不记得在 while 循环中修改的变量 [英] A variable modified inside a while loop is not remembered

问题描述

在下面的程序中，如果我在第一个 if 语句中将变量 $foo 设置为值 1，它的工作原理是它的值在之后被记住if 语句.但是，当我在 while 语句内的 if 中将相同的变量设置为值 2 时，它会在 while 循环之后被遗忘.它的行为就像我在 while 循环中使用了变量 $foo 的某种副本，而我只修改了那个特定的副本.这是一个完整的测试程序:

In the following program, if I set the variable $foo to the value 1 inside the first if statement, it works in the sense that its value is remembered after the if statement. However, when I set the same variable to the value 2 inside an if which is inside a while statement, it's forgotten after the while loop. It's behaving like I'm using some sort of copy of the variable $foo inside the while loop and I am modifying only that particular copy. Here's a complete test program:

#!/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 line
second line
third 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)

推荐答案

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

while 循环在子 shell 中执行.因此，一旦子 shell 退出，您对变量所做的任何更改都将不可用.

The while loop is executed in a subshell. So any changes you do to the variable will not be available once the subshell exits.

相反，您可以使用此处的字符串 重写while循环，使其在主shell进程中；只有 echo -e $lines 会在子 shell 中运行:

Instead you can use a here string to re-write the while loop to be in the main shell process; only echo -e $lines will run in a subshell:

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")"

通过在分配行时立即扩展反斜杠序列，您可以摆脱上面here-string中相当丑陋的echo.可以在此处使用 $'...' 形式的引用:

You can get rid of the rather ugly echo in the here-string above by expanding the backslash sequences immediately when assigning lines. The $'...' form of quoting can be used there:

lines=$'first line
second line
third line'
while read line; do
    ...
done <<< "$lines"

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