#!/bin/bash
# neg-array.sh
# Requires Bash, version -ge 4.2.

array=( zero one two three four five )   # Six-element array.
#         0    1   2    3    4    5
#        -6   -5  -4   -3   -2   -1

# Negative array indices now permitted.
echo ${array[-1]}   # five
echo ${array[-2]}   # four
# ...
echo ${array[-6]}   # zero
# Negative array indices count backward from the last element+1.

# But, you cannot index past the beginning of the array.
echo ${array[-7]}   # array: bad array subscript


# So, what is this new feature good for?

echo "The last element in the array is "${array[-1]}""
# Which is quite a bit more straightforward than:
echo "The last element in the array is "${array[${#array[*]}-1]}""
echo

# And ...

index=0
let "neg_element_count = 0 - ${#array[*]}"
# Number of elements, converted to a negative number.

while [ $index -gt $neg_element_count ]; do
  ((index--)); echo -n "${array[index]} "
done  # Lists the elements in the array, backwards.
      # We have just simulated the "tac" command on this array.

echo

# See also neg-offset.sh.