Passing a Command as a Variable to another Program? - linux

Question

I'm creating an interactive script that allows me to plot specific parts of a data file using Gnuplot. These files contain multiple channels of instrumentation data and I'd like to plot specified channels together on the plot. At the moment I can only plot each channel independently with a script.
for ycolumn in $ycolumns; do
gnuplot -p << EOF
set datafile separator ","
set autoscale
set grid
set xlabel "Time"
set ylabel "Data"
set title "$graphTitle"
plot "$FILE" using $xcolumn:$ycolumn lt rgb "red" with lines t " "
EOF
This is the code that plots each channel independently. To plot them together I need the the plot line to look like this
plot "$FILE" using $xcolumn:$ycolumn1 lt with lines t " ","$FILE" using $xcolumn:$ycolumn2 lt with lines t " ", etc...
My question is how, if possible, could I have a for loop outside gnuplot that appends each column command ("$FILE" using $xcolumn:$ycolumn1 lt with lines t " ") to a variable, and then that variable gets passed to the plot command inside Gnuplot?
I was thinking about using expect but I can't figure out a way to implement that in the way I'm trying to automate this script.

Answer 1

Probably the most straightforward approach is to first assemble the entire plot command (as mentioned in the comments by 'that other guy') and then use it in the here document:
plotCmd=""
plotCmdDelim="plot"
for ycolumn in $ycolumns
do
plotCmd="${plotCmd}${plotCmdDelim} '${FILE}' using $xcolumn:$ycolumn lt rgb 'red' with lines t ' '"
plotCmdDelim=","
done
gnuplot -p << EOF
set datafile separator ","
set autoscale
set grid
set xlabel "Time"
set ylabel "Data"
set title "$graphTitle"
${plotCmd}
EOF