how to send a customized string to linux command in gnuplot

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
}

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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