# Intersection of two arrays in BASH

I have two arrays like this:

``````A=(vol-175a3b54 vol-382c477b vol-8c027acf vol-93d6fed0 vol-71600106 vol-79f7970e vol-e3d6a894 vol-d9d6a8ae vol-8dbbc2fa vol-98c2bbef vol-ae7ed9e3 vol-5540e618 vol-9e3bbed3 vol-993bbed4 vol-a83bbee5 vol-ff52deb2)
B=(vol-175a3b54 vol-e38d0c94 vol-2a19386a vol-b846c5cf vol-98c2bbef vol-7320102b vol-8f6226cc vol-27991850 vol-71600106 vol-615e1222)
``````

The arrays are not sorted and might possibly even contain duplicated elements.

1. I would like to make the intersection of these two arrays and store the elements in another array. How would I do that?

2. Also, how would I get the list of elements that appear in B and are not available in A?

• Use a real programming language, not a shell for this kind of task. – Stéphane Chazelas Dec 12 '13 at 12:53
• Do you need to retain the order of the elements? If there are duplicated elements (e.g. A and B both contain `foo` twice), do you need them duplicated in the result? – Gilles 'SO- stop being evil' Dec 12 '13 at 22:37

## 5 Answers

`comm(1)` is a tool that compares two lists and can give you the intersection or difference between two lists. The lists need to be sorted, but that's easy to achieve.

To get your arrays into a sorted list suitable for `comm`:

``````\$ printf '%s\n' "\${A[@]}" | LC_ALL=C sort
``````

That will turn array A into a sorted list. Do the same for B.

To use `comm` to return the intersection:

``````\$ comm -1 -2 file1 file2
``````

`-1 -2` says to remove entries unique to file1 (A) and unique to file2 (B) - the intersection of the two.

To have it return what is in file2 (B) but not file1 (A):

``````\$ comm -1 -3 file1 file2
``````

`-1 -3` says to remove entries unique to file1 and common to both - leaving only those unique to file2.

To feed two pipelines into `comm`, use the "Process Substitution" feature of `bash`:

``````\$ comm -1 -2 <(pipeline1) <(pipeline2)
``````

To capture this in an array:

``````\$ C=(\$(command))
``````

Putting it all together:

``````# 1. Intersection
\$ C=(\$(comm -12 <(printf '%s\n' "\${A[@]}" | LC_ALL=C sort) <(printf '%s\n' "\${B[@]}" | LC_ALL=C sort)))

# 2. B - A
\$ D=(\$(comm -13 <(printf '%s\n' "\${A[@]}" | LC_ALL=C sort) <(printf '%s\n' "\${B[@]}" | LC_ALL=C sort)))
``````
• This will only work if your values don't contain `\n`. – Chris Down Dec 12 '13 at 16:08
• @ChrisDown: That's right. I always try to write shell scripts that are properly quoted and handle all chars, but I've given up on \n. I have NEVER seen it in a filename, and a large bunch of unix tools work with \n delimited records that you lose a lot if you try to handle \n as a valid char. – camh Dec 13 '13 at 10:17
• I've seen it in filenames when using GUI file managers that do not properly sanitise input filenames that are copied from somewhere else (also, nobody said anything about filenames). – Chris Down Dec 13 '13 at 11:10
• To protect `\n` try this: `arr1=( one two three "four five\nsix\nseven" ); arr2=( \${arr1[@]:1} "four five\\nsix" ); n1=\${#arr1[@]}; n2=\${#arr2[@]}; arr=( \${arr1[@]/ /'-_-'} \${arr2[@]/ /'-_-'} ); arr=( \$( echo "\${arr[@]}"|tr '\t' '-t-'|tr '\n' '-n-'|tr '\r' '-r-' ) ); arr1=( \${arr[@]:0:\${n1}} ); arr2=( \${arr[@]:\${n1}:\${n2}} ); unset arr; printf "%0.s-" {1..10}; printf '\n'; printf '{'; printf " \"%s\" " "\${arr1[@]}"; printf '}\n'; printf "%0.s-" {1..10}; printf '\n'; printf '{'; printf " \"%s\" " "\${arr2[@]}"; printf '}\n'; printf "%0.s-" {1..10}; printf '\n\n'; unset arr1; unset arr2` – Jason R. Mick Aug 18 '15 at 8:19
• One should not set `LC_ALL=C`. Instead set `LC_COLLATE=C` for the same performance gain without other side effects. In order to obtain correct results you will also need to set the same collation for `comm` that was used for `sort`, e.g.: `unset LC_ALL; LC_COLLATE=C ; comm -12 <(printf '%s\n' "\${A[@]}" | sort) <(printf '%s\n' "\${B[@]}" | sort)` – Sorpigal Dec 8 '18 at 18:08

