How to capture error messages thrown by a command?

Question

I am writing a PowerShell script where in I need to capture the error message that it's throwing. Note: according to PowerShell, there is no error and command is executed successfully.

For example: I tried to download a package from SVN Link. The Link actually is not present. The script is showing me error message on the console. However, when I tried to check $_ or $? or $error, I did not see any error message. However, $LASTEXITCODE returned value 1. I need to get the exact error message.

Answer 1

If you get an error message, you need to capture the error stream:

$msg = command 2>&1

or

command 2>error.txt

PowerShell writes its messages to different streams that can be redirected to files for capturing the respective output.

• Stream 1 (default): regular output ("STDOUT")
• Stream 2: error messages ("STDERR"), including error messages from external programs
• Stream 3: warning messages
• Stream 4: verbose messages
• Stream 5: debug messages
• Stream 6: information messages (only PowerShell v5 and newer)

To capture a particular stream in a file you need to redirect the stream number to a file name. For instance

command 2>"C:\path\to\error.log"

would capture all error messages produced by command in the file C:\path\to\error.log. Use 2>> instead of 2> if you want to append to the file instead of overwriting it with each run.

You can also combine other streams with STDOUT to process/redirect all command output:

command >>"C:\path\to\all.log" *>&1

See Get-Help about_Redirection or this Scripting Guy article for more information about streams and redirection.

Things worth of note:

• The *> redirection was introduced with PowerShell v3, hence it won't work in PowerShell v2 and earlier.
• PowerShell v5 introduced a new stream (Information, stream number 6) since people kept misusing Write-Host because they didn't understand what the cmdlet was intended for.

Answer 2

Assuming your executable is named svn.exe and is on the path, you can capture the messages it sends to console this way:

$msg = [string] (svn.exe <your parameters here>)

You can then parse the $msg string to find information you need.