Bourne Shell 条件运算符

Question

我和 Bourne 一起玩得很开心 Shell，但我面临着一个关于条件的相当神秘的情况：

#! /bin/sh

a=1
b=2
c="0 kB/s"

if [ "$a" -eq 1 ] ; then echo "a = 1: true" ; else echo "a =  1: false" ; fi
if [ "$b" -gt 0 ] ; then echo "b > 0: true" ; else echo "b > 0: false" ; fi
if [ "$c" != "0 kB/s" ] ; then echo "c <> 0: true" ; else echo "c <> 0: false" ; fi
if [ "$a" -eq 1 ] || [ "$b" -gt 0 ] ; then echo "a = 1 or b > 0: true" ; else echo "a = 1 or b > 0: false" ; fi
if [ "$a" -eq 1 ] || [ "$b" -gt 0 ] && [ "$c" != "0 kB/s" ] ; then echo "a = 1 or b > 0 and c <> 0: true" ; else echo "a = 1 or b > 0 and c <> 0: false" ; fi
if [ true ] || [ true ] && [ false ] ; then echo "true or true and false: true" ; else echo "true or true and false: false" ; fi

给我以下结果：

a = 1: true
b > 0: true
c <> 0: false
a = 1 or b > 0: true
a = 1 or b > 0 and c <> 0: false
true or true and false: true

简短的问题：为什么我没有得到 a = 1 or b > 0 and c <> 0: true？

非常感谢您的帮助...

Answer 1

|| 和 && 具有相同的优先级，这与逻辑 AND 运算符比逻辑 OR 绑定更紧密的语言不同。这意味着您编写的代码等同于

if { [ "$a" -eq 1 ] || [ "$b" -gt 0 ]; } && [ "$c" != "0 kB/s" ] ; then
  echo "a = 1 or b > 0 and c <> 0: true"
else
  echo "a = 1 or b > 0 and c <> 0: false"
fi

而不是预期的

if [ "$a" -eq 1 ] || { [ "$b" -gt 0 ] && [ "$c" != "0 kB/s" ]; } ; then
  echo "a = 1 or b > 0 and c <> 0: true"
else
  echo "a = 1 or b > 0 and c <> 0: false"
fi

Bourne Shell 条件运算符

Bourne Shell Conditional Operators

conditional

sh