Use a variable in PowerShell to pass multiple arguments to an external program

Question

I downloaded the npm package for merge junit reports - https://www.npmjs.com/package/junit-merge.

The problem is that I have multiple files to merge and I am trying to use string variable to hold file names to merge.

When I write the script myslef like:

junit-merge a.xml b.xml c.xml 

This works, the merged file is being created, but when I do it like

$command = "a.xml b.xml c.xml"
junit-merge $command

This does not work. The error is

Error: File not found

Has anyone faced similar issues?

• notjustme: Try with $command = "a.xml", "b.xml", "c.xml" instead.

Answer 1

# !! WRONG

$command = "a.xml b.xml c.xml"; junit-merge $command

results in command line junit-merge "a.xml b.xml c.xml"^[1], i.e. it passes a string with verbatim value a.xml b.xml c.xml as a single argument to junit-merge, which is not the intent.

PowerShell does not act like POSIX-like shells such as bash do in this regard: In bash, the value of variable $command - due to being referenced unquoted - would be subject to word splitting (one of the so-called shell expansions) and would indeed result in 3 distinct arguments (though even there an array-based invocation would be preferable).
PowerShell supports no bash-like shell expansions^[2]; it has different, generally more flexible constructs, such as the splatting technique discussed below.

Instead, define your arguments as individual elements of an array , as justnotme advises:

# Define the *array* of *individual* arguments.
$command = @('a.xml', 'b.xml', 'c.xml')
# Pass the array to junit-merge, which causes PowerShell
# to pass its elements as *individual arguments*; it is the equivalent of:
# junit-merge a.xml b.xml c.xml
junit-merge $command # or: junit-merge @command (see further below)

This is an application of a PowerShell technique called splatting , where you specify arguments to pass to a command via a variable:

• Either (typically only used for external programs, as in your case):

  • As an array of arguments to pass individually as positional arguments, as shown above. In this case, even an array literal would work.

• Or (more typically when calling PowerShell commands):

  • As a hashtable to pass named parameter values, in which case you must replace the $ sigil in the variable reference with @; e.g., in your case @command; e.g., the following is the equivalent of calling Get-ChildItem C:\ -Directory:

  • $paramVals = @{ LiteralPath = 'C:\'; Directory = $true }; Get-ChildItem @paramVals

---

Caveat re array-based splatting:

Due to a bug detailed in GitHub issue #6280, PowerShell doesn't pass empty arguments through to external programs, in all Windows PowerShell versions and PowerShell (Core) 7 versions up to 7.2.x; the problem is fixed in v7.3+, with selective exceptions on Windows, in conjunction with fixing how arguments with embedded " are passed - see the $PSNativeCommandArgumentPassing preference variable.

E.g., up to PowerShell v7.2.x, foo.exe "" unexpectedly results in just foo.exe being called, without an argument.

This problem equally affects array-based splatting, so that
$cmdArgs = "", "other"; foo.exe $cmdArgs results in foo.exe other rather than the expected foo.exe "" other.

The workaround (which also applies in v7.3+ if $PSNativeCommandArgumentPassing = 'Legacy' is set) is to use '""' (sic).

---

Optional use of @ in array-based splatting with external programs:

As noted, with external programs use of @ in lieu of $ isn't necessary, because passing an array implicitly results in splatting. (By contrast, when calling a PowerShell command to which you want to pass the elements of an array as individual, positional arguments, you must use @)

However, you may choose to use @ with external programs too, and arguably it conveys the splatting intent more clearly:

junit-merge @command

There is a subtle behavioral distinction, however - though it will probably rarely, if ever, surface in practice:

The safer choice is to use $, because it guards against (the however hypothetical) accidental misinterpretation of a array element containing --% that you intend to be passed on as-is.

Only the @ syntax recognizes an array element with verbatim value --% as the special stop-parsing token, --%

Said token tells PowerShell not to parse the remaining arguments as it normally would and instead pass them through as-is - unexpanded, except for expanding cmd.exe-style variable references such as %USERNAME%.

This is normally only useful when not using splatting, typically in the context of being able to use command lines that were written for cmd.exe from PowerShell as-is, without having to account for PowerShell's syntactical differences.

In the context of splatting, however, the behavior resulting from --% is non-obvious and best avoided:

• As in direct argument passing, the --% is removed from the resulting command line.

• Argument boundaries are lost, so that a single array element foo bar, which normally gets placed as "foo bar" on the command line, is placed as foo bar, i.e. effectively as 2 arguments.

---

^{[1] Your call implies the intent to pass the value of variable $command as a single argument, so when PowerShell builds the command line behind the scenes, it double-quotes the verbatim a.xml b.xml c.xml string contained in $command to ensure that. Note that these double quotes are unrelated to how you originally assigned a value to $command. Unfortunately, this automatic quoting is broken for values with embedded " chars. - see this answer, for instance.}

^{[2] As a nod to POSIX-like shells, PowerShell does perform one kind of shell expansion, but (a) only on Unix-like platforms (macOS, Linux) and (b) only when calling external programs: Unquoted wildcard patterns such as *.txt are indeed expanded to their matching filenames when you call an external program (e.g., /bin/echo *.txt), which is feature PowerShell calls native globbing.}

Answer 2

I had a similar problem. This technique from powershell worked for me:

Invoke-Expression "junit-merge $command"

I also tried the following (from a powershell script) and it works:

cmd / c "junit-merge $command"

• mklement0: Invoke-Expression (iex) should generally be avoided; definitely don't use it to invoke an external program or PowerShell script.
• mklement0: In the case at hand, defining $command as an array - $command = "a.xml", "b.xml", "c.xml" - is simpler and safer.