shell编程——if语句

时间：2021-05-25 07:27:41

if
语句格式

if  条件

then

 Command

else

 Command

fi                              别忘了这个结尾
If语句忘了结尾fi

test.sh: line 14: syntax error: unexpected end of fi

if
的三种条件表达式

if

command

then



if

 函数

then		  命令执行成功，等于返回0 （比如grep ,找到匹配）

执行失败，返回非0 （grep,没找到匹配）
if [ expression_r_r_r  ]

then 		  表达式结果为真，则返回0，if把0值引向then
if test expression_r_r_r

then		  表达式结果为假，则返回非0，if把非0值引向then

[
] &&  ——快捷if

[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors"
   && 可以理解为then

    如果左边的表达式为真则执行右边的语句

shell的if与c语言if的功能上的区别

 shell if     c语言if
0为真，走then  正好相反，非0走then
 不支持整数变量直接if

必须:if [ i –ne 0 ]



但支持字符串变量直接if

if [ str ] 如果字符串非0		  支持变量直接if

if (i )

=================================以command作为if 条件===================================

  

    以多条command或者函数作为if
条件

echo –n “input:”

read user



if

多条指令,这些命令之间相当于“and”（与）

grep $user /etc/passwd >/tmp/null      

who -u | grep $user

then             上边的指令都执行成功,返回值$?为0，0为真，运行then

 echo "$user has logged"

else     指令执行失败，$?为1，运行else                            

 echo "$user has not logged"

fi   
# sh test.sh

input : macg

macg     pts/0        May 15 15:55   .          2075 (192.168.1.100)

macg has logged

   

# sh test.sh

input : ddd

ddd has not logged  

以函数作为if条件  (函数就相当于command,函数的优点是其return值可以自定义)

if

以函数作为if条件，

getyn

then   函数reture值0为真，走then

echo " your answer is yes"

else  函数return值非0为假，走else

echo "your anser is no"

fi  

if
command  等价于 command+if $?

$ vi testsh.sh

#!/bin/sh



if

cat 111-tmp.txt | grep ting1

then

echo found

else

echo "no found"

fi		  $ vi testsh.sh

#!/bin/sh



cat 111-tmp.txt | grep ting1



if [ $? -eq 0 ]

then

echo $?

echo found

else

echo $?

echo "no found"

fi
$ sh testsh.sh

no found  		 $ sh testsh.sh

1

no found
$ vi 111-tmp.txt

that is 222file

thisting1 is 111file



$ sh testsh.sh

thisting1 is 111file

found		 $ vi 111-tmp.txt

that is 222file

thisting1 is 111file



$ sh testsh.sh

thisting1 is 111file

0

found

========================================以条件表达式作为 if条件=============================



    传统if
从句子——以条件表达式作为 if条件

if [ 条件表达式 ]

then

 command

 command

 command

else

 command

 command

fi

   

   条件表达式

  • 文件表达式

if [ -f  file ]    如果文件存在

if [ -d ...   ]    如果目录存在

if [ -s file  ]    如果文件存在且非空 

if [ -r file  ]    如果文件存在且可读

if [ -w file  ]
   如果文件存在且可写

if [ -x file  ]
   如果文件存在且可执行

  • 整数变量表达式

if [ int1 -eq int2 ]    如果int1等于int2   

if [ int1 -ne int2 ]    如果不等于    

if [ int1 -ge int2 ]       如果>=

if [ int1 -gt int2 ]       如果>

if [ int1 -le int2 ]       如果<=

if [ int1 -lt int2 ]       如果<

  • 字符串变量表达式

If  [
$a = $b ]                 如果string1等于string2

                                字符串允许使用赋值号做等号

if  [
$string1 !=  $string2
]   如果string1不等于string2       

if  [ -n $string  ]             如果string
非空(非0），返回0(true)  

if  [
-z $string  ]             如果string
为空

if  [ $sting ]                  如果string
非空，返回0 (和-n类似)    





    条件表达式引用变量要带$

if [ a = b ] ;then    

echo equal

else

echo no equal

fi
[macg@machome ~]$ sh test.sh

input a:

5

input b:

5

no equal  （等于表达式没比较$a和$b,而是比较和a和b,自然a!=b)

改正：

if [ $a = $b ] ;then       

echo equal

else

echo no equal

fi
[macg@machome ~]$ sh test.sh

input a:

5

input b:

