Bourne Shell的條件運算

Question

我有很多的樂趣與Bourne Shell中玩，但我現在面臨一個相當神祕的有關情況及條件：

#! /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 

給我下面的結果：

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 

簡短的問題：爲什麼我不能得到a = 1 or b > 0 and c <> 0: true？

非常感謝您的幫助...

Answer 1

||和&&具有相同的優先級，在不同地方的語言邏輯與操作的優先級比邏輯或更緊密。這意味着你的代碼編寫相當於

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 

而不是預期的

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