Can't decrement counter with fpsystem while loop

[hakelm]

In the little test program below I am trying to run a while loop controlled by a decrementing counter.
The result is, however, not what I had expected, the counter i is not decremented, I get an endless loop.
What is wrong?
H
I am using lazarus-ide-2.2.0 on ubuntu

Code: Pascal  [Select][+][-]

1. [
2. program project1;
3. {$mode objfpc}{$H+}
4. uses
5.   {$IFDEF UNIX}
6.   cthreads,
7.   {$ENDIF}
8.   Classes,unix
9.   { you can add units after this };
10. //{$R *.res}
11. var cmd:string;
12. begin
13.     //cmd:='i=100; while [ 1 -le "$i" ]; do amixer -c 1 set Master playback "$i"%;   ((i--)); done|grep dB > r';
14.     cmd:='i=100; while [ 1 -le "$i" ]; do echo $i;   ((i--)); done';
15.     writeln(cmd);
16.     fpsystem(cmd);
17. end.
18. { output:
19. i=100; while [ 1 -le "$i" ]; do echo $i;   ((i--)); done
20. 100
21. /bin/sh: 1: i--: not found
22. 100
23. /bin/sh: 1: i--: not found
24. 100
25. /bin/sh: 1: i--: not found
26. 100
27. /bin/sh: 1: i--: not found
28. 100
29. ......
30. }/code]

[TRon]

The result is, however, not what I had expected, the counter i is not decremented, I get an endless loop.
What is wrong?

In principle there seems nothing wrong. But i suspect that the decrement of the counter is an internal bash-ishm and therefor fails (you should invoke bash then instead). I get "i-- not found" as if it where a command.

Having said that, this seems to work for me:

Code: [Select]

    cmd:='i=100; while [ 1 -le "$i" ]; do echo $i; i=$((i-1)) ; done';


[hakelm]

Thanks. i-1 did the trick.
But it is a bit annoying not understanding what is going on.
H

[TRon]

But it is a bit annoying not understanding what is going on.

Some expressions/commands are only available inside bash itself (so called internals). Because you did not invoke bash itself in the fpsystem call these internals are not available/accessible in your script.
see also https://bash.cyberciti.biz/guide/Built-in_commands (click below for the different available shells). I am sure there are websites that have a more comprehensive lists and other gory details available :-)

Other than that, it is always a good idea to use posix compliant scripts.

[Warfley]

To make things more complicated, fpSystem calls the shell that is registered as your systems /bin/sh, which *can* be a symlink to bash, but also to any other shell such as the Bourne Shell (sh), Z-Shell (zsh), Fish, etc.
So you cannot assume any shell specific functionality when calling system. If you need such functionality, you should be calling the respective shell directly. In this case you also do not need fpsystem but can also just rely on TProcess or RunCommand

[TRon]

Thank you Warfley for that information as I was not aware of that.

[hakelm]

Interesting.
On my system sh points neither to sh nor to bash but to dash and man dash says:

     dash is the standard command interpreter for the system.  The current
     version of dash is in the process of being changed to conform with the
     POSIX 1003.2 and 1003.2a specifications for the shell.  This version has
     many features which make it appear similar in some respects to the Korn
     shell, but it is not a Korn shell clone (see ksh(1)).  Only features des‐
     ignated by POSIX, plus a few Berkeley extensions, are being incorporated
     into this shell.  This man page is not intended to be a tutorial or a
     complete specification of the shell.