Cdtjkyj

If I have an array with 5 elements, for example:

[a][b][c][d][e]

Using echo ${myarray[4]} I can see what it holds.

But what if I didn't know the number of elements in a given array? Is there a way of reading the last element of an unknown length array?
i.e. The first element reading from the right to the left for any array?

I would like to know how to do this in bash.

You can just use a negative index ${myarray[-1]} to get the last element. You can do the same thing for the second-last, and so on; in Bash:

If the subscript used to reference an element of an indexed array
evaluates to a number less than zero, it is interpreted as relative to
one greater than the maximum index of the array, so negative indices
count back from the end of the array, and an index of -1 refers to the
last element.

The same also works for assignment. When it says "expression" it really means an expression; you can write in any arithmetic expression there to compute the index, including one that computes using the length of the array ${#myarray[@]} explicitly.

Modern bash (v4.1 or better)

You can read the last element at index -1:

$ a=(a b c d e f)

$ echo ${a[-1]}

f

Support for accessing numerically-indexed arrays from the end using negative indexes started with bash version 4.1-alpha.

Older bash (v4.0 or earlier)

You must get the array length from ${#a[@]} and then subtract one to get the last element:

$ echo ${a[${#a[@]}-1]}

f

Since bash treats array subscripts as an arithmetic expression, there is no need for additional notation, such as $((...)), to force arithmetic evaluation.

bash array assignment, reference, unsetting with negative index were only added in bash 4.3. With older version of bash, you can use expression in index array[${#array[@]-1}]

Another way, also work with older version of bash (bash 3.0 or better):

$ a=([a] [b] [c] [d] [e])

$ printf %s\n "${a[@]:(-1)}"

[e]

or:

$ printf %s\n "${a[@]: -1}"

[e]

Using negative offset, you need to separate : with - to avoid being confused with the :- expansion.

Also you can do this:

$ a=(a b c d e f)

$ echo ${a[$(expr ${#a[@]} - 1)]}

Result:

$ f

What you're doing is getting all the count of elements in the array and subtract -1 due you're getting all the elements, not starting from the array index that is 0.