sorting numbers read in from input file in bash

Question

So I have the following bash script, which is supposed to read a .txt file of an n amount of integers, add it to an array, sort the array using selection sort, then print the array:

a=()
filename="$1"
while IFS='' read -r line || [[ -n "$line" ]]; do
    a+=($line)
done < "$filename"

for((i=0; i<${#a[@]}; i++))
do
  min=$i
  for((j=$i+1; j<${#a[@]}; j++))
  do
    if (( ${a[$j]} <= ${a[$min]} ))
    then
      $min=$j
      echo "$min"
    fi
  done
  temp=a[$i]
  a[$i]=a[$min]
  a[$min]=$temp
done

for i in ${a[@]}
do
  echo $i
done

The problem is that the conditional within the inner loop throws an exception, where the problem I believe is that the array elements are all strings, and I'm not sure how to cast them to strings in this situation. Also, I don't think I'm swapping the two values properly. The output:

sh selectionsort.sh 10.txt
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 152ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 436ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 756ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 391ax error: invalid arithmetic operator (error token is "
 ") 435ax error: invalid arithmetic operator (error token is "
 ") 435ax error: invalid arithmetic operator (error token is "
 ") 435ax error: invalid arithmetic operator (error token is "
 ") 435ax error: invalid arithmetic operator (error token is "
 ") 435ax error: invalid arithmetic operator (error token is "
 ") 404ax error: invalid arithmetic operator (error token is "
 ") 404ax error: invalid arithmetic operator (error token is "
 ") 404ax error: invalid arithmetic operator (error token is "
 ") 404ax error: invalid arithmetic operator (error token is "
 ") 853ax error: invalid arithmetic operator (error token is "
 ") 853ax error: invalid arithmetic operator (error token is "
 ") 853ax error: invalid arithmetic operator (error token is "
 ") 278ax error: invalid arithmetic operator (error token is "
 ") 278ax error: invalid arithmetic operator (error token is "
 ") 643ax error: invalid arithmetic operator (error token is "
a[0]
a[1]
a[2]
a[3]
a[4]
a[5]
a[6]
a[7]
a[8]
a[9]

Any help is appreciated! Note: I know there's easier way to sort arrays, I'm using this script to implement selection sort.

selection sort in bash? how would you expect people to answer your question? — Jason Hu
– Jason Hu, Commented Oct 29, 2017 at 2:52
It's jut an implementation, the sort itself is not the issue; it's the read from the file I believe and the conditional. — Anthony B
– Anthony B, Commented Oct 29, 2017 at 3:09

Renaud Pacalet · Accepted Answer · 2017-10-30 05:20:15Z

1

$min=$j is definitely an error. It should be min=$j. The way you access the cells of your array (temp=a[$i], a[$i]=a[$min]) too. Use temp=${a[$i]}, a[$i]=${a[$min]}, instead. But there are a few more things that you could improve:

declare -ai a=()
declare -i i j min temp
declare line

filename="$1"
while IFS='' read -r line; do
  [[ $line =~ ^[0-9]+$ ]] && a+=($line) || printf "Warning: not a number (%s)\n" "$line"
done < "$filename"

for (( i=0; i<${#a[@]}; i++ ))
do
  min=$i
  for (( j=i+1; j<${#a[@]}; j++ ))
  do
    if (( a[j] <= a[min] ))
    then
      min=$j
      echo "$min"
    fi
  done
  temp=${a[i]}
  a[i]=${a[min]}
  a[min]=$temp
done

for i in "${a[@]}"
do
  echo "$i"
done

In the context of arithmetic evaluation (((...))) and array indexing (a[...]), variable names are interpreted as their numeric values (no need of $).

I also added variable declarations and a test to eliminate non-integer entries.

edited Oct 30, 2017 at 5:20

answered Oct 29, 2017 at 7:20

Renaud Pacalet

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5 Comments

Anthony B Over a year ago

Thanks! I'm still confused on why it's not $min =$j. I've declared min prior to this line, don't you always need to put a $ when accessing variables that were already initialized?

Renaud Pacalet Over a year ago

min is the variable name.$min is its value.

Anthony B Over a year ago

And don't we need to change it's value by saying $min rather than min? F.e. a=5, b=1, $a=2, $a=$b is how we change the value of variables, from what I've learned.

Renaud Pacalet Over a year ago

If the value of min is 5 and you execute $min=6 the expansion transforms it into 5=6 and you get an error message because it does not make sense. Think at $x as value(x) and x as the name of the variable.

Anthony B Over a year ago

Oh! so $ is similar to the concept of dereferencing a pointer in C, at least that's how I'm understanding it. Thanks!

agc · Accepted Answer · 2017-10-29 16:01:19Z

1

If you are using bash you can take advantage of its builtin functions like sort and readarray. (notice readarray is available since bash 4+, no OS X support by default.)

You can remove your sorting code and use the following code as described here

readarray -t sorted < <(for b in "${a[@]}"; do echo "$b"; done | sort)

$a will be transformed to a sorted array.

edited Oct 29, 2017 at 16:01

agc

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answered Oct 29, 2017 at 0:12

Mr. bug

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6 Comments

Anthony B Over a year ago

The point of this script is just to implement the selection sort algorithm using the shell. I know it can be done in an easier method, my only question is what the issue is with the current script.

Mr. bug Over a year ago

I ran it and it worked, the only problem I might figure is $min=$j should be min=$j

Anthony B Over a year ago

Why would you not put the $? I initialized it prior.

Mr. bug Over a year ago

In my bash version it failed with ./test.sh: line 15: 2=3: command not found, I was assigning an int to an int

Anthony B Over a year ago

The initialization of the for loop? There seems to be no issue there.

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