shell编程——if语句 if -z -n -f -eq -ne -lt

shell编程中条件表达式的使用

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