Bash - reverse an array

Question

Is there a simple way to reverse an array?

#!/bin/bash

array=(1 2 3 4 5 6 7)

echo "${array[@]}"

so I would get: 7 6 5 4 3 2 1
instead of: 1 2 3 4 5 6 7

Answer 1

Another unconventional approach:

#!/bin/bash

array=(1 2 3 4 5 6 7)

f() { array=("${BASH_ARGV[@]}"); }

shopt -s extdebug
f "${array[@]}"
shopt -u extdebug

echo "${array[@]}"

Output:

7 6 5 4 3 2 1

If extdebug is enabled, array BASH_ARGV contains in a function all positional parameters in reverse order.

Answer 2

Unconventional approach (all not pure bash):

• if all elements in an array are just one characters (like in the question) you can use rev:

  echo "${array[@]}" | rev
  

• otherwise:

  printf '%s\n' "${array[@]}" | tac | tr '\n' ' '; echo
  

• and if you can use zsh:

  echo ${(Oa)array}
  

Answer 3

I have answered the question as written, and this code reverses the array. (Printing the elements in reverse order without reversing the array is just a for loop counting down from the last element to zero.) This is a standard "swap first and last" algorithm.

array=(1 2 3 4 5 6 7)

min=0
max=$(( ${#array[@]} -1 ))

while [[ min -lt max ]]
do
    # Swap current first and last elements
    x="${array[$min]}"
    array[$min]="${array[$max]}"
    array[$max]="$x"

    # Move closer
    (( min++, max-- ))
done

echo "${array[@]}"

It works for arrays of odd and even length.

Answer 4

If you actually want the reverse in another array:

reverse() {
    # first argument is the array to reverse
    # second is the output array
    declare -n arr="$1" rev="$2"
    for i in "${arr[@]}"
    do
        rev=("$i" "${rev[@]}")
    done
}

Then:

array=(1 2 3 4)
reverse array foo
echo "${foo[@]}"

Gives:

4 3 2 1

This should correctly handle cases where an array index is missing, say you had array=([1]=1 [2]=2 [4]=4), in which case looping from 0 to the highest index may add additional, empty, elements.

Answer 5

To swap the array positions in place (even with sparse arrays)(since bash 3.0):

#!/bin/bash
# Declare an sparse array to test:
array=([5]=101 [6]=202 [10]=303 [11]=404 [20]=505 [21]=606 [40]=707)
echo "Initial array values"
declare -p array

swaparray(){ local temp; temp="${array[$1]}"
             array[$1]="${array[$2]}"
             array[$2]="$temp"
           }

ind=("${!array[@]}")                         # non-sparse array of indexes.

min=-1; max="${#ind[@]}"                     # limits to one before real limits.
while [[ min++ -lt max-- ]]                  # move closer on each loop.
do
    swaparray "${ind[min]}" "${ind[max]}"    # Exchange first and last
done

echo "Final Array swapped in place"
declare -p array
echo "Final Array values"
echo "${array[@]}"

On execution:

./script
Initial array values
declare -a array=([5]="101" [6]="202" [10]="303" [11]="404" [20]="505" [21]="606" [40]="707")

Final Array swapped in place
declare -a array=([5]="707" [6]="606" [10]="505" [11]="404" [20]="303" [21]="202" [40]="101")

Final Array values
707 606 505 404 303 202 101

For older bash, you need to use a loop (in bash (since 2.04)) and using $a to avoid the trailing space:

#!/bin/bash

array=(101 202 303 404 505 606 707)
last=${#array[@]}

a=""
for (( i=last-1 ; i>=0 ; i-- ));do
    printf '%s%s' "$a" "${array[i]}"
    a=" "
done
echo

For bash since 2.03:

#!/bin/bash
array=(101 202 303 404 505 606 707)
last=${#array[@]}

a="";i=0
while [[ last -ge $((i+=1)) ]]; do 
    printf '%s%s' "$a" "${array[ last-i ]}"
    a=" "
done
echo

Also (using the bitwise negation operator) (since bash 4.2+):

#!/bin/bash
array=(101 202 303 404 505 606 707)
last=${#array[@]}

a=""
for (( i=0 ; i<last ; i++ )); do 
    printf '%s%s' "$a" "${array[~i]}"
    a=" "
done
echo

Answer 6

Ugly, unmaintainable, but one-liner:

eval eval echo "'\"\${array['{$((${#array[@]}-1))..0}']}\"'"

Answer 7

To reverse an arbitrary array (which may contain any number of elements with any values):

With zsh:

array_reversed=("${(@Oa)array}")

With bash 4.4+, given that bash variables can't contain NUL bytes anyway, you can use GNU tac -s '' on the elements printed as NUL delimited records:

readarray -td '' array_reversed < <(
  ((${#array[@]})) && printf '%s\0' "${array[@]}" | tac -s '')

Note however that bash arrays were inspired from ksh arrays instead of csh/zsh arrays, and are more like associative arrays with keys limited to positive integers (so called sparse arrays), and that method doesn't preserve the keys of the arrays. For instance, for an array like:

array=( [3]=a [12]=b [42]=c )

You get

array_reversed=( [0]=c [1]=b [2]=a )

POSIXly, to reverse the one and only POSIX shell array ($@, made of $1, $2...) in place:

code='set --'
n=$#
while [ "$n" -gt 0 ]; do
  code="$code \"\${$n}\""
  n=$((n - 1))
done
eval "$code"

Answer 8

Pure bash solution, would work as a one-liner.

