Unable to edit with bcdedit filelds in powershell

问题:

Why can't I edit the description field using bcdedit in PowerShell?

For example, in cmd.exe the following command:

bcdedit /set {GUID} description "OS2"

completes successfully, changing the description field for the specified GUID, but when I do the same thing from Powershell, I get the following error:

The set command specified is not valid.
Run "bcdedit /?" for command line assistance.
The parameter is incorrect.

Can someone explain this to me?

回答1:

To pass a value enclosed in {...} as a literal (as-is) in PowerShell, you must quote it; e.g.:

bcdedit /set "{340E0E1A-01EC-4A33-A850-8D6A09FD4CE9}" description "OS2"

{ and }, unlike in cmd.exe, have special meaning in PowerShell when used unquoted (they enclose a script block), which in this case happens to result in { and } simply getting removed.
Quoting prevents that.

A generic alternative, available since PSv3, is to use the so-called stop-parsing symbol, --%, which passes all remaining arguments as-is, without interpretation by PowerShell (with the exception of expanding %...%-enclosed environment-variable references):

bcdedit --% /set {340E0E1A-01EC-4A33-A850-8D6A09FD4CE9} description "OS2"

---

Optional Background Information

Unless interpolation of PowerShell variables and expression is needed, --% allows reuse of cmd.exe command lines as-is, without having to worry about PowerShell's quoting (escaping) requirements.

Generally, PowerShell's metacharacters (characters that have special meaning when unquoted) are different from cmd.exe's and much more numerous:

In addition to cmd.exe's metachars.,

& | < >

PowerShell has:

( ) , { } ; @ # 

^{<, >, @ and # only have special meaning at the start of a token.
< and &, as of PSv5.1, are reserved for future use.}

Aside from that, with respect to variable expansion (interpolation):

• cmd.exe only expands %...%-enclosed variable names (e.g., %PATH%), whereas PowerShell requires $-prefixed variable names (e.g., $env:PATH or $HOME) or $(...)-enclosed expressions (subexpression operator)

  • In both interpreters, variable expansion (and, in PowerShell, also subexpression expansion) is also performed inside "..." (double-quoted strings).

• '...' (single-quoted strings) are literal strings in PowerShell (contents is used as-is, without interpolation), whereas ' has no special meaning to cmd.exe at all.

To treat metacharacters as literals, you have two options:

• Enclose them in quoted strings:

  • Both cmd.exe and PowerShell: enclose them in "..." (but potentially with interpolation of any variable references / subexpressions also enclosed in the string); e.g., "|".
  • PowerShell only: enclose them in '...'; e.g., '|'

• Escape them individually:

  • PowerShell: `-escape them (backtick); e.g., `|
    • This also works inside "...", although there it is only needed to escape $ so as to prevent variable / subexpression expansion.
  • cmd.exe: ^-escape them (caret); e.g., ^|
    • This only works outside of "...", and sadly, doesn't work for escaping % to suppress variable expansion - see this answer of mine for the full story.