SSM send-command with Powershell does not complete until background job finishes

Question

I am using SSM send-command to start a Tomcat process on a Windows EC2 instance.

for example:

aws ssm send-command --instance-id i-xxxx --document-name "AWS-RunPowerShellScript"
   --parameters 'commands=["startup.bat"]'

This does indeed start Tomcat but I noticed that SSM command invocation itself is stuck in progress until I eventually shut Tomcat down, at which point it immediately succeeds.

My hunch is that something about the way SSM runs Powershell blocks on any spawned child processes ( start java... in the case of Tomcat). I don't think this is unique to Tomcat or even Java, though.

Is there a way I can get Powershell to exit, leaving the child processes running in the background?

Answer 1

If you want to start an independent process in PowerShell - one that stays alive even after the creating process terminates - use the Start-Process cmdlet:

Start-Process aws -ArgumentList @'
send-command --instance-id i-xxxx --document-name AWS-RunPowerShellScript --parameters commands=["startup.bat"]
'@

^{Note the use of a (literal) here-string ( @'<newline>...<newline>'@ ) to make embedding quotes easier; see the bottom section of this answer for information about PowerShell string literals.}

While -ArgumentList can accept an array of arguments, the way these are translated into a command line is broken as of PowerShell 7.0 (and will likely not get fixed due to backward compatibility concerns; see this GitHub issue ).

Therefore, it is best to use a single argument with embedded quoting .

Note that you should only use double quotes ( " ) as argument delimiters when calling external programs (such as aws in this case). ^[1]

---

^{[1] With direct invocation of an external program, you're free to use either quoting style, because PowerShell, when it rebuilds the command line behind the scenes, uses only " to quote arguments (if needed at all). In the case of Start-Process , the -ArgumentList value is passed through to a .NET API ( System.Diagnostics.ProcessStartInfo ), where no such translation is performed.}