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 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.
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
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"