Piping Text To An External Program Appends A Trailing Newline

tl;dr:

When PowerShell pipes a string to an external program:

• It encodes it using the character encoding stored in the $OutputEncoding preference variable
• It invariably appends a trailing (platform-appropriate) newline.

Therefore, the key is to avoid PowerShell's pipeline in favor of the native shell's, so as to prevent implicit addition of a trailing newline:

• If you're running your command on a Unix-like platform (using PowerShell Core):

sh -c "printf %s 'string' | openssl dgst -sha256 -hmac authcode"

^{printf %s is the portable alternative to echo -n. If the string contains ' chars., double them or use `"...`" quoting instead.}

• In case you need to do this on Windows via cmd.exe, things get even trickier, because cmd.exe doesn't directly support echoing without a trailing newline:

cmd /c "<NUL set /p =`"string`"| openssl dgst -sha256 -hmac authcode"

^{Note that there must be no space before | for this to work. For an explanation and the limitations of this solution, see this answer.}

Encoding issues would only arise if the string contained non-ASCII characters and you're running in Windows PowerShell; in that event, first set $OutputEncoding to the encoding that the target utility expects, typically UTF-8: $OutputEncoding = [Text.Utf8Encoding]::new()

• PowerShell, as of Windows PowerShell v5.1 / PowerShell (Core) v7.2, invariably appends a trailing newline when you send a string without one via the pipeline to an external utility, which is the reason for the difference you're observing (that trailing newline will be a LF only on Unix platforms, and a CRLF sequence on Windows).

  • You can keep track of efforts to address this problem in GitHub issue #5974, opened by the OP.

• Additionally, PowerShell's pipeline is invariably text-based when it comes to piping data to external programs; the internally UTF-16LE-based PowerShell (.NET) strings are transcoded based on the encoding stored in the automatic $OutputEncoding variable, which defaults to ASCII-only encoding in Windows PowerShell, and to UTF-8 encoding in PowerShell Core (both on Windows and on Unix-like platforms).

  • In PowerShell Core, a change is being discussed for piping raw byte streams between external programs.

• The fact that echo -n in PowerShell does not produce a string without a trailing newline is therefore incidental to your problem; for the sake of completeness, here's an explanation:

  • echo is an alias for PowerShell's Write-Output cmdlet, which - in the context of piping to external programs - writes text to the standard input of the program in the next pipeline segment (similar to Bash / cmd.exe's echo).
  • -n is interpreted as an (unambiguous) abbreviation for Write-Output's -NoEnumerate switch.
  • -NoEnumerate only applies when writing multiple objects, so it has no effect here.
  • Therefore, in short: in PowerShell, echo -n "string" is the same as Write-Output -NoEnumerate "string", which - because only a single string is output - is the same as Write-Output "string", which, in turn, is the same as just using "string", relying on PowerShell's implicit output behavior.
  • Write-Output has no option to suppress a trailing newline, and even if it did, using a pipeline to pipe to an external program would add it back in.

Linux terminals and PowerShell use different encodings. So real bytes produced by echo -n "string" are different. I tried it on my Linux Mint terminal and Windows 10 PowerShell. Here what I got:

Linux Mint:

73 74 72 69 6E 67

Windows 10:

FF FE 73 00 74 00 72 00 69 00 6E 00 67 00 0D 00 0A 00

It seems that Linux terminals use UTF-8 and Windows PowerShell uses UTF-16 with a BOM. Also in PowerShell you cannot use '-n' parameter for echo. So echo places newline characters \r\n (0D 00 0A 00) at the end of the "string".

Edit: As mklement0 said below, Windows PowerShell uses ASCII by default when piping.