Unique possible combinations bash

unique_combinations.sh

#!/usr/bin/env bash

set -euo pipefail

function unique_combinations() {
    # Returns all unique possible combinations
    # of a collection of elements of a given length, 
    # to return all possible unique combinations, 
    # set the last argument equal to the length of 
    # the input collection, see example below: 
    # @Input $1 to (N-1): Set of items to permutate
    # @Input $N: Cardinality of the unique combination sets
    # @Example: 
    # $ unique_combinations A B C 3
    # > A
    # > B
    # > C
    # > A B
    # > A C
    # > B C
    # > A B C
    # Check if GNU parallel is installed
    command -V parallel &> /dev/null || \
        { cat <<< 'Error: missing GNU parallel, exiting now!' 1>&2; return 1; }
    # Get all the args except the last
    items=("${@:1:$(($#-1))}")
    # bash get length of an array
    n="${@: -1}" 
    ( for i in $(seq 1 $n);  do
        # Building GNU parallel 
        # commands to generate
        # unique combinations 
        # of cardinality N
        echo -ne "\nparallel --plus echo {choose_k}"
        for j in $(seq 1 $i); do 
            echo -n " ::: ${items[@]}";  
        done
    # Add a trailing newline
    # char, remove empty line,
    # exec the GNU parallel cmd
    done; echo ) | awk 'NF' | bash
}

Python-based implementation

This python-based implementation is orders of magnitude (20-30x times) faster than the GNU parallel implementation above.

#!/usr/bin/env bash

set -euo pipefail

function unique_combinations() {
    # Returns all unique possible combinations
    # of a collection of elements of a given length, 
    # to return all possible unique combinations, 
    # set the last argument equal to the length of 
    # the input collection, see example below: 
    # @Input $1 to (N-1): Set of items to permutate
    # @Input $N: Cardinality of the unique combination sets, can include a range or a set number
    # @Example: 
    # $ unique_combinations A B C 2-3
    # > A B
    # > A C
    # > B C
    # > A B C
    # Check if python is installed
    command -V python &> /dev/null || \
        { cat <<< 'Error: missing python, exiting now!' 1>&2; return 1; }
        
cat << EOF | python /dev/stdin "$@"
from __future__ import print_function
import sys

def unique_n_len_combinations(items, n):
    if n==0: yield []
    else:
      for i in range(len(items)):
            for cc in unique_n_len_combinations(items[i+1:], n-1):
                yield [str(items[i])] + cc

# Parse command line arguments
s = sys.argv[1:-1]
n = sys.argv[-1]
range_len = len([ _ for _ in n.split('-') if _ ])
# Check if range is specified
start = 0 if '-' not in n else int(n.split('-')[0])-1
stop = int(n) if '-' not in n \
else len(s) if range_len==1 else int(n.split('-')[1])
# Generate unique combinations
# differing set lengths
for i in range(start,stop):
    for comb in unique_n_len_combinations(s, i+1):
        print(' '.join(comb))
EOF
}