嘗試為“eval”中的關聯陣列賦值不起作用

當試圖定義輔助函式來構建關聯串列時，出現錯誤，我無法自行解決 (BASH 4.4)：

/tmp/foo.sh: 第 18 行：'key'：語法錯誤：預期運算元（錯誤標記為“'key'”）

對于 BASH 4.3，我得到了：

/tmp/foo.sh: 第 18 行：鍵：未系結變數

這是測驗用例：

#!/bin/bash
set -u

# add services list
add_list()
{
    local list="$1"

    eval "declare -a ${list}=(); declare -A ${list}_A=()"
}

# add services to list of services
add_service()
{
    local list="$1" def="$2"
    local s="${def%%:*}"

    eval "${list} =('$def'); ${list}_A['$s']='$def'"
}

add_list TEST
add_service TEST 'key:value'

使用兩個陣列的原因是我想按鍵訪問元素，并且我想保留原始順序（實際上${list} =('$s')就足夠了）。

這是輸出bash -x：

> bash -x /tmp/foo.sh 
  set -u
  add_list TEST
  local list=TEST
  eval 'declare -a TEST=(); declare -A TEST_A=()'
   TEST=()
   declare -a TEST
   TEST_A=()
   declare -A TEST_A
  add_service TEST key:value
  local list=TEST def=key:value
  local s=key
  eval 'TEST =('\''key:value'\''); TEST_A['\''key'\'']='\''key:value'\'''
   TEST =('key:value')
   TEST_A['key']=key:value
/tmp/foo.sh: line 18: 'key': syntax error: operand expected (error token is "'key'")

uj5u.com熱心網友回復：

筆記：

• 跳過討論為什么eval可能不是最好的方法
• 跳過對使用 namerefs 的替代方法的討論
• 將關注 OP 的當前代碼如何（不）創建所需的陣列和快速修復

在函式中宣告的陣列保持區域范圍，除非陣列也用global 標志宣告；考慮以下：

$ mytest() { typeset -a myarray; typeset -p myarray; echo "##### mytest(): exit"; }
                     ^^
$ unset myarray
$ mytest
declare -a myarray                           # array exists while inside the function
##### mytest(): exit

$ typeset -p myarray
-bash: typeset: myarray: not found           # array no longer exists once outside the function

現在添加-g標志：

$ mytest() { typeset -ag myarray; typeset -p myarray; echo "##### mytest(): exit"; }
                     ^^^
$ unset myarray
$ mytest
declare -a myarray                           # array exists while inside the function
##### mytest(): exit

$ typeset -p myarray
declare -a myarray                           # array still exists after leaving function

將-g標志添加到OP 當前函式中的兩個陣列宣告：

add_list()
{
    local list="$1"

    eval "declare -ag ${list}=(); declare -Ag ${list}_A=()"
    #             ^^^                     ^^^
}

注意：函式定義現在add_service可以保持原樣

運行 OP 的測驗：

$ unset TEST TEST_A
$ add_list TEST
$ add_service TEST 'key:value'
$ typeset -p TEST TEST_A
declare -a TEST=([0]="key:value")
declare -A TEST_A=([key]="key:value" )

至于為什么OP的當前代碼會產生錯誤......

在命令提示符下，我們將模擬add_service操作......

$ unset TEST TEST_A                                       # just to make these variables are undefined before calling add_service ...

$ typeset -p TEST TEST_A                                  # verify variables are not set
-bash: typeset: TEST: not found
-bash: typeset: TEST_A: not found

$ TEST =('key:value')                                     # bash recognizes this as valid array syntax and will automagically create a normal (-a) array named TEST
$ typeset -p TEST
declare -a TEST=([0]="key:value")

$ TEST_A['key']='key:value'                               # bash recognizes this as the correct syntax for an integer-keyed array but has problems processing the string `key` as an integer so ...
-bash: 'key': syntax error: operand expected (error token is "'key'")

$ TEST_A[key]='key:value'                                 # again, looks like correct syntax but in this case no error ...
$ typeset -p TEST_A
declare -a TEST_A=([0]="key:value")
        ^^

        # in this case bash considers key as a variable (ie, bash treats it as $key)
        # but since $key is undefined it defaults to 0 and a normal array (-a) is
        # created with index 0

$ TEST_A[xxx]='keyX:valueX'
$ typeset -p TEST_A
declare -a TEST_A=([0]="keyX:valueX")                     # $xxx is undefined, treated as 0, and we end up overwriting previous 0-indexed entry in array

$ key=9
$ TEST_A[key]='key:value'
$ typeset -p TEST_A
declare -a TEST_A=([0]="keyX:valueX" [9]="key:value")     # $key is defined (9) so we get a new array entry with index=9

uj5u.com熱心網友回復：

在函式中使用declare會使變數成為區域變數，因此可能無法通過呼叫自定義函式將區域變數轉換為關聯陣列。

對于一個普通的陣列，這是可能的，檢查這個結果：

#!/bin/bash

make_it_an_array() {
    [[ $1 =~ ^[[:alpha:]_][[:alnum:]_]*$ ]] || return 1
    eval "$1=()"
}

fun1() {
    local arr # arr isn't an array, so what will happen?
    make_it_an_array arr
    printf '"%s" in %s: %s\n' "$1" "$FUNCNAME" "$(declare -p arr 2> /dev/null)"
}

#############################################################

fun1 arr
printf '"arr" in main: %s\n' "$(declare -p arr 2> /dev/null)"

"arr" in fun1: declare -a arr='()'
"arr" in main: 

如果替換eval "$1=()"為declare -ga "$1=()"結果將是：

"arr" in fun1: declare -a arr='()'
"arr" in main: declare -a arr='()'

這意味著父函式中變數的區域作用域已經丟失。

在旁邊：

如果$list僅包含變數的名稱，則可以替換：

eval "declare -a ${list}=(); declare -A ${list}_A=()"

和：

declare -a "$list=()"
declare -A "${list}_A=()"

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