Linux: Search for coincidences in four different files - bash

Question

Scenario: Four files with 300 lines on each one. I want to know which lines are in all four files using bash only (no perl/python/ruby please)
Quick sample
$cat bad_domains.urlvoid
a
b
c
d
e
$cat bad_domains.alienvault
f
g
a
c
h
$cat bad_domains.hphosts
i
j
k
a
h
$cat bad_domains.malwaredomain
l
b
m
f
a
j
I only want to match the "a" i tried with stuff like this but it's slow as hell:
for void in $(cat bad_domains.urlvoid)
do
for vault in $(cat bad_domains.alienvault)
do
for hphosts in $(cat bad_domains.hphosts)
do
for malwaredomain in $(cat bad_domains.malwaredomain)
do
if [ $void == $vault -a $void == $hphosts -a $void == $malwaredomain -a $vault == $hphosts -a $vault == $malwaredomain -a $hphosts == $malwaredomain ]
then
echo $void
fi
done
done
done
done
Any good tips for optimizing my code? I read something about Dichotomic search that maybe could work.

Answer 1

Using comm:
comm -12 <(awk 'FNR==NR{a[$0];next} $0 in a' f1 f2) <(awk 'FNR==NR{a[$0];next} $0 in a' f3 f4)
a
Which works using these 3 steps:
Get common strings from file1 and file2
Get common strings from file3 and file4
Get common strings from above 2 steps thus getting intersection of 4 sets
EDIT: Pure awk solution:
awk 'FNR==NR{a[$0];next} $0 in a' <(awk 'FNR==NR{a[$0];next} $0 in a' f1 f2) <(awk 'FNR==NR{a[$0];next} $0 in a' f3 f4)

Answer 2

If the lines are unique within each file:
cat file1 file2 file3 file4 | sort | uniq -c | grep '^ *4 '

Answer 3

For bash 4.x (and ksh93)
Create an associative array indexed by the lines of one of the files (master).
For each of the remaining files, create a second array (work) indexed by the file's lines, then iterate over the master
array removing any entry with a key which does not also appear in the work array.
Any keys left in master[] after processing must have been in all files.
list=( bad_domains.* )
typeset -A master
while IFS= read -r key ; do master[$key]=1 ; done < "${list[0]}"
unset list[0]
for file in "${list[#]}" ; do
typeset -A work
while IFS= read -r key ; do work[$key]=1 ; done < "$file"
for key in "${!master[#]}" ; do [[ ${work[$key]+set} = set ]] || unset master[$key] ; done
unset work
done
for key in "${!master[#]}" ; do printf '%s\n' "$key" ; done