Does Powershell string interpolation support format specifiers

Question

Typical string formatting in powershell for instance to use padding or specifying number can be written like this:

>>> "x={0,5} and y={1:F3}" -f $x, $y
x=   10 and y=0.333

But in Powershell you can also use string interpolation like

>>> $x=10
>>> $y=1/3
>>> "x=$x and y=$y"
x=10 and y=0.333333333333333

And in C# string interpolation also supports the formatting specifiers:

> var x = 10;
> var y = 1.0/3.0;
> $"x={x,5} and y = {y:F2}";
"x=   10 and y = 0.33"

Is there a way to have that in Powershell? I've tried many combinations like

>>> "var=$($var, 10)"
var=10 10

but none of them work. Is this supported? Or is there a succinct way to call into C# to use it?

update as Mathias answers and as confirmed on Powershell's github this is currently not supported, so I made a feature request here

I assume you mean "var={0,10}" -f $var like in my second code example. Well, because I like inline string interpolation, am used to the syntax from other languages, it's somewhat shorter and I find it more readable (usually). — stijn
– stijn, Commented Apr 4, 2020 at 11:05
Not clear what you are asking here.. Do you mean to left-pad the variable $var? In that case, you could use the String method PadLeft, something like "var=$($var.ToString().PadLeft(5))" — Theo
– Theo, Commented Apr 4, 2020 at 11:41
@Theo yes, padding, formatting numbers and dates etc, the usual C# things. See edit for clarification. Something like what you propse works of course, but it's not very succinct. — stijn
– stijn, Commented Apr 4, 2020 at 12:49
@stijn - from what i can tell ... NO. the only way to use string composition is to use the -f string format operator. there is [apparently] no other way in powershell to do that stuff. — Lee_Dailey
– Lee_Dailey, Commented Apr 4, 2020 at 13:42

Mathias R. Jessen · Accepted Answer · 2020-04-04 15:29:09Z

7

Is this supported?

No, formatting is not supported during string expansion

As you might have noticed, string expansion in PowerShell works by naively resolving subexpressions nested in double-quoted strings - there are no {} placeholder constructs.

If you want string formatting, -f is the way to go.

FWIW, $s -f $a is directly translated to a String.Format($s, $a) call

For value types that support string formatting you can usually also call ToString() with a format string (just like in C#):

PS C:\> $a = 1 / 3 
PS C:\> $a.ToString("F2")
0.33

answered Apr 4, 2020 at 15:29

Mathias R. Jessen

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Comments

Martin Brown · Accepted Answer · 2023-08-15 09:05:37Z

The way to go with this is to use the -f operator. The -f operator takes a format string as it's first operand and the items you want to format as the second. The format string must contain tokens like {n:f} where n is the index of the item in the items to be formatted and f is the format specifier. For example {0:D4} will take the first item and format it as a Decimal that is a minimum of 4 digits. So '{0:D4}-{1:D3}' -f 24,3 gives the output 0024-003. ref

Inside an expandable string this can be used within a subexpression operator $().

A real world example is to split an input file into multiple files with ten lines in each while making the the output file name contain a serial number wich is padded with zeros to make it four digits. This can be done like this:

$i = 0; Get-Content input.txt -ReadCount 10 | %{ $i++; $_ | Out-File "output_$('{0:D4}' -f $i).txt" }

Creates files

output_0001.txt
output_0002.txt
...

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No, formatting is not supported during string expansion

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