$: for (( i=${#array[@]}-1; i>=0; i-- ))
>  do rev[${#rev[@]}]=${array[i]}
>  done
$: echo  "${rev[@]}"
7 6 5 4 3 2 1

Answer 9

#!/bin/bash
(a=(1 2 3 4 5) r=(); for e in "${a[@]}"; do r=("$e" "${r[@]}"); done; declare -p a r)

prints

declare -a a=([0]="1" [1]="2" [2]="3" [3]="4" [4]="5")
declare -a r=([0]="5" [1]="4" [2]="3" [3]="2" [4]="1")

Answer 10

Try this

#!/bin/bash

array=(1 2 3 4 5 6)
index=$((${#array[@]}-1))

for e in "${array[@]}"; do
  result[$((index--))]="$e"
done

echo "${result[@]}"

Answer 11

According to TIMTOWDI (There Is More Than One Way To Do It), here is my solution, reversing array a into r:

#!/bin/bash
set -u
a=(1 2 3)
t=("${a[@]}")
declare -p a t
r=()
while [ "${#t[@]}" -gt 0 ]
do
    r+=("${t[-1]}")
    unset 't[-1]'
done
echo "${r[@]}"
declare -p r

When executing with BASH "4.4.23(1)-release (x86_64-suse-linux-gnu)" I got:

+ set -u
+ a=(1 2 3)
+ t=("${a[@]}")
+ declare -p a t
declare -a a=([0]="1" [1]="2" [2]="3")
declare -a t=([0]="1" [1]="2" [2]="3")
+ r=()
+ '[' 3 -gt 0 ']'
+ r+=("${t[-1]}")
+ unset 't[-1]'
+ '[' 2 -gt 0 ']'
+ r+=("${t[-1]}")
+ unset 't[-1]'
+ '[' 1 -gt 0 ']'
+ r+=("${t[-1]}")
+ unset 't[-1]'
+ '[' 0 -gt 0 ']'
+ echo 3 2 1
3 2 1
+ declare -p r
declare -a r=([0]="3" [1]="2" [2]="1")

Answer 12

Two different versions

Inspired from Cyrus's answer and wiki.wooledge.org. Very quick as there are no loop and no forks! ... On not too large bunch of data!!, regarding Stéphane Chazelas's comment, if you plan to manipulate big bunch of datas, you'd better mandate specialised tools!

And here, confined into one single function.

Print submited array in reversed order:

printReverseArray () {
    if shopt -q extdebug; then
        printf "%s " "${BASH_ARGV[@]}"
    else
        shopt -s extdebug
        "${FUNCNAME}" "$@"
        shopt -u extdebug
    fi
}

Sample run:

printReverseArray world! good Hello
Hello good world!

printReverseArray baz "Foo bar"
Foo bar baz

Reverse array variable:

reverseArray() { 
    if shopt -q extdebug; then
        _ArrayToReverse=("${BASH_ARGV[@]}")
    else
        local -n _ArrayToReverse=$1
        shopt -s extdebug
        "${FUNCNAME}" "${_ArrayToReverse[@]}"
        shopt -u extdebug
    fi
}

Then

myArray=({a..d}{1,2})
echo ${myArray[@]}
a1 a2 b1 b2 c1 c2 d1 d2

reverseArray myArray
echo ${myArray[@]}
d2 d1 c2 c1 b2 b1 a2 a1

Answer 13

Bash

array=(1 2 3 4 5 6 7)
echo "${array[@]} " | tac -s ' '

Or

array=(1 2 3 4 5 6 7)
reverse=$(echo "${array[@]} " | tac -s ' ')
echo ${reverse[@]}

Result

7 6 5 4 3 2 1

Version

$ tac --version
tac (GNU coreutils) 8.28

Answer 14

You can also consider using seq:

array=(1 2 3 4 5 6 7)

for i in $(seq $((${#array[@]} - 1)) -1 0); do
    echo "${array[$i]}"
done

In FreeBSD you can omit the -1 increment parameter:

for i in $(seq $((${#array[@]} - 1)) 0); do
    echo "${array[$i]}"
done