Unshift array element in Bash script

Question

I would like to add elements to the beginning of an array instead of to the end. Is this possible in Bash?

Answer 1

If your array is contiguous, you can use the "${array[@]}" syntax to construct a new array:

array=('a' 'b' 'c');
echo "${array[@]}"; # prints: a b c
array=('d' "${array[@]}");
echo "${array[@]}"; # prints: d a b c

As chepner mentions, the above method will collapse indices of sparse arrays:

array=([5]='b' [10]='c');
declare -p array; # prints: declare -a array='([5]="b" [10]="c")'
array=('a' "${array[@]}");
declare -p array; # prints: declare -a array='([0]="a" [1]="b" [2]="c")'

(Fun fact: PHP does that too - but then again, it's PHP :P)

If you need to work with sparse arrays, you can iterate over the indices of the array manually (${!array[@]}) and increase them by one (with $((...+1))):

old=([5]='b' [10]='c');
new=('a');
for i in "${!old[@]}"; do
    new["$(($i+1))"]="${old[$i]}";
done;
declare -p new; # prints: declare -a new='([0]="a" [6]="b" [11]="c")'

Answer 2

Non-bash version: POSIX shells don't really have arrays, excepting shell parameters, (i.e. $1, $2, $3, ...),but for those parameters this should work:

set - a b c ; echo $1 $3

Output:

a c

Now add "foo" to the beginning:

set - foo "$@" ; echo $1 $3

Output:

foo b