Question

linux shell 关联数组的一个小问题

array.sh文件中：
#! /bin/bash
declare -a array
array[apple]=100
array[banana]=200
echo ${array[apple]}
echo ${array[banana]}
运行./array.sh
输出：
200
200
为什么会输出两个200呢？求解答

Answer 1

shell 并不支持关联数组。

只有awk才支持关联数组。

shell只支持index数字类型的数组，凡是不是数字的，都会当做0，或者-1，也就是最后的那个元素。

详细参见bash的man手册。

Arrays
Bash provides one-dimensional array variables. Any variable may be used as an array; the declare builtin
will explicitly declare an array. There is no maximum limit on the size of an array, nor any requirement
that members be indexed or assigned contiguously. Arrays are indexed using integers and are zero-based.

An array is created automatically if any variable is assigned to using the syntax name[subscript]=value.
The subscript is treated as an arithmetic expression that must evaluate to a number greater than or equal
to zero. To explicitly declare an array, use declare -a name (see SHELL BUILTIN COMMANDS below).
declare -a name[subscript] is also accepted; the subscript is ignored. Attributes may be specified for
an array variable using the declare and readonly builtins. Each attribute applies to all members of an
array.

Arrays are assigned to using compound assignments of the form name=(value1 ... valuen), where each value
is of the form [subscript]=string. Only string is required. If the optional brackets and subscript are
supplied, that index is assigned to; otherwise the index of the element assigned is the last index
assigned to by the statement plus one. Indexing starts at zero. This syntax is also accepted by the
declare builtin. Individual array elements may be assigned to using the name[subscript]=value syntax
introduced above.

Any element of an array may be referenced using ${name[subscript]}. The braces are required to avoid
conflicts with pathname expansion. If subscript is @ or *, the word expands to all members of name.
These subscripts differ only when the word appears within double quotes. If the word is double-quoted,
${name[*]} expands to a single word with the value of each array member separated by the first character
of the IFS special variable, and ${name[@]} expands each element of name to a separate word. When there
are no array members, ${name[@]} expands to nothing. If the double-quoted expansion occurs within a
word, the expansion of the first parameter is joined with the beginning part of the original word, and
the expansion of the last parameter is joined with the last part of the original word. This is analogous
to the expansion of the special parameters * and @ (see Special Parameters above). ${#name[subscript]}
expands to the length of ${name[subscript]}. If subscript is * or @, the expansion is the number of ele-
ments in the array. Referencing an array variable without a subscript is equivalent to referencing ele-
ment zero.
The unset builtin is used to destroy arrays. unset name[subscript] destroys the array element at index
subscript. Care must be taken to avoid unwanted side effects caused by filename generation. unset name,
where name is an array, or unset name[subscript], where subscript is * or @, removes the entire array.

The declare, local, and readonly builtins each accept a -a option to specify an array. The read builtin
accepts a -a option to assign a list of words read from the standard input to an array. The set and
declare builtins display array values in a way that allows them to be reused as assignments.

Answer 2

因为关联数组是 declare -A array。大写的

Answer 3

declare -A xxx

-A Each name is an associative array variable