How to Get The Union of Two Arrays in Bash

Array intersection in Bash

The elements of list 1 are used as regular expression looked up in list2 (expressed as string: ${list2[*]} ):

list1=( 1 2 3 4   6 7 8 9 10 11 12)
list2=( 1 2 3   5 6   8 9    11 )

l2=" ${list2[*]} "                    # add framing blanks
for item in ${list1[@]}; do
  if [[ $l2 =~ " $item " ]] ; then    # use $item as regexp
    result+=($item)
  fi
done
echo  ${result[@]}

The result is

1 2 3 6 8 9 11

Merging two arrays in Bash

Since Bash supports sparse arrays, it's better to iterate over the array than to use an index based on the size.

a=(0 1); b=(2 3)
i=0
for z in ${a[@]}
do
    for y in ${b[@]}
    do
        c[i++]="$z:$y"
    done
done
declare -p c   # dump the array

Outputs:

declare -a c='([0]="0:2" [1]="0:3" [2]="1:2" [3]="1:3")'

Bash - iterate over a union of two (disjoint) arrays

Just put one after the other:

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

Bash: How do I combine two arrays into a third array?

From The Advanced Bash Scripting Guide example 27-10, with a correction:

declare -a array1=( zero1 one1 two1 )
declare -a array2=( [0]=zero2 [2]=two2 [3]=three2 )
dest=( "${array1[@]}" "${array2[@]}" )

Thus, for my case, it's:

array3=( "${array1[@]}" "${array2[@]}" )

Find items common between two Bash arrays

You can use comm with process substitution instead of looping:

mapfile -t new_number1 < <(comm -12 <(printf '%s\n' "${number1[@]}" | sort) <(printf '%s\n' $range_f | sort))
mapfile -t new_number2 < <(comm -12 <(printf '%s\n' "${number2[@]}" | sort) <(printf '%s\n' $range_f | sort))

mapfile -t name reads from the nested process substitution into the named array
printf ... | sort pair provides the sorted input streams for comm
comm -12 emits the items common to the two streams

Intersection of two lists in Bash

comm -12  <(ls 1) <(ls 2)

Compare/Difference of two arrays in Bash

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.

How can I join elements of an array in Bash?

A 100% pure Bash function that supports multi-character delimiters is:

function join_by {
  local d=${1-} f=${2-}
  if shift 2; then
    printf %s "$f" "${@/#/$d}"
  fi
}

For example,

join_by , a b c #a,b,c
join_by ' , ' a b c #a , b , c
join_by ')|(' a b c #a)|(b)|(c
join_by ' %s ' a b c #a %s b %s c
join_by $'\n' a b c #a<newline>b<newline>c
join_by - a b c #a-b-c
join_by '\' a b c #a\b\c
join_by '-n' '-e' '-E' '-n' #-e-n-E-n-n
join_by , #
join_by , a #a

The code above is based on the ideas by @gniourf_gniourf, @AdamKatz, @MattCowell, and @x-yuri. It works with options errexit (set -e) and nounset (set -u).

Alternatively, a simpler function that supports only a single character delimiter, would be:

function join_by { local IFS="$1"; shift; echo "$*"; }