gnuplot: Can you set shell variables in a script and then later call them?

I want to be able to issue shell commands from a gnuplot script, such as setting a variable, but it appears that the system command spawns a new shell instance as opposed to issuing commands to the shell instance which is running the gnuplot script. See the following script where the first line allows assignment of $foo, but the second line cannot access that variable. In this case, $foo is assigned as an arbitrary string as opposed to a reference to the directory, hence the "

#!/usr/bin/gnuplot -p
system "export foo="$HOME/path/to/dir";echo $foo"
system "echo $foo"
set datafile separator "t"
#plot "`echo $foo`/bar.dat" using 2:3
plot "<( sed '5p' $foo/bar.dat )" using 2:3

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Author: T. Zack Crawford