How to pass a bash variable (list of numeric data) to gnuplot *without using a file*

Question

I am using netcat to get a series of points which I would like to plot with gnuplot. However, I am having trouble passing the data to gnuplot without writing it to a file first.

In the simplest case, I want to plot a 3-point line that I've defined in a variable first as follows:

#!/bin/bash

DATA="10
12
15"

echo "$DATA"

# Data looks good in terminal (single vertical column of numbers 10, 12, 15)
# However, the following step doesn't work:

gnuplot -persist <<-EOFMarker
    plot $DATA w lines
EOFMarker

The actual use case is to get data from a networked Rigol oscilloscope and show it in a plot. Here is the full script so far:

#!/bin/bash
SERVER=192.168.0.12
PORT=5555

RIGOLDATA="$(echo ":TRAC:DATA? TRACE1" | netcat -q1 $SERVER $PORT | cut -c 13- | tr ',' '\n' | tr -d [:blank:])"
echo "$RIGOLDATA"

# RigolData looks good in terminal (single vertical column of numbers).
# However, the following does not work:

gnuplot -persist <<-EOFMarker
    plot $RIGOLDATA w lines
EOFMarker

I can get the desired result by passing the data into a temp file first as follows, but I am trying to avoid the use of a temp file:

#!/bin/bash
SERVER=192.168.0.12
PORT=5555

RIGOLDATA="$(echo ":TRAC:DATA? TRACE1" | netcat -q1 $SERVER $PORT | cut -c 13- | tr ',' '\n' | tr -d [:blank:])"

echo "$RIGOLDATA" > tempfile.csv


gnuplot -persist <<-EOFMarker
    plot "tempfile.csv" w lines
EOFMarker

Answer 1

plot $DATA w lines

expands to

plot 10
12
15 w lines

i.e. whatever $DATA contains gets put there literally, newlines and all. That likely breaks the command. (I'm not sure if gnuplot has a way to give the data directly on the plot command line, but it'd probably need to be in some other format.)

The shell has process substitution that makes the output of a command available as a "file", and expands to the filename, but you might be better off passing the data through stdin, and giving the plot command to gnuplot as an option.

Based on the example command used here, something like this might work:

echo "$DATA" | gnuplot -e 'plot "-" w lines'

or rather by piping the data directly from where you're getting it, without collecting it in the shell in the first place:

echo ":TRAC:DATA? TRACE1" | netcat -q1 "$SERVER" "$PORT" | 
  cut -c 13- | tr ',' '\n' | tr -d "[:blank:]" |
  gnuplot -e 'plot "-" w lines'

---

Then again, I don't think using a temporary file is that bad. A lot of shell scripts use them, and so do non-shell programs, too. A lot of the programs commonly called from shell scripts can read files directly, while passing data from a shell variable requires more complications. (E.g. the pipeline echo "$var" | somecmd requires launching an extra copy of the shell to write the data, and here-docs and here-strings like somecmd <<< "$var" just might create a temporary file in background anyway...)

Of course you usually want to remove any temporary files when you're done, and you should make sure they're created with a unique name so there are no collisions. But that's not hard to do:

#!/bin/bash
server=192.168.0.12
port=5555
tempfile=$(mktemp)

echo ":TRAC:DATA? TRACE1" | netcat -q1 "$server" "$port" |
   cut -c 13- | tr ',' '\n' | tr -d "[:blank:]" > "$tempfile"


gnuplot -persist <<-EOFMarker
    plot "$tempfile" w lines
EOFMarker

rm -f -- "$tempfile"

Answer 2

Ok I want to share the simple answer that I figured out, which is to install and use feedgnuplot.

This will plot a simple line connecting Y-coordinates 4, 8, 5, 7:

echo -e "4\n8\n5\n7" | feedgnuplot -w lines

This will plot a simple line connecting the (x,y) points (3,11), (4,15), (5,12):

echo -e "3 11\n4 15\n5 12" | feedgnuplot -w lines --domain

It is also possible to redraw the plot frequently for a live-view graph with the --stream option.

This is basically what I was looking for, and I am using feedgnuplot successfully now in a stable and quickly updating bash script for the Rigol DSA815 as follows:

while true; do
   printf "clear\n"
   echo ":TRAC:DATA? TRACE1" | netcat -N 192.168.0.12 5555 | cut -c 13- | tr ',' '\n' | tr -d [:blank:]
   printf "replot\n"
done | feedgnuplot --with 'lines lw 3' --stream 0 --exit --xlen 601

I hope this helps anyone trying to make a gnuplot graph without using a temp file!