You can get all elements that are in both A and B by looping through both arrays and comparing:

``````A=(vol-175a3b54 vol-382c477b vol-8c027acf vol-93d6fed0 vol-71600106 vol-79f7970e vol-e3d6a894 vol-d9d6a8ae vol-8dbbc2fa vol-98c2bbef vol-ae7ed9e3 vol-5540e618 vol-9e3bbed3 vol-993bbed4 vol-a83bbee5 vol-ff52deb2)
B=(vol-175a3b54 vol-e38d0c94 vol-2a19386a vol-b846c5cf vol-98c2bbef vol-7320102b vol-8f6226cc vol-27991850 vol-71600106 vol-615e1222)

intersections=()

for item1 in "\${A[@]}"; do
for item2 in "\${B[@]}"; do
if [[ \$item1 == "\$item2" ]]; then
intersections+=( "\$item1" )
break
fi
done
done

printf '%s\n' "\${intersections[@]}"
``````

You can get all elements in B but not in A in a similar manner:

``````A=(vol-175a3b54 vol-382c477b vol-8c027acf vol-93d6fed0 vol-71600106 vol-79f7970e vol-e3d6a894 vol-d9d6a8ae vol-8dbbc2fa vol-98c2bbef vol-ae7ed9e3 vol-5540e618 vol-9e3bbed3 vol-993bbed4 vol-a83bbee5 vol-ff52deb2)
B=(vol-175a3b54 vol-e38d0c94 vol-2a19386a vol-b846c5cf vol-98c2bbef vol-7320102b vol-8f6226cc vol-27991850 vol-71600106 vol-615e1222)

not_in_a=()

for item1 in "\${B[@]}"; do
for item2 in "\${A[@]}"; do
[[ \$item1 == "\$item2" ]] && continue 2
done

# If we reached here, nothing matched.
not_in_a+=( "\$item1" )
done

printf '%s\n' "\${not_in_a[@]}"
``````
• Exercise: if you interchange `A` and `B`, is `intersections` always the same up to reordering? – Gilles 'SO- stop being evil' Dec 12 '13 at 22:39
• @Gilles If the arrays may contain duplicate elements, no. – Chris Down Dec 14 '13 at 5:04

There is rather elegant and efficient approach to do that, using `uniq` — but, we will need to eliminate duplicates from each array, leaving only unique items. If you want to save duplicates, there is only one way "by looping through both arrays and comparing".

Consider we have two arrays:

``````A=(vol-175a3b54 vol-382c477b vol-8c027acf vol-93d6fed0 vol-71600106 vol-79f7970e vol-e3d6a894 vol-d9d6a8ae vol-8dbbc2fa vol-98c2bbef vol-ae7ed9e3 vol-5540e618 vol-9e3bbed3 vol-993bbed4 vol-a83bbee5 vol-ff52deb2)
B=(vol-175a3b54 vol-e38d0c94 vol-2a19386a vol-b846c5cf vol-98c2bbef vol-7320102b vol-8f6226cc vol-27991850 vol-71600106 vol-615e1222)
``````

First of all, lets transform these arrays into sets. We will do it because there is mathematical operation intersection which is known like intersection of sets, and set is a collection of distinct objects, distinct or unique. To be honest, I don't know what is "intersection" if we speak about lists or sequences. Though we can pick out a subsequence from sequence, but this operation (selection) has slightly different meaning.

So, lets transform!

``````\$ A=(echo \${A[@]} | sed 's/ /\n/g' | sort | uniq)
\$ B=(echo \${B[@]} | sed 's/ /\n/g' | sort | uniq)
``````
1. Intersection:

``````\$ echo \${A[@]} \${B[@]} | sed 's/ /\n/g' | sort | uniq -d
``````

If you want to store the elements in another array:

``````\$ intersection_set=\$(echo \${A[@]} \${B[@]} | sed 's/ /\n/g' | sort | uniq -d)

\$ echo \$intersection_set
vol-175a3b54 vol-71600106 vol-98c2bbef
``````

`uniq -d` means show only duplicates (I think, `uniq` is rather fast because of its realisation: I guess that it is done with `XOR` operation).

2. Get the list of elements that appear in `B` and are not available in `A`, i.e. `B\A`

``````\$ echo \${A[@]} \${B[@]} | sed 's/ /\n/g' | sort | uniq -d | xargs echo \${B[@]} | sed 's/ /\n/g' | sort | uniq -u
``````

Or, with saving in a variable:

``````\$ subtraction_set=\$(echo \${A[@]} \${B[@]} | sed 's/ /\n/g' | sort | uniq -d | xargs echo \${B[@]} | sed 's/ /\n/g' | sort | uniq -u)

\$ echo \$subtraction_set
vol-27991850 vol-2a19386a vol-615e1222 vol-7320102b vol-8f6226cc vol-b846c5cf vol-e38d0c94
``````

Thus, at first we have got intersection of `A` and `B` (which is simply the set of duplicates between them), say it is `A/\B`, and then we used operation of inverting intersection of `B` and `A/\B` (which is simply only unique elements), so we get `B\A = ! (B /\ (A/\B))`.

