How to reverse array in bash onliner FOR loop?

How can I reverse the order in which I perform a for loop for a defined array

To iterate through the array I am doing this:

$ export MYARRAY=("one" "two" "three" "four")
$ for i in ${MYARRAY[@]}; do echo $i;done
one
two
three
four

Is there a function where I can reverse the order of the array?

One thought I had is to generate a sequence of inverted indexes and call the elements by using this reversed index but maybe there is a quicker alternative, or at least easier to read.

Solution 1:

You can use the C-style for loop:

for (( idx=${#MYARRAY[@]}-1 ; idx>=0 ; idx-- )) ; do
    echo "${MYARRAY[idx]}"
done

For an array with "holes", the number of elements ${#arr[@]} doesn't correspond to the index of the last element. You can create another array of indices and walk it backwards in the same way:

#! /bin/bash
arr[2]=a
arr[7]=b

echo ${#arr[@]}  # only 2!!

indices=( ${!arr[@]} )
for ((i=${#indices[@]} - 1; i >= 0; i--)) ; do
    echo "${arr[indices[i]]}"
done

Solution 2:

You can use tac, which is an opposite of cat in sense that it reverses the lines.

MYARRAY=("one" "two" "three" "four")
for item in "$MYARRAY"; do
   echo "$item"; 
done | tac

# four
# three
# two
# one

Solution 3:

_arr+=( '"${_arrev} is an actual "${array[@]}"' )  ⏎
_arr+=( '"${_arrev} is created as a result"' )
_arr+=( '"of reversing the key order in"' )
_arr+=( '"this "${_arr}. It handles zsh and"' )
_arr+=( '"bash arrays intelligently by tracking"' )
_arr+=( '"shell "$ENV." quotes=fine ( i hope ) "' )

. <<REVERSE /dev/stdin                    ⏎
    _arrev=( $(: $((l=${#_arr[@]}${ZSH_VERSION++1})) ; printf '"${_arr[$(('$l'-%d))]}" ' `seq 1 $l`) )
REVERSE

echo ; printf %s\\n ${_arrev}

"shell "$ENV." quotes=fine ( i hope ) "
"bash arrays intelligently by tracking"
"this "${_arr}. It handles zsh and"
"of reversing the key order in"
"${_arrev} is created as a result"
"${_arrev} is an actual "${array[@]}"

This should handle any possible array, I think.

If you're interested in what's going on up there, I suggest you have a look here first. Then maybe here, definitely here, and, if you've got the time, here and here.

In all of those answers I discuss different aspects of the here-document (and in many others) which you can use to your advantage. For instance I discuss twice-evaluating variables, which is done above, and in one declare a function that globally declares another function named "_$1" in just 5 or 6 lines - most of which were _$1() { func body ; }. It's pretty handy if you use it correctly.

Regarding the auto-switch between bash/zsh, well that's something else, but very simple as well. See here.

Solution 4:

How about this:

for i in `printf '%s\n' "${MYARRAY[@]}"|tac`; do echo $i; done

The output is:

four
three
two
one

Limitation: doesn't work if array contain newlines. As a workaround you may implement function:

reverse(){ reversed=();local i;for ((i=$#;i>0;i--)); do reversed+=("${!i}");done; }

and use it this way:

reverse "${MYARRAY[@]}" && for i in "${reversed[@]}"; do echo $i; done

Solution 5:

Simple as a string:

% unset c; a="1 2 3 4 5"; for b in $a; do c="$b $c"; done; echo $c

5 4 3 2 1

You sure you want array syntax??:

 % unset c; declare -a c; a=(1 2 3 4 5); i=0; for b in ${a[*]}; \
    do c[$((${#a[@]}-$i))]=$b; i=$(($i+1)); done; echo ${c[*]}

5 4 3 2 1