Powershell call cmd.exe command like copy /b - powershell

Question

I saw this already Fast and simple binary concatenate files in Powershell
I'm not interested by the answer above I'm interested about what's wrong with syntax below :
when I call a cmd.exe command like copy /b:
function join-file {
copy /b $($args[0])+$($args[1]) $($args[2])
}
I get an error Copy-Item : A positional parameter cannot be found

Answer 1

As the error alludes to, copy is actually just an alias for Copy-Item and it does not have a /b parameter. You can call cmd to use its copy command.
function join-file {
cmd /c copy /b $($args[0])+$($args[1]) $($args[2])
}

Answer 2

Note: This answer complements Doug Maurer's helpful answer, which provides an effective solution (for file names without spaces).
There's a subtlety in how PowerShell parses unquoted compound tokens such as $($args[0])+$($args[1]) (by compound token I mean directly concatenated distinct syntax constructs):
$($args[0])+$($args[1]) results in two arguments[1] - although with the specific command at hand (cmd.exe's internal copy command) that happens not to be a problem:
Argument 1: The value of $($args[0])
Argument 2: A verbatim + directly followed by the value of $($args[1])
To avoid this problem, enclose the whole compound token in "...", so as to predictably treat it as an expandable string.
The upshot:
To be safe, use double-quoting ("...") explicitly to enclose compound tokens that involve variable references or subexpressions.
By contrast, to reference a variable or even method call in isolation, neither quoting nor enclosing in $(...), the subexpression operator, are needed.
Applied naively to your command (see the better solution below):
# Note: See better solution below.
function join-file {
# Note the "..." around the first argument, and the absence of quoting
# and $(...) around the second.
cmd /c copy /b "$($args[0])+$($args[1])" $args[2]
}
However, if $args[0] or $($args[1]) contained spaces, the copy command would malfunction; it is therefore more robust to pass the file names and the + as separate arguments, which copy also supports:
function join-file {
# Pass the arguments individually, which obviates the need for quoting
# and $(...) altogether:
cmd /c copy /b $args[0] + $args[1] $args[2]
}
[1] You can verify this as follows: $arr='foo', 'bar'; cmd /c echo $($arr[0])+$($arr[1]), which yields: foo +bar (note the space).