Question

I have a very simple script:

#!/bin/bash
gnuplot << EOF
set term postscript portrait
set output 'out.ps'

plot 'data_file' u 3:($2==0.0 ? $2:1/0)

EOF

where data_file looks like this:

  O4     -1.20     -0.33     -5.20  
O9.5     -1.10     -0.30     -3.60  
  B0     -1.08     -0.30     -3.25  
B0.5     -1.00     -0.28     -2.60  
B1.5     -0.90     -0.25     -2.10  
B2.5     -0.80     -0.22     -1.50  
  B3     -0.69     -0.20     -1.10  
....

I have tried several combinations and I still get the same error always:

gnuplot> plot 'S-K_data' u 3:(==0.0 ? :1/0)
                          ^
         line 0: invalid expression 

As you can see, it’s not reading the $2 characters and I just can’t figure out why.

Answer 1

In an unquoted here doc, variables are expanded. Try

gnuplot << 'EOF'

or

gnuplot << \EOF

Which you choose is a matter of personal preference (you can use any quoting mechanism, so EOF"" works, but the two above are the most common). When the heredoc delimiter is unquoted, the shell is expanding $2 to the second parameter passed to the script. (Try calling the script with 2 arguments.) When the heredoc delimiter is quoted, no such expansion takes place.