以查找文件为例：同级目录下存在find 不存在find1

#!/bin/bash
if test -e find;then
        echo "ok" 
else
        echo "no"
fi

if [ -e find ];then
        echo "ok" 
else
        echo "no"
fi

ok
ok

#!/bin/bash
if test -e find1;then
        echo "ok" 
else
        echo "no"
fi

if [ -e find1 ];then
        echo "ok" 
else
        echo "no"
fi

no
no

#!/bin/bash
test -e find
echo $?
test -e find1
echo $?
[ -e find ]
echo $?
[ -e find1 ]
echo $?

0
1
0
1

结论

if 是通过其后的命令执行完成的结果

$? = 0 #执行成功
$? = 1 #执行失败

从而判断条件的真/假

#!/bin/bash
test -e find
echo $?
test -e find1
echo $?
[ -e find ]
echo $?
[ -e find1 ]
echo "干扰" #执行成功 $?=0  
echo $?

0
1
0
干扰
0