bash 脚本中比较运算符设置的退出状态

Question

以下 bash 脚本打印 "ERROR!" 而不是 "Server error response" 即使 wget returns 8:

#!/bin/bash

wget -q "www.google.com/unknown.html"
if [ $? -eq 0 ]
then
    echo "Fetch successful!"
elif [ $? -eq 8 ]
then
    echo "Server error response"
else
    echo "ERROR!"
fi

当上面的脚本是运行带-x的时候，第一次和0比较好像是设置退出状态为1：

+ wget www.google.com/unknown.html
+ '[' 8 -eq 0 ']'
+ '[' 1 -eq 8 ']'
+ echo 'ERROR!'
ERROR!

我通过使用一个变量来存储 wget 退出状态来解决这个问题，但是我找不到任何关于 $?已设置。 Bash 详情：

$ bash --version
GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

有人可以指点我吗？

Answer 1

$? 在 man bash:

的 特殊参数 参数部分进行了解释，虽然非常简短

   ?      Expands to the exit status of the most recently  executed  fore-
          ground pipeline.

@chepner 在他的评论中说得最好：

The key thing to understand is that each [ ... ] is a separate foreground pipeline, not part of the if statement's syntax, and they are executed in order, updating $? as you go along.

如果要使用 if-else 链，则将 $? 的值保存在一个变量中，并对该变量使用条件：

wget -q "www.google.com/unknown.html"
x=$?
if [ $x -eq 0 ]
then
    echo "Fetch successful!"
elif [ $x -eq 8 ]
then
    echo "Server error response"
else
    echo "ERROR!"
fi

但在这个例子中，case 会更实用：

wget -q "www.google.com/unknown.html"
case $? in
    0)
        echo "Fetch successful!" ;;
    8)
        echo "Server error response" ;;
    *)
        echo "ERROR!"
esac

Answer 2

尝试在 $? 上使用 switch case或存储 $?在变量中。

bash 脚本中比较运算符设置的退出状态

Exit status set by comparison operator in bash script

bash

status

exit

setting