Remove an element from a Bash array

Question

I need to remove an element from an array in bash shell. Generally I'd simply do:

array=("${(@)array:#<element to remove>}")

Unfortunately the element I want to remove is a variable so I can't use the previous command. Down here an example:

array+=(pluto)
array+=(pippo)
delete=(pluto)
array( ${array[@]/$delete} ) -> but clearly doesn't work because of {}

Any idea?

Answer 1

This answer is specific to the case of deleting multiple values from large arrays, where performance is important.

The most voted solutions are (1) pattern substitution on an array, or (2) iterating over the array elements. The first is fast, but can only deal with elements that have distinct prefix, the second has O(n*k), n=array size, k=elements to remove. Associative array are relative new feature, and might not have been common when the question was originally posted.

For the exact match case, with large n and k, possible to improve performance from O(nk) to O(n+klog(k)). In practice, O(n) assuming k much lower than n. Most of the speed up is based on using associative array to identify items to be removed.

Performance (n-array size, k-values to delete). Performance measure seconds of user time

   N     K     New(seconds) Current(seconds)  Speedup
 1000   10     0.005        0.033             6X
10000   10     0.070        0.348             5X
10000   20     0.070        0.656             9X
10000    1     0.043        0.050             -7%

As expected, the current solution is linear to N*K, and the fast solution is practically linear to K, with much lower constant. The fast solution is slightly slower vs the current solution when k=1, due to additional setup.

The 'Fast' solution: array=list of input, delete=list of values to remove.

        declare -A delk
        for del in "${delete[@]}" ; do delk[$del]=1 ; done
                # Tag items to remove, based on
        for k in "${!array[@]}" ; do
                [ "${delk[${array[$k]}]-}" ] && unset 'array[k]'
        done
                # Compaction
        array=("${array[@]}")

Benchmarked against current solution, from the most-voted answer.

    for target in "${delete[@]}"; do
        for i in "${!array[@]}"; do
            if [[ ${array[i]} = $target ]]; then
                unset 'array[i]'
            fi
        done
    done
    array=("${array[@]}")

Answer 2

This is a quick-and-dirty solution that will work in simple cases but will break if (a) there are regex special characters in $delete, or (b) there are any spaces at all in any items. Starting with:

array+=(pluto)
array+=(pippo)
delete=(pluto)

Delete all entries exactly matching $delete:

array=(`echo $array | fmt -1 | grep -v "^${delete}$" | fmt -999999`)

resulting in echo $array -> pippo, and making sure it's an array: echo $array[1] -> pippo

fmt is a little obscure: fmt -1 wraps at the first column (to put each item on its own line. That's where the problem arises with items in spaces.) fmt -999999 unwraps it back to one line, putting back the spaces between items. There are other ways to do that, such as xargs.

Addendum: If you want to delete just the first match, use sed, as described here:

array=(`echo $array | fmt -1 | sed "0,/^${delete}$/{//d;}" | fmt -999999`)

Answer 3

There is also this syntax, e.g. if you want to delete the 2nd element :

array=("${array[@]:0:1}" "${array[@]:2}")

which is in fact the concatenation of 2 tabs. The first from the index 0 to the index 1 (exclusive) and the 2nd from the index 2 to the end.

Answer 4

To avoid conflicts with array index using unset - see https://stackoverflow.com/a/49626928/3223785 and https://stackoverflow.com/a/47798640/3223785 for more information - reassign the array to itself: ARRAY_VAR=(${ARRAY_VAR[@]}).

#!/bin/bash

ARRAY_VAR=(0 1 2 3 4 5 6 7 8 9)
unset ARRAY_VAR[5]
unset ARRAY_VAR[4]
ARRAY_VAR=(${ARRAY_VAR[@]})
echo ${ARRAY_VAR[@]}
A_LENGTH=${#ARRAY_VAR[*]}
for (( i=0; i<=$(( $A_LENGTH -1 )); i++ )) ; do
    echo ""
    echo "INDEX - $i"
    echo "VALUE - ${ARRAY_VAR[$i]}"
done

exit 0

[Ref.: https://tecadmin.net/working-with-array-bash-script/ ]

Answer 5

If anyone finds themselves in a position where they need to remember set -e or set -x values and be able to restore them, please check out this gist which uses the first array deletion solution to manage it's own stack:

https://gist.github.com/kigster/94799325e39d2a227ef89676eed44cc6

Answer 6

How about something like:

array=(one two three)
array_t=" ${array[@]} "
delete=one
array=(${array_t// $delete / })
unset array_t