Re: how to send a customized string to linux command in gnuplot

On Monday, 11 February 2019 11:50:12 Jon wrote:
> Hi,
> 
> I need to extract a value from each file and combine a column in that
> file and then plot. The gnuplot script looks something like
> 
> reset; set macro
> list="2.35 2.4 2.5 2.6 2.7 "
> do for [i in list]{
>   file="/dir/to/".i."/target/file"
>   cmd="head -n 6 ".@file."  | tail -n 1 | awk '{printf $4}'"
>   print file
>   print cmd
>   ef=system(@cmd)
>   print ef
>   plot @file u ($1-ef):2 ev ::7::301 w l ti i
> }

I do not know the reason for that exact error message, 
but the code shown cannot work as written because it is not possible
to set and reevaluate macros inside a loop.  See for example
 
  https://sourceforge.net/p/gnuplot/bugs/1176/
  https://sourceforge.net/p/gnuplot/mailman/message/32364975/

Fortunately as with issue #1176 there is no need to use macros at all
in your case. The solution may be as simple as removing all those @ signs. 
I suggest you rewrite the code without macros and report back
if there is still an error.

	Ethan



> The expected screen output is, say,
> /dir/to/2.35/target/file
> head -n 6 /dir/to/2.35/target/file | tail -n 1 | awk '{printf $4}'
> 3.243  (some intended value coming out of linux command, differing at each i)
> the intended plot with x axis shifted 3.243
> 
> The execution failed complaining  "invalid expression"
> 
> Why is the error message? Is this possibly done with gnuplot? I am
> using gnuplot 5.2.6 on linux mint 8.1 installed in a virtualbox hosted
> with windows 7
> 
> Thanks for your help!
> 
> Sincerely,
> John
> 
> 
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entia non sunt multiplicanda praeter necessitatem


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