How do I shift a bash array at some index in the middle?

Question

1  #!/bin/bash
2  # query2.sh
3
4  numbers=(53 8 12 9 784 69 8 7 1)
5  i=4
6
7  echo ${numbers[@]} # <--- this echoes "53 8 12 9 784 69 8 7 1" to stdout.
8  echo ${numbers[i]} # <--- this echoes "784" to stdout.
9
10 unset numbers[i]
11
12 echo ${numbers[@]} # <--- this echoes "53 8 12 9 69 8 7 1" to stdout.
13 echo ${numbers[i]} # <--- stdout is blank.

Why, in line 13, is the stdout blank, considering that the array seems to have been updated judging by line 12's stdout?

And therefore, what should I do to get the intended answer, "69"?

Answer 1

unset removes an element. It doesn't renumber the remaining elements.

We can use declare -p to see exactly what happens to numbers:

$ unset "numbers[i]"
$ declare -p numbers
declare -a numbers=([0]="53" [1]="8" [2]="12" [3]="9" [5]="69" [6]="8" [7]="7" [8]="1")

Observe the numbers no longer has an element 4.

Another example

Observe:

$ a=()
$ a[1]="element 1"
$ a[22]="element 22"
$ declare -p a
declare -a a=([1]="element 1" [22]="element 22")

Array a has no elements 2 through 21. Bash does not require that array indices be consecutive.

Suggested method to force a renumbering of the indices

Let's start with the numbers array with the missing element 4:

$ declare -p numbers
declare -a numbers=([0]="53" [1]="8" [2]="12" [3]="9" [5]="69" [6]="8" [7]="7" [8]="1")

If we would like the indices to change, then:

$ numbers=("${numbers[@]}")
$ declare -p numbers
declare -a numbers=([0]="53" [1]="8" [2]="12" [3]="9" [4]="69" [5]="8" [6]="7" [7]="1")

There is now an element number 4 and it has value 69.

Alternate method to remove an element & renumber array in one step

Again, let's define numbers:

$ numbers=(53 8 12 9 784 69 8 7 1)

As suggested by Toby Speight in the comments, a method to remove the fourth element and renumber the remaining elements all in one step:

$ numbers=("${numbers[@]:0:4}" "${numbers[@]:5}")
$ declare -p numbers
declare -a numbers=([0]="53" [1]="8" [2]="12" [3]="9" [4]="69" [5]="8" [6]="7" [7]="1")

As you can see, the fourth element was removed and all remaining elements were renumbered.

${numbers[@]:0:4} slices array numbers: it takes the first four elements starting with element 0.

Similarly, ${numbers[@]:5} slice array numbers: it takes all elements starting with element 5 and continuing to the end of the array.

Obtaining the indices of an array

The values of an array can be obtained with ${a[@]}. To find the indices (or keys) that correspond to those values, use ${!a[@]}.

For example, consider again our array numbers with the missing element 4:

$ declare -p numbers
declare -a numbers=([0]="53" [1]="8" [2]="12" [3]="9" [5]="69" [6]="8" [7]="7" [8]="1")

To see which indices are assigned:

$ echo "${!numbers[@]}"
0 1 2 3 5 6 7 8

Again, 4 is missing from the list of indices.

Documentation

From man bash:

该unset内建是用来摧毁阵列。 unset name[subscript]销毁 index 处的数组元素subscript。索引数组的负下标按上述解释。必须小心避免由路径名扩展引起的不必要的副作用。 unset name, where nameis an array, or unset name[subscript], where subscriptis * or @, 删除整个数组。

Answer 2

bash像 inksh这样的数组并不是真正的数组，它们更像是键限制为正整数的关联数组（或所谓的稀疏数组）。对于具有真实数组的外壳，您可以查看诸如rc, es, fish, yash, 之类的外壳zsh（甚至csh/tcsh尽管这些外壳有很多问题，但最好避免）。

在zsh：

a=(1 2 3 4 5)
a[3]=() # remove the 3rd element
a[1,3]=() # remove the first 3 elements
a[-1]=() # remove the last element

（注意，在 zsh 中，unset 'a[3]'实际上将其设置为空字符串以提高与 的兼容性ksh）

在yash：

a=(1 2 3 4 5)
array -d a 3 # remove the 3rd element
array -d a 1 2 3 # remove the first 3 elements
array -d a -1 # remove the last element

in fish（不是与bash/相反的类似 Bourne 的外壳zsh）：

set a 1 2 3 4 5
set -e a[3] # remove the 3rd element
set -e a[1..3] # remove the first 3 elements
set -e a[-1] # remove the last element

in es（基于rc，不是 Bourne 式的）

a = 1 2 3 4 5
a = $a(... 2 4 ...) # remove the 3rd element
a = $a(4 ...) # remove the first 3 elements
a = $a(... `{expr $#a - 1}) # remove the last element
# or a convoluted way that avoids forking expr:
a = $a(... <={@{*=$*(2 ...); return $#*} $a})

在ksh和bash

如果您这样做，您可以将数组用作普通数组：

a=("${a[@]}")

在每次删除或插入操作之后，可能使索引列表不连续或不从 0 开始。还要注意ksh/bash数组从 0 开始，而不是 1（除了$@（在某些方面））。

这实际上会整理元素并将它们按顺序移动到索引 0、1、2...。

另请注意，您需要引用number[i]in：

unset 'number[i]'

否则，unset numberi如果numberi在当前目录中调用了一个文件，它将有效地运行。