How to slice an array in Bash

Question

Looking the "Array" section in the bash(1) man page, I didn't find a way to slice an array.

So I came up with this overly complicated function:

#!/bin/bash

# @brief: slice a bash array
# @arg1:  output-name
# @arg2:  input-name
# @args:  seq args
# ----------------------------------------------
function slice() {
   local output=$1
   local input=$2
   shift 2
   local indexes=$(seq $*)

   local -i i
   local tmp=$(for i in $indexes 
                 do echo "$(eval echo \"\${$input[$i]}\")" 
               done)

   local IFS=$'\n'
   eval $output="( \$tmp )"
}

Used like this:

$ A=( foo bar "a  b c" 42 )
$ slice B A 1 2
$ echo "${B[0]}"  # bar
$ echo "${B[1]}"  # a  b c

Is there a better way to do this?

Answer 1

See the Parameter Expansion section in the Bash man page. A[@] returns the contents of the array, :1:2 takes a slice of length 2, starting at index 1.

A=( foo bar "a  b c" 42 )
B=("${A[@]:1:2}")
C=("${A[@]:1}")       # slice to the end of the array
echo "${B[@]}"        # bar a  b c
echo "${B[1]}"        # a  b c
echo "${C[@]}"        # bar a  b c 42
echo "${C[@]: -2:2}"  # a  b c 42 # The space before the - is necesssary

Note that the fact that a b c is one array element (and that it contains an extra space) is preserved.

Answer 2

There is also a convenient shortcut to get all elements of the array starting with specified index. For example "${A[@]:1}" would be the "tail" of the array, that is the array without its first element.

version=4.7.1
A=( ${version//\./ } )
echo "${A[@]}"    # 4 7 1
B=( "${A[@]:1}" )
echo "${B[@]}"    # 7 1

Answer 3

Array slicing like in Python (From the rebash library):

array_slice() {
    local __doc__='
    Returns a slice of an array (similar to Python).

    From the Python documentation:
    One way to remember how slices work is to think of the indices as pointing
    between elements, with the left edge of the first character numbered 0.
    Then the right edge of the last element of an array of length n has
    index n, for example:
    ```
    +---+---+---+---+---+---+
    | 0 | 1 | 2 | 3 | 4 | 5 |
    +---+---+---+---+---+---+
    0   1   2   3   4   5   6
    -6  -5  -4  -3  -2  -1
    ```

    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice 1:-2 "${a[@]}")
    1 2 3
    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice 0:1 "${a[@]}")
    0
    >>> local a=(0 1 2 3 4 5)
    >>> [ -z "$(array.slice 1:1 "${a[@]}")" ] && echo empty
    empty
    >>> local a=(0 1 2 3 4 5)
    >>> [ -z "$(array.slice 2:1 "${a[@]}")" ] && echo empty
    empty
    >>> local a=(0 1 2 3 4 5)
    >>> [ -z "$(array.slice -2:-3 "${a[@]}")" ] && echo empty
    empty
    >>> [ -z "$(array.slice -2:-2 "${a[@]}")" ] && echo empty
    empty

    Slice indices have useful defaults; an omitted first index defaults to
    zero, an omitted second index defaults to the size of the string being
    sliced.
    >>> local a=(0 1 2 3 4 5)
    >>> # from the beginning to position 2 (excluded)
    >>> echo $(array.slice 0:2 "${a[@]}")
    >>> echo $(array.slice :2 "${a[@]}")
    0 1
    0 1

    >>> local a=(0 1 2 3 4 5)
    >>> # from position 3 (included) to the end
    >>> echo $(array.slice 3:"${#a[@]}" "${a[@]}")
    >>> echo $(array.slice 3: "${a[@]}")
    3 4 5
    3 4 5

    >>> local a=(0 1 2 3 4 5)
    >>> # from the second-last (included) to the end
    >>> echo $(array.slice -2:"${#a[@]}" "${a[@]}")
    >>> echo $(array.slice -2: "${a[@]}")
    4 5
    4 5

    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice -4:-2 "${a[@]}")
    2 3

    If no range is given, it works like normal array indices.
    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice -1 "${a[@]}")
    5
    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice -2 "${a[@]}")
    4
    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice 0 "${a[@]}")
    0
    >>> local a=(0 1 2 3 4 5)
    >>> echo $(array.slice 1 "${a[@]}")
    1
    >>> local a=(0 1 2 3 4 5)
    >>> array.slice 6 "${a[@]}"; echo $?
    1
    >>> local a=(0 1 2 3 4 5)
    >>> array.slice -7 "${a[@]}"; echo $?
    1
    '
    local start end array_length length
    if [[ $1 == *:* ]]; then
        IFS=":"; read -r start end <<<"$1"
        shift
        array_length="$#"
        # defaults
        [ -z "$end" ] && end=$array_length
        [ -z "$start" ] && start=0
        (( start < 0 )) && let "start=(( array_length + start ))"
        (( end < 0 )) && let "end=(( array_length + end ))"
    else
        start="$1"
        shift
        array_length="$#"
        (( start < 0 )) && let "start=(( array_length + start ))"
        let "end=(( start + 1 ))"
    fi
    let "length=(( end - start ))"
    (( start < 0 )) && return 1
    # check bounds
    (( length < 0 )) && return 1
    (( start < 0 )) && return 1
    (( start >= array_length )) && return 1
    # parameters start with $1, so add 1 to $start
    let "start=(( start + 1 ))"
    echo "${@: $start:$length}"
}
alias array.slice="array_slice"