Compare/Difference of two arrays in Bash

arrays, bash, compare, diff

Solution

If you strictly want `Array1 - Array2`, then

Array1=( "key1" "key2" "key3" "key4" "key5" "key6" "key7" "key8" "key9" "key10" )
Array2=( "key1" "key2" "key3" "key4" "key5" "key6" )

Array3=()
for i in "${Array1[@]}"; do
    skip=
    for j in "${Array2[@]}"; do
        [[ $i == $j ]] && { skip=1; break; }
    done
    [[ -n $skip ]] || Array3+=("$i")
done
declare -p Array3

Runtime might be improved with associative arrays, but I personally wouldn't bother. If you're manipulating enough data for that to matter, shell is the wrong tool.

For a symmetric difference like Dennis's answer, existing tools like `comm` work, as long as we massage the input and output a bit (since they work on line-based files, not shell variables).

Here, we tell the shell to use newlines to join the array into a single string, and discard tabs when reading lines from `comm` back into an array.

$ oldIFS=$IFS IFS=$'\n\t'
$ Array3=($(comm -3 <(echo "${Array1[*]}") <(echo "${Array2[*]}")))
comm: file 1 is not in sorted order
$ IFS=$oldIFS
$ declare -p Array3
declare -a Array3='([0]="key7" [1]="key8" [2]="key9" [3]="key10")'

It complains because, by lexographical sorting, `key1 < … < key9 > key10`. But since both input arrays are sorted similarly, it's fine to ignore that warning. You can use `--nocheck-order` to get rid of the warning, or add a `| sort -u` inside the `<(…)` process substitution if you can't guarantee order&uniqueness of the input arrays.

Problem

Original source

Related problems

• Intersection of two lists in Bash
• Array intersection in Bash