P.S. `uniq` was written by Richard M. Stallman and David MacKenzie.

Ignoring efficiency, here is an approach:

``````declare -a intersect
declare -a b_only
for bvol in "\${B[@]}"
do
in_both=""
for avol in "\${A[@]}"
do
[ "\$bvol" = "\$avol" ] && in_both=Yes
done
if [ "\$in_both" ]
then
intersect+=("\$bvol")
else
b_only+=("\$bvol")
fi
done
echo "intersection=\${intersect[*]}"
echo "In B only=\${b_only[@]}"
``````

### My pure bash way

As this variables contain only `vol-XXX` where `XXX` is an hexadecimal number, there is a quick way using bash arrays

``````unset A B a b c i                    # Only usefull for re-testing...

A=(vol-175a3b54 vol-382c477b vol-8c027acf vol-93d6fed0 vol-71600106 vol-79f7970e
vol-e3d6a894 vol-d9d6a8ae vol-8dbbc2fa vol-98c2bbef vol-ae7ed9e3 vol-5540e618
vol-9e3bbed3 vol-993bbed4 vol-a83bbee5 vol-ff52deb2)
B=(vol-175a3b54 vol-e38d0c94 vol-2a19386a vol-b846c5cf vol-98c2bbef vol-7320102b
vol-8f6226cc vol-27991850 vol-71600106 vol-615e1222)

for i in \${A[@]#vol-};do
[ "\${a[\$((16#\$i))]}" ] && echo Duplicate vol-\$i in A
((a[\$((16#\$i))]++))
((c[\$((16#\$i))]++))
done
for i in \${B[@]#vol-};do
[ "\${b[\$((16#\$i))]}" ] && echo Duplicate vol-\$i in B
((b[\$((16#\$i))]++))
[ "\${c[\$((16#\$i))]}" ] && echo Present in A and B: vol-\$i
((c[\$((16#\$i))]++))
done
``````

This must output:

``````Present in A and B vol-175a3b54
Present in A and B vol-98c2bbef
Present in A and B vol-71600106
``````

At this state, you bash environment contain:

``````set | grep ^c=
c=(="2" ="1" ="1" ="1" ="1"
="1" ="2" ="1" ="1" ="1"
="1" ="1" ="1" ="2" ="1"
="1" ="1" ="1" ="1" ="1"
="1" ="1" ="1")
``````

So you could:

``````for i in \${!b[@]};do
[ \${c[\$i]} -eq 1 ] &&
printf "Present only in B: vol-%8x\n" \$i
done
``````

This will render:

``````Present only in B: vol-27991850
Present only in B: vol-2a19386a
Present only in B: vol-615e1222
Present only in B: vol-7320102b
Present only in B: vol-8f6226cc
Present only in B: vol-b846c5cf
Present only in B: vol-e38d0c94
``````

But this is numericaly sorted! If you want original order, you could:

``````for i in \${B[@]#vol-};do
[ \${c[((16#\$i))]} -eq 1 ] && printf "Present in B only: vol-%s\n" \$i
done
``````

So you dislay vols in same order as submited:

``````Present in B only: vol-e38d0c94
Present in B only: vol-2a19386a
Present in B only: vol-b846c5cf
Present in B only: vol-7320102b
Present in B only: vol-8f6226cc
Present in B only: vol-27991850
Present in B only: vol-615e1222
``````

or

``````for i in \${!a[@]};do
[ \${c[\$i]} -eq 1 ] && printf "Present only in A: vol-%8x\n" \$i
done
``````

for showing only in A:

``````Present only in A: vol-382c477b
Present only in A: vol-5540e618
Present only in A: vol-79f7970e
Present only in A: vol-8c027acf
Present only in A: vol-8dbbc2fa
Present only in A: vol-93d6fed0
Present only in A: vol-993bbed4
Present only in A: vol-9e3bbed3
Present only in A: vol-a83bbee5
Present only in A: vol-ae7ed9e3
Present only in A: vol-d9d6a8ae
Present only in A: vol-e3d6a894
Present only in A: vol-ff52deb2
``````

or even:

``````for i in \${!b[@]};do
[ \${c[\$i]} -eq 2 ] && printf "Present in both A and B: vol-%8x\n" \$i
done
``````

will re-print:

``````Present in both A and B: vol-175a3b54
Present in both A and B: vol-71600106
Present in both A and B: vol-98c2bbef
``````
• Of course, if `Duplicate` lines are useless, they could simply be dropped. – F. Hauri Dec 12 '13 at 12:49