How to run external commands?

[notooth]

Can anyone guide me run an external command and get its return output? I failed with process_create:

?- process_create(path(ls), ['-l'], []).
false.

[EricGT]

Have you looked at the test cases for process_create/3 in test_process.pl?

[peter.ludemann]

Which OS are you using?

What do you get from this query:
absolute_file_name(path(ls), Abs).

[notooth]

I am using Debian 10.6, swi-prolog 8.0.2.

?- absolute_file_name(path(ls), Abs).
Abs = ‘/usr/bin/ls’.

[peter.ludemann]

Hmmm … it works for me on Ubuntu, SWI-Prolog 8.3.10. And presumably you get the same result with process_create('/bin/ls', [], [])? (or '/usr/bin/ls').

process_create/3 is documented as det, so it shouldn’t fail.
I wonder if this works for you: process_create(path(echo), [hello], []).
How about shell(ls)?

[notooth]

I’ve just restarted the Prolog shell, and it gives me some different results:

?- process_create(path(ls), ['-l'], []).
total 4
-rw-r--r-- 1 notooth notooth 106 Oct 28 08:30 test.pl
true.

?- process_create(path(ls), [], [stdout(pipe(Var))]).
Var = <stream>(0x55b96a953a40).

[jan]

This looks normal. Wondering what could have caused a failure before the restart. I guess we’ll never know

[notooth]

I put the code in test.pl file and run $ swipl test.pl. Then the shell caused that failure.

[jan]

Tried this (file p.pl):

:- initialization process_create(path(ls), ['-l'], []).

Then in the shell

swipl p.pl
total 10084
drwxr-xr-x  2 janw janw    4096 Feb 12  2020 a
-rwxr-xr-x  1 janw janw    8361 Apr 29 11:05 aa.pl
-rw-r--r--  1 janw janw    1528 Jun  5  2019 ac2_tests.P
...
-rw-r--r--  1 janw janw       7 Aug  6 10:33 хелло.pl
Welcome to SWI-Prolog (threaded, 64 bits, version 8.3.10-11-g6801fa53e-DIRTY)
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software.
Please run ?- license. for legal details.

    CMake built from "/home/janw/src/swipl-devel/linux"

For online help and background, visit https://www.swi-prolog.org
For built-in help, use ?- help(Topic). or ?- apropos(Word).

101 ?- ^D

Looks good to me. Please be very precise in your instructions. Quite likely you found some problem. The (in my view) most obvious way to reproduce it doesn’t work though.

[Boris]

Note that @notooth is also using 8.0.2

[jan]

Could indeed be an old bug. I don’t have 8.0.2 around. I’d need to rebuild it