How to produce cartesian product in bash?

Question

I want to produce such file (cartesian product of [1-3]X[1-5]):

1 1
1 2
1 3
1 4
1 5
2 1
2 2
2 3
2 4
2 5
3 1
3 2
3 3
3 4
3 5

I can do this using nested loop like:

for i in $(seq 3) 
do
  for j in $(seq 5)
  do
      echo $i $j
  done
done

is there any solution without loops?

Answer 1

The best alternative for cartesian product in bash is surely -- as pointed by @fedorqui -- to use parameter expansion. However, in case your input that is not easily producible (i.e., if {1..3} and {1..5} does not suffice), you could simply use join.

For example, if you want to peform the cartesian product of two regular files, say "a.txt" and "b.txt", you could do the following. First, the two files:

$ echo -en {a..c}"\tx\n" | sed 's/^/1\t/' > a.txt
$ cat a.txt
1    a    x
1    b    x
1    c    x

$ echo -en "foo\nbar\n" | sed 's/^/1\t/' > b.txt
$ cat b.txt
1    foo
1    bar

Notice the sed command is used to prepend each line with an identifier. The identifier must be the same for all lines, and for all files, so the join will give you the cartesian product -- instead of putting aside some of the resultant lines. So, the join goes as follows:

$ join -j 1 -t $'\t' a.txt b.txt | cut -d $'\t' -f 2-
a    x    foo
a    x    bar
b    x    foo
b    x    bar
c    x    foo
c    x    bar

After both files are joined, cut is used as an alternative to remove the column of "1"s formerly prepended.