-storage-internal/staging/returns/$($line.Substring(0,16))`" --profile DER")
    # Echo
    $sb.ToString()
    # Execute, using &, the call operator.
    & $sb
    

  • Invoke-Expression (whose built-in alias is iex), offers a slightly simpler alternative, though note that Invoke-Expression should generally be avoided, due to potential security risks:

     $str = "s3 rm `"s3://Derwent-storage-internal/staging/returns/$($line.Substring(0,16))`" --profile DER"
     # Echo
     $str
     # Execute, using iex (Invoke-Expression)
     iex $str