Re: [Gnuplot-info] failure to change directory and execute linux command

[Ethan M. <eam...@gm...>]

On Tuesday, 23 June 2020 14:26:41 PDT Jon wrote:
> Hi,
> 
> I am trying to execute a script ssss.gn within gnuplot 5.2.2 in
> linuxmint 18.2 including the following lines
> pwdd="`pwd`"
> do for [ dir in "4.1 4.15 4.2 4.25 4.3 "] {
>   print dir
>   cd dir."/right/here/waiting/"
>   ef=`head -n 1 DOSCAR | awk '{print $5}'`
>   print ef
>   cd pwdd
> }
> 
> but it complains

I do not fully understand this myself, but I do know how avoid the problem.
There is a subtle difference between
      foo = "`some command`"
and
      foo = system("some command")

in that the back-quote version is evaluated during command parsing,
while the system() command is evaluated during execution.
I am not sure why exactly that matters in your case but apparently it does.

You can fix it by replacing the back-quotes with a system command

pwdd = system("pwd")
do for [ dir in "4.1 4.15 4.2 4.25 4.3 "] {
  print dir
  cd dir."/right/here/waiting/"
  ef = system("head -n 1 DOSCAR | awk '{print $5}'")
  print ef
  cd pwdd
}

> gnuplot> load "ssss.gn"
> head: cannot open 'DOSCAR' for reading: No such file or directory
> 4.1
>          line 5: constant expression required
> 
> but the file does exist under the directory, why would it complain
> file not existing
> 
> Another confusing observation is the error concerning nonexistence of
> DOSCAR comes before the print statement in the script. Why is that?

It is a symptom of the back-quote evaluation occuring during an
earlier stage of processing than the loop execution.

	Ethan


> 
> Thanks,
> 
> Sincerely,
> Jon