bash – 修改while循环中的变量不记住

在下面的程序中，如果我将变量$ foo设置为第一个if语句中的值1，它的意思是它的值在if语句之后被记住。但是，当我将一个相同的变量设置为一个if语句中的if里面的值2，它在while循环后被遗忘。它的行为像我在while循环中使用某种类型的变量$ foo的副本，我只修改那个特定的副本。这里有一个完整的测试程序：

#!/bin/bash

set -e
set -u

foo=0
bar="hello"

if [[ "$bar" == "hello" ]]
then
    foo=1
    echo "Setting $foo to 1: $foo"
fi
echo "Variable $foo after if statement: $foo"

lines="first linensecond linenthird line"

echo -e $lines | while read line
do
    if [[ "$line" == "second line" ]]
    then
    foo=2
    echo "Variable $foo updated to $foo inside if inside while loop"
    fi
    echo "Value of $foo in while loop body: $foo"
done

echo "Variable $foo after while loop: $foo"

# Output:
# $ ./testbash.sh
# Setting $foo to 1: 1
# Variable $foo after if statement: 1
# Value of $foo in while loop body: 1
# Variable $foo updated to 2 inside if inside while loop
# Value of $foo in while loop body: 2
# Value of $foo in while loop body: 2
# Variable $foo after while loop: 1

# bash --version
# GNU bash,version 4.1.10(4)-release (i686-pc-cygwin)

感谢您的阅读和提前感谢任何帮助！

echo -e $lines | while read line 
...
done

while read line
do
    if [[ "$line" == "second line" ]]
    then
    foo=2
    echo "Variable $foo updated to $foo inside if inside while loop"
    fi
    echo "Value of $foo in while loop body: $foo"
done <<< "$(echo -e "$lines")"