5

equal

-eq  -ne  -lt  -nt只能用于整数，不适用于字符串，字符串等于用赋值号=

[macg@machome ~]$ vi test.sh

echo -n "input your choice:"

read var

if  [ $var -eq "yes" ]

then

echo $var

fi

[macg@machome ~]$ sh -x test.sh

input your choice:

y

test.sh: line 3: test: y: integer expression_r_r_r expected

                       期望整数形式，即-eq不支持字符串

  =放在别的地方是赋值,放在if
[ ] 里就是字符串等于,shell里面没有==的,那是c语言的等于





   无空格的字符串，可以加"
",也可以不加

[macg@machome ~]$ vi test.sh

echo "input a:"

read a

echo "input is $a"

if [ $a = 123 ] ; then

echo equal123

fi
[macg@machome ~]$ sh test.sh

input a:

123

input is 123

equal123 

   =
作为等于时，其两边都必须加空格，否则失效

等号也是操作符，必须和其他变量，关键字，用空格格开 (等号做赋值号时正好相反，两边不能有空格）

[macg@machome ~]$ vi test.sh



echo "input your choice:"

read var

if [ $var="yes" ]

then

echo $var

echo "input is correct"

else

echo $var

echo "input error"

fi		 [macg@machome ~]$ vi test.sh



echo "input your choice:"

read var

if [ $var = "yes" ]   在等号两边加空格

then

echo $var

echo "input is correct"

else

echo $var

echo "input error"

fi
[macg@machome ~]$ sh test.sh

input your choice:

y

y

input is correct

[macg@machome ~]$ sh test.sh

input your choice:

n    

n

input is correct 

输错了也走then,都走then,为什么?

因为if把$var="yes"连读成一个变量，而此变量为空，返回1，则走else 		 [macg@machome ~]$ sh test.sh

input your choice:

y

y

input error

[macg@machome ~]$ sh test.sh

input your choice:

no                       

no

input error

一切正常

If  [  $ANS  ]     等价于  if
[ -n $ANS ]

      如果字符串变量非空（then）
, 空(else)

echo "input your choice:"

read ANS



if [ $ANS ]

then

echo no empty

else

echo empth

fi 
[macg@machome ~]$ sh test.sh

input your choice:                       回车

                                                

empth                                    说明“回车”就是空串

[macg@machome ~]$ sh test.sh

input your choice:

34

no empty 

整数条件表达式，大于，小于，shell里没有>
和< ,会被当作尖括号，只有-ge,-gt,-le,lt

[macg@machome ~]$ vi test.sh



echo "input a:"

read a

if  [ $a -ge 100 ] ; then

echo 3bit

else

echo 2bit

fi
[macg@machome ~]$ sh test.sh

input a:

123

3bit

[macg@machome ~]$ sh test.sh

input a:

20

2bit

整数操作符号-ge,-gt,-le,-lt,
别忘了加-

if  test $a  ge 100 ; then



[macg@machome ~]$ sh test.sh

test.sh: line 4: test: ge: binary operator expected
if  test $a -ge 100 ; then



[macg@machome ~]$ sh test.sh

input a:

123

3bit

============================逻辑表达式=========================================



    逻辑非 !                   条件表达式的相反

if [ ! 表达式 ]

if [ ! -d $num ]                        如果不存在目录$num





    逻辑与 –a                    条件表达式的并列

if [ 表达式1  –a  表达式2
]





    逻辑或
-o                    条件表达式的或

if [ 表达式1  –o
表达式2 ]



   

   逻辑表达式

  • 表达式与前面的=  != -d –f –x -ne -eq -lt等合用
  • 逻辑符号就正常的接其他表达式，没有任何括号（ ），就是并列

if [ -z "$JHHOME" -a -d $HOME/$num ]

  • 注意逻辑与-a与逻辑或-o很容易和其他字符串或文件的运算符号搞混了

最常见的赋值形式，赋值前对=两边的变量都进行评测

左边测变量是否为空，右边测目录(值)是否存在（值是否有效）

 
[macg@mac-home ~]$ vi test.sh

:

echo "input the num:"

read num

echo "input is $num"



if [ -z "$JHHOME" -a -d $HOME/$num ]   如果变量$JHHOME为空，且$HOME/$num目录存在

then

JHHOME=$HOME/$num                      则赋值

fi



echo "JHHOME is $JHHOME"  
-----------------------

[macg@mac-home ~]$ sh test.sh

input the num:

