How to slice an array in Bash
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 arrayecho "${B[@]}" # bar a b cecho "${B[1]}" # a b cecho "${C[@]}" # bar a b c 42echo "${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.
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=4.7.1A=( ${version//\./ } )echo "${A[@]}" # 4 7 1B=( "${A[@]:1}" )echo "${B[@]}" # 7 1
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"