Shell编程中括号判断中赋值语句和判断语句

#!/bin/bash

declare var="xxx"
# without space and use one =
#1.judge whether the assignment statement returns true
echo "----------------------test assignment in bracket --------------------------------"
[ var="yyyy" ] && echo "success"
echo "end"
    # success
[ "$var"="yyyy" ] && echo "success"
echo "end"
    # success
[ "var"="yyyy" ] && echo "success"
echo "end"
    # success
[ var1="yyyy" ] && echo "success"
echo "end"
    # success

# result: yes

#2. judge the assignment whether be taken in bracket
echo "--------------------test assignment whether been taken -----------------------"
[ var="yyyy" ] && echo "$var"
echo "end"
    #xxx
    #end
#result: assignment can‘t be taken in bracket

#3. whether the effect of single = and double = similary
echo "-------------------test single = and double = ------------------------"
[ "$var" == "yyyy" ] && echo "$var"
echo "end"
    #end
[ "$var" = "yyyy" ] && echo "$var"
echo "end"
    #end
var="yyyy"
[ "$var" == "yyyy" ] && echo "$var"
echo "end"
    #yyyy
    #end
[ "$var" = "yyyy" ] && echo "$var"
echo "end"
    #yyyy
    #end
# result: effect same
#4. whether the effect of having space and not space similary
echo "----------------------test space -------------------------------------"
[ "$var"=="yyyy" ] && echo "$var"
echo "end"
    #yyyy
    #end
[ "$var"=="zzzz" ] && echo "$var"
echo "end"
    #yyyy
    #end
#result: always true with no space 

#conclusion:
# with space at both side of = will judge in normal
# don‘t space at both side of = or ==  will always true 

通过上面的测试得出的结论是：

在中括号判断中不使用空格例如  "$var"="xx" 或 "$var"=="xxx"（在= 和== 两边没有空格）则这个判断一直是真

在中括号中使用空格 例如  "$var" = "xx" 或 "$var" == "xxx"（在= 和== 两边有空格） 则这个判断会按照正常的值进行判断，值相等返回true否则返回false

可能会因使用的shell及其版本不同而导致结果不同，我使用的shell是：GNU bash, version 4.1.2(1)-release (i686-redhat-linux-gnu)