PowerShell formatting for a string

Question

I have a string that I want to insert dynamically a variable. Example;

$tag = '{"number" = "5", "application" = "test","color" = "blue", "class" = "Java"}'

I want to accomplish:

$mynumber= 2
$tag = '{"number" = "$($mynumber)", "application" = "test","color" = "blue", "class" = "Java"}'

I want to have the variable inserted on the string, but it is not going through. I guess the '' sets all as a string. Any recommendations on how should I approach this?

PowerShell test and trial and error. Also Google.

Answer 1

The reason your current attempt doesn't work is that single-quoted (') string literals in PowerShell are verbatim strings - no attempt will be made at expanding subexpression pipelines or variable expressions.

If you want an expandable string literal without having to escape all the double-quotes (") contained in the string itself, use a here-string:

$mynumber = 2

$tag = @"
{"number" = "$($mynumber)", "application" = "test","color" = "blue", "class" = "Java"}
"@

Answer 2

To add to Mathias' helpful answer:

• Mistakenly expecting string interpolation inside '...' strings (as opposed to inside "...") has come up many times before, and questions such as yours are often closed as a duplicate of this post.

• However, your question is worth answering separately, because:

  • Your use case introduces a follow-up problem, namely that embedded " characters cannot be used as-is inside "...".

  • More generally, the linked post is in the context of argument-passing, where additional rules apply.

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Note: Some links below are to the relevant sections of the conceptual about_Quoting_Rules help topic.

In PowerShell:

• only "..." strings (double-quoted, called expandable strings) perform string interpolation, i.e. expansion of variable values (e.g. "... $var" and subexpressions (e.g., "... $($var.Prop)")

• not '...' strings (single-quoted, called verbatim strings), whose values are used verbatim (literally).

With "...", if the string value itself contains " chars.:

• either escape them as `" or ""

  • E.g., with `"; note that while use of $(...), the subexpression operator never hurts (e.g. $($mynumber)), it isn't necessary with stand-alone variable references such as $mynumber:

    $mynumber= 2
    $tag = "{`"number`" = `"$mynumber`", `"application`" = `"test`",`"color`" = `"blue`", `"class`" = `"Java`"}"
    

  • Similarly, if you want to selectively suppress string interpolation, escape $ as `$

    # Note the ` before the first $mynumber.
    # -> '$mynumber = 2'
    $mynumber = 2; "`$mynumber` = $mynumber"
    

  • See the conceptual about_Special_Characters help topic for info on escaping and escape sequences.

  • If you need to embed ' inside '...', use '', or use a (single-quoted) here-string (see next).

• or use a double-quoted here-string instead (@"<newline>...<newline>"@):

  • See Mathias' answer, but generally note the strict, multiline syntax of here-strings:
    • Nothing (except whitespace) must follow the opening delimiter on the same line (@" / @')
    • The closing delimiter ("@ / '@) must be at the very start of the line - not even whitespace may come before it.

Related answers:

• Overview of PowerShell's expandable strings

• Overview of all forms of string literals in PowerShell

• When passing strings as command arguments, they are situationally implicitly treated like expandable strings (i.e. as if they were "..."-enclosed); e.g.
  Write-Output $HOME\projects - see this answer.

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Alternatives to string interpolation:

Situationally, other approaches to constructing a string dynamically can be useful:

• Use a (verbatim) template string with placeholders, with -f, the format operator:

  $mynumber= 2
  # {0} is the placeholder for the first RHS operand ({1} for the 2nd, ...)
  '"number" = "{0}", ...' -f $mynumber # -> "number" = "2", ...
  

• Use simple string concatenation with the + operator:

  $mynumber= 2
  '"number" = "' + $mynumber + '", ...' # -> "number" = "2", ...