sliced arrays flatten to len=1?-CodePudding

I am pulling my hair out manipulating arrays in bash. I have an array of strings, which contain spaces. I would like an array containing all but the first element of my input array.

input=("first string" "second string" "third string")
echo ${#input[@]}
# len(input)=3
# get slice of all except for first element of input
slice=${input[@]:1}
echo ${#slice[@]}
# expect 2, but get 1
echo $slice
# second string third string
# slice should contain ("second string" "third string"), but instead is "second string third string"

Slicing the array clearly works to eliminate the first element, but the result appears to be a concatenation of all remaining strings, rather than an array. Is there a way to slice an array in bash and get an array as a result?

(sorry, I'm not new to bash, but I've never used it for much before, and I can't find any documentation showing why my slice is flattened)

CodePudding user response：

Indexes are reset, element 1 is now element 0:

slice=("${input[@]:1}")

Element and index are removed, the first element is now index 1, not index 0:

unset input[0]

${#slice[@]} or ${#input[@]} will now be 1 less than the previous value of ${#input[@]}. Starting out with three elements in slice, the values of "${!slice[@]}" and "${!input[@]}", will be 0 1 and 1 2 respectively (for either the first or second approach)

If you don't quote slice=("${input[@]:1}"), each array element is split on whitespace, creating many more elements.

CodePudding user response：

First off, you should always quote variable expansions. Be very wary of any solution that relies on unquoted expansions. ShellCheck.net is a great tool for catching bugs related to quoting (among many other issues).

To your specific issue, slice=${input[@]:1} does not do what you want. It defines a single scalar variable slice rather than an array, meaning the array expansion (denoted by the [@]) will first be munged into a single string using the current IFS. Here's a demo:

$ arr=(1 2 '3 4')
$ IFS=,
$ var="${arr[@]:1}"
$ echo "$var"
2,3 4

To instead declare and populate an array use the =() notation, like so:

$ var=("${arr[@]:1}")
$ printf '%s\n' "${var[@]}"
2
3 4