How to slice an array in Bash - arrays

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"

Answer 4

At the risk of beating a dead horse, I was inspired by #jandob's answer and made this version that
Is simpler (doesn't have so much shift logic or rewriting of variables as often).
Respects quoted strings without dealing with IFS (-r mode only).
Allows the user to specify [start, end) slicing or [start, length] slicing via -l flag.
Allows you to echo the resulting array (default behavior), or "return" it into a new array for use in the calling parent (via -r slicedArray).
Note: namerefs are only supported in Bash >= 4.3. To support earlier versions of Bash (i.e. Mac without Brew's bash), you'll need to use indirection instead: use a temp var to access array parameters, e.g. declare arrValuesCmd="$1[#]"; declare arr=("${!arrValuesCmd}"), and use eval for return values, e.g. eval $retArrName='("${newArr[#]}")' (note the single quotes around the array declaration).
array.slice() {
# array.slice [-l] [-r returnArrayName] myArray 3 5
# Default functionality is to use second number as end index for slice (exclusive).
# Can instead use second number as length by passing `-l` flag.
# `echo` doesn't maintain quoted entries, so pass in `-r returnArrayName` to keep them.
declare isLength
declare retArrName
declare OPTIND=1
while getopts "lr:" opt; do
case "$opt" in
l)
# If `end` is slice length instead of end index
isLength=true
;;
r)
retArrName="$OPTARG"
;;
esac
done
shift $(( OPTIND - 1 ))
declare -n arr="$1"
declare start="$2"
declare end="$3"
declare arrLength="${#arr[#]}"
declare newArr=()
declare newArrLength
# Bash native slicing:
# Positive index values: ${array:start:length}
# Negative index values: ${array: start: length}
# To use negative values, a space is required between `:` and the variable
# because `${var:-3}` actually represents a default value,
# e.g. `myVar=${otherVal:-7}` represents (pseudo-code) `myVar=otherVal || myVar=7`
if [[ -z "$end" ]]; then
# If no end is specified (regardless of `-l`/length or index), default to the rest of the array
newArrLength="$arrLength"
elif [[ -n "$isLength" ]]; then
# If specifying length instead of end-index, use native bash array slicing
newArrLength="$(( end ))"
else
# If specifying end-index, use custom slicing based on a range of [start, end):
newArrLength="$(( end - start ))"
fi
newArr=("${arr[#]: start: newArrLength}")
if [[ -n "$retArrName" ]]; then
declare -n retArr="$retArrName"
retArr=("${newArr[#]}")
else
echo "${newArr[#]}"
fi
}
Examples:
myArray=(x y 'a b c' z 5 14) # length=6
array.slice myArray 2 4
# > a b c z
array.slice -l myArray 3 2
# > z 5
# Note: Output was manually quoted to show the result more clearly.
# Actual stdout content won't contain those quotes, which is
# why the `-r returnArray` option was added.
array.slice -r slicedArray myArray -5 -3 # equivalent of [2, 4)
# > (null)
echo -e "myArray (length=${#myArray[#]}): ${myArray[#]} \nslicedArray (length=${#slicedArray[#]}): ${slicedArray[#]}"
# > myArray (length=6): x y 'a b c' z 5 14
# > slicedArray (length=2): 'a b c' z
array.slice -lr slicedArray myArray -5 3 # length instead of index, equivalent of [2, 5)
# > (null)
echo -e "myArray (length=${#myArray[#]}): ${myArray[#]} \nslicedArray (length=${#slicedArray[#]}): ${slicedArray[#]}"
# > myArray (length=6): x y 'a b c' z 5 14
# > slicedArray (length=3): 'a b c' z 5