在while循环内修改的变量不被记住 [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$foo设置为值1，则它的工作方式是在if语句之后记住其值.但是，当我在while语句内的if中将相同的变量设置为值2时，在while循环之后将其忘记.就像我在while循环中使用某种类型的变量$foo的副本一样，而我仅在修改该特定副本.这是完整的测试程序:

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\nsecond line\nthird 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 此处为字符串 重写在主shell进程中的while循环；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")"

您可以通过在分配lines时立即展开反斜杠序列来摆脱上面的此处字符串中难看的echo.可以在其中使用$'...'报价形式:

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=$'first line\nsecond line\nthird line'
while read line; do
    ...
done <<< "$lines"

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