[squeak-dev] OSProcess Question

David T. Lewis lewis at mail.msen.com
Sat Aug 27 03:19:04 UTC 2022

Hi Eric and Tom,

On Fri, Aug 26, 2022 at 04:27:56PM -0400, Eric Gade wrote:
> I believe what is happening is that the various #command: messages in
> OSProcess classes are executing without searching my PATH. Even if I add a
> copy of the current running process environment and set that as the
> environment of a new PipeableOSProcess, for example, I can't even get a
> result for `which node`. I can see that /usr/bin and /usr/sbin are on the
> path from within Squeak.

Yes that is correct. On a unix system, the interpretation of PATH (and
lots of other things) are done by the shell, which is typically an
executable program such as /bin/bash. The actual programs and commands
that you run on a unix system are subprocesses of that shell program.
So the shell interprets the PATH and uses it to find the program that
you want to run.

In Squeak with OSProcess, your Squeak virtual machine is the "shell"
and you can directly run programs by creating subprocesses of the VM
process. Of course, you can chose to run the executable program
/bin/bash, in which case you have a real unix shell, and you can let
that shell run other commands in the normal way.

You can also run any other executable directly (and I think this is
what you have been doing). In that case there is no unix shell program
involved, and you would need to specify the fully qualified path to
the program, and you would need to do any necessary command parameter
expansion that would otherwise have been done by the unix shell.

In OSProcess/CommandShell there is also a pure Smalltalk emulation
of a simple unix shell. This is by no means a complete emulation of
/bin/bash but it is sufficient to handle things like PATH expansion
and command pipeline composition without requiring the actual
/bin/bash shell.

So to tie the pieces together, if you want to run the command "which node"
you can do this:

  OSProcess outputOf: 'which node'

This uses a CommandShell in Squeak to do the shell processing to
locate and run the program /usr/bin/which (so no need for /bin/bash).
It takes some patience but if you step through this it a debugger
you will be able to see what is going on. The Smalltalk command
shell emulation sets everything up so that OSProcess can run the
/usr/bin/which program as a direct subprocess of the Squeak VM.

> Another example is that `PipeableOSProcess class >> #unixCommandPipeLine`
> responds with an empty string when I inspect it.
> Might this all be related to newer macOS settings somehow?

This method is just an example to show how to compose a command pipeline
with some programs that would be found on a typical Unix/Linux system.
I'm not sure what it does on Mac.  It uses the programs 'ps', 'grep',
and 'cut', so if any of these is not found on your system, it probably
will just give an empty string in the output (plus some error output
that you can probably find if you dig through it in a debugger).


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