Add a new element to an array without specifying the index in Bash
BashArraysBash Problem Overview
Is there a way to do something like PHPs $array[] = 'foo';
in bash vs doing:
array[0]='foo'
array[1]='bar'
Bash Solutions
Solution 1 - Bash
Yes there is:
ARRAY=()
ARRAY+=('foo')
ARRAY+=('bar')
> In the context where an assignment statement is assigning a value to a shell variable or array index (see Arrays), the ‘+=’ operator can be used to append to or add to the variable's previous value.
Also: > When += is applied to an array variable using compound assignment (see Arrays below), the variable's value is not unset (as it is when using =), and new values are appended to the array beginning at one greater than the array's maximum index (for indexed arrays)
Solution 2 - Bash
As Dumb Guy points out, it's important to note whether the array starts at zero and is sequential. Since you can make assignments to and unset non-contiguous indices ${#array[@]}
is not always the next item at the end of the array.
$ array=(a b c d e f g h)
$ array[42]="i"
$ unset array[2]
$ unset array[3]
$ declare -p array # dump the array so we can see what it contains
declare -a array='([0]="a" [1]="b" [4]="e" [5]="f" [6]="g" [7]="h" [42]="i")'
$ echo ${#array[@]}
7
$ echo ${array[${#array[@]}]}
h
Here's how to get the last index:
$ end=(${!array[@]}) # put all the indices in an array
$ end=${end[@]: -1} # get the last one
$ echo $end
42
That illustrates how to get the last element of an array. You'll often see this:
$ echo ${array[${#array[@]} - 1]}
g
As you can see, because we're dealing with a sparse array, this isn't the last element. This works on both sparse and contiguous arrays, though:
$ echo ${array[@]: -1}
i
Solution 3 - Bash
$ declare -a arr
$ arr=("a")
$ arr=("${arr[@]}" "new")
$ echo ${arr[@]}
a new
$ arr=("${arr[@]}" "newest")
$ echo ${arr[@]}
a new newest
Solution 4 - Bash
If your array is always sequential and starts at 0, then you can do this:
array[${#array[@]}]='foo'
# gets the length of the array
${#array_name[@]}
If you inadvertently use spaces between the equal sign:
array[${#array[@]}] = 'foo'
Then you will receive an error similar to:
array_name[3]: command not found
Solution 5 - Bash
With an indexed array, you can to something like this:
declare -a a=()
a+=('foo' 'bar')