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分类: LINUX

2012-11-28 16:28:15

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
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