Bourne Shell Conditional Operators - conditional-statements

Question

I am having a lot of fun playing with Bourne Shell, but I am facing a quite cryptic situation regarding conditions:
#! /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
gives me the following result:
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
Short question: why don't I get a = 1 or b > 0 and c <> 0: true?
Thanks a lot for your help ...

Answer 1

|| and && have equal precedence, unlike in languages where the logical AND operator binds more tightly than the logical OR. This means your code as written is equivalent to
if { [ "$a" -eq 1 ] || [ "$b" -gt 0 ]; } && [ "$c" != "0 kB/s" ] ; then
echo "a = 1 or b > 0 and c <> 0: true"
else
echo "a = 1 or b > 0 and c <> 0: false"
fi
rather than the expected
if [ "$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