为什么管道输出到 /dev/null 和 grep 会污染我的 bash 数组？

Question

我有两个 shell 脚本（Linux，#!/bin/bash），一个有一些常用的功能来查找文本等，另一个利用常用的功能做一些“实际”的工作。

在检查数组变量（test2.sh 内的oranges）以测试/查看它是否是一个数组（declare -p oranges 2>/dev/null | grep -q '^declare \-a' && printf "$fmt" "oranges is an indexed array"）时，我偶然发现另一个数组（LINES 内test1.sh）突然得到带有随机数的“污染”。

我不明白为什么会这样。有人有答案吗？ 如果您在使用脚本时使用 echo 或 printf ，则只有当您将错误传递到 2>/dev/null 并 grep 输出时，它才会产生零差异。我已将脚本缩减为基本要素以显示问题。 此外，通过将单行分开并将每个部分的输出分配给一个变量（参见test3.sh），这个问题很容易解决，所以我不会被困在脚本编写或进一步的进展上。我一直试图回答为什么会发生LINES 的污染。

test1.sh

#!/bin/bash

declare -a LINES

echo "::: DECLARE ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"

test2.sh

#!/bin/bash

source ./test1.sh

declare -a oranges=()
echo "::: test2-1 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"


declare -p oranges 2>/dev/null
echo "::: test2-2 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"


declare -p oranges 2>/dev/null | grep -E -i '^declare \-a'
echo "::: test2-3 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"


declare -p oranges 2>/dev/null | grep -E -i '^declare \-a' && printf "$fmt" "oranges is an indexed array"
echo "::: test2-4 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"

declare -p LINES
declare -p oranges 2>/dev/null | grep -E -i '^declare \-a' && printf "$fmt" "oranges is an indexed array" || printf "$fmt" "oranges is not an indexed array"
echo "::: test2-5 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"


declare -p oranges 2>/dev/null
echo "::: test2-6 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"

并且在运行时会出现以下输出：

::: DECLARE ::: Length of [LINES] is: 0, :::Content of [LINES] is: 
::: test2-1 ::: Length of [LINES] is: 0, :::Content of [LINES] is: 
declare -a oranges=()
::: test2-2 ::: Length of [LINES] is: 0, :::Content of [LINES] is: 
declare -a oranges=()
::: test2-3 ::: Length of [LINES] is: 1, :::Content of [LINES] is: 81
declare -a oranges=()
::: test2-4 ::: Length of [LINES] is: 1, :::Content of [LINES] is: 81
declare -a LINES=([0]="81")
declare -a oranges=()
::: test2-5 ::: Length of [LINES] is: 1, :::Content of [LINES] is: 81
declare -a oranges=()
::: test2-6 ::: Length of [LINES] is: 1, :::Content of [LINES] is: 81

test3.sh - 解决方法

#!/bin/bash

source ./test1.sh

declare -a oranges=()
echo "::: test2-1 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"


someResult="$(declare -p oranges 2>/dev/null)"
thePattern="^declare \-a"

nextResult=`grep -E -i "$thePattern" <<< "$someResult"` || nextResult=""

if [[ -n "$nextResult" ]]
then
    echo "oranges is an indexed array"
else
    echo "oranges is NOT an indexed array"
fi

echo "::: test2-2 ::: Length of [LINES] is: ${#LINES[@]}, :::Content of [LINES] is: ${LINES[@]}"

test3.sh 的输出 ..

$ ./test3.sh 
::: DECLARE ::: Length of [LINES] is: 0, :::Content of [LINES] is: 
::: test2-1 ::: Length of [LINES] is: 0, :::Content of [LINES] is: 
oranges is an indexed array
::: test2-2 ::: Length of [LINES] is: 0, :::Content of [LINES] is: 

Answer 1

LINES 是由 shell 填充的保留变量，其中包含终端窗口中可以容纳的行数。使用不同的变量名（shell 不使用该变量名；请参阅Shell Variables），它会按预期工作。

正如 David C. Rankin 在他的评论中提到的，所有 UPPER-CASE 中的变量通常都是为 shell 保留的，这就是为什么建议使用小写变量名的原因。

如果您出于某种原因必须为变量使用名称 LINES，您可以通过以下命令禁用 checkwinsize 选项作为解决方法；但这可能会产生副作用，我不能保证它会一直有效。

shopt -u checkwinsize

Answer 2

这可以解释这种行为：

d。非交互式 shell 现在对 checkwinsize 的设置作出反应 并设置 前台作业退出后的行和列。

来自http://www.softpanorama.org/Scripting/Shellorama/Bash_history/bash43.shtml