Linux shell：条件语句中的空格

Question

我最近学习了如何在shell中编程。 我不明白为什么这两个声明产生不同的输出。看起来如果没有空格，测试将10==11作为一个字符串并始终返回true。

$test 10==11 && echo yes || echo no 
$yes 
$test 10 == 11 && echo yes || echo no 
$no 

Answer 1

# single string, not null so true, and result is yes 
test 10==11 && echo yes || echo no 

# there exist space 10 == 11 not equal string comparison so result no 
test 10 == 11 && echo yes || echo no 

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等同

if test 10==11; then echo yes; else echo no; fi 
yes 

if test 10 == 11; then echo yes; else echo no; fi 
no 

# or this same as above 
if test 10 = 11; then echo yes; else echo no; fi 
no 

从http://tldp.org/LDP/abs/html/comparison-ops.html

string comparison 

= 

    is equal to 

    if [ "$a" = "$b" ] 

    Caution 

    Note the whitespace framing the =. 

    if [ "$a"="$b" ] is not equivalent to the above. 
== 

    is equal to 

    if [ "$a" == "$b" ]