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bash条件测试及变量比较

一、条件表达式

1. 基本的if语句

if [ … ]

then

….

Fi

2. 多个if语句

if […]

then

…

Elif […]

Then

…

Fi

3. 嵌套if语句

If […]

Then

…

If[…]

Then

…

Fi

Else

…

fi

二、字符串和数字比较

	Number	String
大于	-gt	/>
小于	-lt	/<
等于	-eq	==
大于等于	-ge	Single[] doesnot support
小于等于	-le	Single[] doesnot support
不等于	-ne	！=

三、 关键点

1.if和elif后面有then

2.else if是非法的

3.单个中括号[]里面的”<”和“ >”需要转义，且不支持”>=” 和“<=”

4.“z” 测试字符串是否为NULL

5.“n” 测试字符串是否不为NULL

四、代码

#! /bin/bash

echo "test you"

a=12

b=13

# number compare

# -eq: equal

# -ne: not equal

# -gt: great

# -ge: great and equal

# -lt: little

# -le: little and equal

if [ "$a" -ne "$b" ]

then

echo "$a is not equel to $b"

fi

if [ "$a" -eq "$b" ]

then

echo "$a is equal to $b"

else

echo "$a is not equal to $b"

fi

if [ "$a" -eq "$b" ]

then

echo "$a is equal to $b"

elif [ "$a" -lt "$b" ]

then

echo "$a is little to $b"

elif [ "$a" -gt "$b" ]

then

echo "$a is great to $b"

else

echo "Default"

fi

#########Key Point##############

#########1."then" after "if" or "elif"############

#########2."elif" is ok, not "else if"############

c=abc

d=abd

e=

#string compare

if [ "$c" == "$d" ]

then

echo "string $c is equal to $d"

elif [ "$c" /> "$d" ]

then

echo "string $c is great to $d"

elif [ "$c" /< "$d" ]

then

echo "string $c is little to $d"

elif [ "$c" != "$d" ]

then

echo "string $c is not equal to $d"

fi

if [ -z "$e" ]

then

echo " $e is NULL"

else

echo " $e is not NULL"

fi

if [ -n "$e" ]

then

echo "$e is not NULL"

else

echo "$e is NULL"

fi

###########KEy point#################

###########1.For string comparing, ">" "<"need escape symbol

###########2."z": zero is used to test whether the string is NULL

###########3."n": not zero is used to test whether the string is not NULL

f=8

if [ "$f" -gt 0 ]

then

echo "f is positive"

if [ "$f" -gt 5 ]

then

echo "f is a big positive"

else

echo "f is a small positive"

fi

else

echo "f is negative"

if [ "$f" -gt -5 ]

then

echo "f is a big negative"

else

echo "f is a small negative"

fi

fi