将 get-process 用于未运行的进程时，如何防止出现错误消息？

Question

When I execute

get-process noSuchProcess

I get the error message

get-process : Cannot find a process with the name "noSuchProcess". Verify the process name and call the cmdlet again.
At line:1 char:5
+     get-process noSuchProcess
+     ~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : ObjectNotFound: (noSuchProcess:String) [Get-Process], ProcessCommandException
    + FullyQualifiedErrorId : NoProcessFoundForGivenName,Microsoft.PowerShell.Commands.GetProcessCommand

Is it possible to prevent such an error message if a process does not exist?

Answer 1

Running:

get-process noSuchProcess 2>$null

will redirect stderr to null, so the error will not be printed. See this detailed answer.

Answer 2

To complement Yosev Arbiv's helpful answer with PowerShell-idiomatic alternatives, using the
-ErrorAction ( -ea ) common parameter :

# Ignores the error altogether.
# Note the use of the -ea alias for -ErrorAction
Get-Process noSuchProcess -ea Ignore

# Silences the error, but still records it in $Error (see next section)
Get-Process noSuchProcess -ea SilentlyContinue

# If you call an *external program*, you must use 2>$null to silence
# its stderr output.
whoami badarg 2>$null

Note:

• -ErrorAction (and -ErrorVariable , see below) can only be used with PowerShell cmdlets and advanced functions or scripts, not with external programs.

• To silence stderr output from external programs, use 2>$null .

  • 2>$null is virtually equivalent to -ErrorAction SilentlyContinue , except for the situationally different behavior introduced by the bug reported in this GitHub issue .

• Both -ErrorAction and 2> by default only act on non-terminating errors (the typical kind), not terminating ones, though, confusingly, the seemingly equivalent $ErrorActionPreference preference variable acts on all errors.

  • For a comprehensive overview of PowerShell's error handling and its pitfalls, see this GitHub docs issue .

---

Background information:

^{Note: The term cmdlets below is used to refer not just to true (compiled) cmdlets, but also to their (written-in-PowerShell) counterparts, advanced functions and scripts .}

• PowerShell's analog to stderr (the standard error stream) is the error [output] stream , which is one of 6 available streams, documented in about_Redirection .

• By default, all errors that occur in a session are recorded in the automatic $Error variable , in reverse chronological order (most recent one first) - whether they are printed to the console at the time they occur or not.

  • As an alternative to examining $Error , the common -ErrorVariable ( -ev ) parameter allows you to capture a command's errors in a variable; eg:

     # Silence any errors, but capture them in variable $err Get-Process noSuchProcess -ea SilentlyContinue -ev err

  • There is no equivalent mechanism when you call external programs, though one has been proposed in this GitHub feature request .

• Using -ErrorAction in a cmdlet call:

  • -ErrorAction SilentlyContinue silences (doesn't print to the console) any error output, but still records it in $Error .

  • -ErrorAction Ignore both silences and suppresses addition to $Error

• Stderr output from external programs is not recorded in $Error by default (stderr output cannot generally assumed to represent errors , given that many programs, such as git , also use it to print status information).

  • Somewhat paradoxically, using 2>$null does cause the stderr lines to be recorded in $Error ; this unexpected behavior is discussed in this GitHub issue .