Integer expression expected bash 2

Question

I'm just learning bash scripting. Tried to sort the array, it says "integer expression expected for lines 10 and 15. What am I doing wrong? Here is my script:

#!/bin/bash
array=('5' '9' '0' '20' '2' '15' '6' '25' '1')
b=0
n=${#array[@]}
i=0
while [ "$i" -lt "$n" ]
do
    c=${array[$i]}
    d=${array[$i+1]}
    if [ "$c" -lt "$d" ]; then
        j=0
        while [ "$j" -le "$i" ]
        do
            f=${b[$j]}
                if [ "$f" -gt "$c" ];
                    then b[$j]=$c
                    echo "${b[$j]}"
                fi
            j=$(( j+1 ))
        done
    fi
    i=$(( i+1 ))
done

You seem to be using $b first as a scalar (b=0) and then later as an array (${b[$j]}). — Stéphane Chazelas
– Stéphane Chazelas, Commented May 4, 2018 at 9:19
What is the significance of the "2" in the title of this question? — Kusalananda
– Kusalananda ♦, Commented May 4, 2018 at 9:33
thank you Stephane , how can I declare the array currectly ? — Anna
– Anna, Commented May 4, 2018 at 9:52
Running your script with bash -x ./the-script should make it apparent what the problem is. Again, look at your usage the $b variable. — Stéphane Chazelas
– Stéphane Chazelas, Commented May 4, 2018 at 12:57

Stéphane Chazelas · Accepted Answer · 2018-05-04 13:57:22Z

You're calling the [ command with the -lt/-gt decimal integer comparison operators on operands that are not always decimal integers.

You can see what happens if you run the script with bash -x. You'll see things like:

+ f=
+ '[' '' -gt 0 ']'
./myscript: line 15: [: : integer expression expected

With:

while [ "$i" -lt "$n" ]
do
[...]
   d=${array[$i+1]}

On the last pass in that loop, you'll try to access beyond the last element of the array, so $d will be empty.

You're also initialising $b as a 0 string, and later on accessing it as an array. See also how f=${b[$j]} will get you an empty $f except for when $j is 0.

I don't know what you're trying to do with that code, but it seems like you need to go back to the drawing board.

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