Question

I am trying to perform a simple integer comparison in bash, which I am new to using, and my methods are causing an error. Any advice would be appreciated.

My basic logic is that I am reading in hurricane track points. There may be multiple entries for the same track_id, with different pressure values. What I want to do is store only one entry per track_id into the array track_id_poly; the case with the lowest pressure. So I am looping through each line, and attempting to compare the current pressure (for $int), with the previous track pressure ($int – 1), and if it is lower, replace the previous array value with the new lower pressure. I hope that makes sense. My code is below.

int=0
    while read track_id ppres_inter 
    do
      printf -v pres_inter "%.0f" "$pres_inter"
      echo pressure $pres_inter
      case $int in
      0)
        Track_id_poly[$int]=$track_id
        Pres_inter_poly[$int]=$pres_inter
        ((int=int+1)) ;;
      *)  
       if [[ $track_id == ${Track_id_poly[$int-1]} ]]
       then
         if (( $pres_inter -lt ${Pres_inter_poly[$int-1]} ))
         then
           Track_id_poly[$int-1]=$track_id
           Pres_inter_poly[$int-1]=$pres_inter
         fi
       else  
         Track_id_poly[$int]=$track_id
         Pres_inter_poly[$int]=$pres_inter
         ((int=int+1))
       fi ;;
      esac
    done <$file_poly
    int_poly=$int
    echo Number of polygon crossings from set $i is $int_poly

The line that is causing me problems is the integer comparison for $pres_inter.

if (( $pres_inter -lt ${Pres_inter_poly[$int-1]} ))

I get the following error:

line 41:  96800 -lt 98759 : syntax error in expression (error token is "98759 ")

Any tips to fix this problem would be appreciated. Probably a simple fix!

Answer 1

The ((...)) operator does not accept the same syntax as the test, [..], or [[...]] commands. To compare two numbers in ((...)), you would use actual > or < symbols:

$ (( 4 > 2 )) && echo '4 is bigger!'
4 is bigger!

See the ARITHMETIC EVALUATION section of the bash(1) man page for more information (or here).