[Gambas-user] Exec statement not working though equivalent Terminal command works

[Tobias Boege]

On Wed, 09 Jul 2014, John Rose wrote:
> I have this code:
>      Print "mplex", "-f 8", "-o " & "'" & sCombinedPath & "'", "'" & 
> sVideoPath & "'", "'" & sAudioPath & "'"
>      Exec ["mplex", "-f 8", "-o " & "'" & sCombinedPath & "'", "'" & 
> sVideoPath & "'", "'" & sAudioPath & "'"] To sOutput
> I'm using the single quotes since they are required when entering the 
> equivalent commands in a Terminal e.g. mplex   -f 8    -o 
> '/home/john/Videos/q.mpg'    '/home/john/Videos/The Story of Science_ 
> Power, Proof____20140621_2000.m2v' '/home/john/Videos/The Story of 
> Science_ Power, Proof____20140621_2000.mp2'.
> 
> The Print statement shows (at runtime):
> mplex   -f 8    -o '/home/john/Videos/q.mpg' '/home/john/Videos/The 
> Story of Science_ Power, Proof____20140621_2000.m2v' 
> '/home/john/Videos/The Story of Science_ Power, Proof____20140621_2000.mp2'
> i.e. the exact equivalent of the working command when entered in a 
> Terminal.
> 
> However, the Exec statement does nothing at runtime, though the 
> equivalent Terminal command works. There is no feedback on why by 
> Gambas. I've also tried the statements without the single quote usage & 
> it also did nothing. I'm fairly sure that this used to work on the 
> previous version of Gambas: I'm using kendek's Gambas3 ppa 
> (https://code.launchpad.net/~nemh/+archive/ubuntu/gambas3) which 
> installed Gambas 3.5.3. The code without the single quote usage is:
>      Print "mplex", "-f 8", "-o " & sCombinedPath, sVideoPath, sAudioPath
>      Exec ["mplex", "-f 8", "-o " sCombinedPath, sVideoPath, sAudioPath] 
> To sOutput
> 
> The above sOutput variable has the value "". What I am doing wrong or is 
> it an issue in Gambas 3.5.3?
> 

The fundamental thing to remember is: Exec is way not the terminal/shell.

The shell needs you to quote your arguments in different manners so that it
can see where actual spaces are and where spaces belong to, e.g., file
names. The space is a control sequence for the shell - not for Exec! In
Exec, you must not quote stuff because the data you give is literally
transferred into the kernel. If you write Exec ["program", "'file name'"],
the kernel will look for a file literally named 'file name', the apostrophes
being part of that file name!

Just make sure that each argument is in its own array element; that is
enough for Exec.

This applies also to "-f 8" where you put two arguments into one array
element. You'll confuse the program's command line parser because it does
not know the switch "-f 8", it just knows the switch "-f" which may be
followed by an argument "8", a *distinct* argument.

To sum up, I'm pretty sure your Exec line should be like:

  Exec ["mplex", "-f", "8", "-o", sCombinedPath, sVideoPath, sAudioPath]

Regards,
Tobi

-- 
"There's an old saying: Don't change anything... ever!" -- Mr. Monk