ppp

input is ppp

JHHOME is



目录-d $HOME/$num   不存在，所以$JHHOME没被then赋值
[macg@mac-home ~]$ mkdir ppp

[macg@mac-home ~]$ sh test.sh

input the num:

ppp

input is ppp

JHHOME is /home/macg/ppp

   一个-o的例子，其中却揭示了”=”必须两边留空格的问题

echo "input your choice:"

read ANS



if [ $ANS="Yes" -o $ANS="yes" -o $ANS="y" -o $ANS="Y" ]

then

ANS="y"

else

ANS="n"

fi



echo $ANS
[macg@machome ~]$ sh test.sh

input your choice:

n

y

[macg@machome ~]$ sh test.sh

input your choice:

no

y

为什么输入不是yes,结果仍是y(走then）

因为=被连读了，成了变量$ANS="Yes"，而变量又为空，所以走else了
[macg@machome ~]$ vi test.sh



echo "input your choice:"

read ANS    echo "input your choice:"

read ANS



if [ $ANS = "Yes" -o $ANS = "yes" -o $ANS = "y" -o $ANS = "Y" ]

then

ANS="y"

else

ANS="n"

fi



echo $ANS
[macg@machome ~]$ sh test.sh

input your choice:

no

n

[macg@machome ~]$ sh test.sh

input your choice:

yes

y

[macg@machome ~]$ sh test.sh

input your choice:

y

y

===================以  test
条件表达式 作为if条件===================================



    if
test $num -eq 0      等价于   if [ $num –eq 0 ]



    test  表达式,没有
[  ]

if test $num -eq 0                

then

echo "try again"

else

echo "good"

fi



    man test

[macg@machome ~]$ man test

[(1)                             User Commands                            [(1)



SYNOPSIS

       test EXPRESSION

       [ EXPRESSION ]





       [-n] STRING

              the length of STRING is nonzero          -n和直接$str都是非0条件



       -z STRING

              the length of STRING is zero



       STRING1 = STRING2

              the strings are equal



       STRING1 != STRING2

              the strings are not equal



       INTEGER1 -eq INTEGER2

              INTEGER1 is equal to INTEGER2



       INTEGER1 -ge INTEGER2

              INTEGER1 is greater than or equal to INTEGER2



       INTEGER1 -gt INTEGER2

              INTEGER1 is greater than INTEGER2



       INTEGER1 -le INTEGER2

              INTEGER1 is less than or equal to INTEGER2



       INTEGER1 -lt INTEGER2

              INTEGER1 is less than INTEGER2



       INTEGER1 -ne INTEGER2

              INTEGER1 is not equal to INTEGER2



       FILE1 -nt FILE2

              FILE1 is newer (modification date) than FILE2



       FILE1 -ot FILE2

              FILE1 is older than FILE2



       -b FILE

              FILE exists and is block special



       -c FILE

              FILE exists and is character special



       -d FILE

              FILE exists and is a directory



       -e FILE

              FILE exists                                 文件存在



       -f FILE

              FILE exists and is a regular file     文件存在且是普通文件



       -h FILE

              FILE exists and is a symbolic link (same as -L)



       -L FILE

              FILE exists and is a symbolic link (same as -h)



       -G FILE

              FILE exists and is owned by the effective group ID



       -O FILE

              FILE exists and is owned by the effective user ID



       -p FILE

              FILE exists and is a named pipe





       -s FILE

              FILE exists and has a size greater than zero



       -S FILE

              FILE exists and is a socket



       -w FILE

              FILE exists and is writable



       -x FILE

FILE exists and is executable

 

======================if简化语句=================================



    最常用的简化if语句

   && 如果是“前面”，则“后面”

[ -f /var/run/dhcpd.pid ] && rm /var/run/dhcpd.pid    检查 文件是否存在，如果存在就删掉
   ||   如果不是“前面”，则后面

[ -f /usr/sbin/dhcpd ] || exit 0    检验文件是否存在，如果存在就退出

用简化
if 和$1,$2,$3来检测参数，不合理就调用help

[ -z "$1" ] && help                 如果第一个参数不存在（-z  字符串长度为0
）

[ "$1" = "-h" ] && help                        如果第一个参数是-h,就显示help





例子

#!/bin/sh



[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0

cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0



[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1

cp ifcfg-eth1.bridge /etc/sysconfig/network-scripts/ifcfg-eth1

[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0:1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0:1

转至：http://blog.sina.com.cn/s/blog_6151984a0100ekl6.html

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