shell编程――if语句 if -z -n -f -eq -ne -lt

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if条件
then
Command
else
Command
@H_502_14@fi别忘了这个结尾
If语句忘了结尾fi
test.sh: line 14: Syntax error:@H_502_14@unexpected end of fi




@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@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


@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@[ ] &&――快捷if

[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors"
@H_502_14@&& 可以理解为then
如果左边的表达式为真则执行右边的语句

@H_502_14@

@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@shell的if与c语言if的功能上的区别

shell if c语言if
@H_502_14@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 �Cne 0 ]

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 )


@H_502_14@
@H_502_14@=================================以command作为if 条件===================================
@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@以多条command或者函数作为if 条件

echo �Cn “input:”
read user

if
多条指令,这些命令之间相当于@H_502_14@“and”(与)
grep $user /etc/passwd >/tmp/null
who -u | grep $user
then上边的指令@H_502_14@都执行成功,返回值$?为0,@H_502_14@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
macgpts/0May 15 15:55.2075 (192.168.1.100)
macg has logged

# sh test.sh
input : ddd
ddd has not logged



@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@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
函数作为if条件,
getyn
then函数reture值@H_502_14@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



@H_502_14@@H_502_14@@H_502_14@@H_502_14@if command等价于 command+if $?

$ vi testsh.sh
#!/bin/sh

@H_502_14@if
@H_502_14@cat 111-tmp.txt | grep ting1
@H_502_14@then
echo found
else
echo "no found"
fi
$ vi testsh.sh
#!/bin/sh

@H_502_14@cat 111-tmp.txt | grep ting1

@H_502_14@if [ $? -eq 0 ]
@H_502_14@then
@H_502_14@echo $?
echo found
else
@H_502_14@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


@H_502_14@@H_502_14@@H_502_14@
@H_502_14@========================================以条件表达式作为 if条件=============================

@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@传统if 从句子――以条件表达式作为 if条件
@H_502_14@if [ 条件表达式 ]
@H_502_14@then
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@else
@H_502_14@@H_502_14@command
@H_502_14@@H_502_14@command
@H_502_14@fi
@H_502_14@@H_502_14@@H_502_14@
@H_502_14@条件表达式

@H_502_14@if [ -ffile ]如果文件存在
@H_502_14@if [ -d ...]如果目录存在
@H_502_14@if [ -s file]如果文件存在且非空
@H_502_14@if [ -r file@H_502_14@@H_502_14@]@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果文件存在且可读
@H_502_14@if [ -w file@H_502_14@@H_502_14@] @H_502_14@@H_502_14@@H_502_14@如果文件存在且可写
@H_502_14@if [ -x file@H_502_14@@H_502_14@] @H_502_14@@H_502_14@@H_502_14@如果文件存在且可执行 @H_502_14@

  • @H_301_410@整数变量表达式

@H_502_14@if [ int1 -eq int2 ]如果int1等于int2
@H_502_14@if [ int1 -ne int2 ]如果不等于 @H_502_14@
@H_502_14@if [ int1 -ge int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果>=
@H_502_14@if [ int1 -gt int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果>
@H_502_14@if [ int1 -le int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果<=
@H_502_14@if [ int1 -lt int2 ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果<
@H_502_14@@H_502_14@@H_502_14@

  • @H_301_410@字符串变量表达式

@H_502_14@If@H_502_14@@H_502_14@[ $a = $b ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果string1等于string2
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@字符串允许使用赋值号做等号
@H_502_14@if@H_502_14@@H_502_14@[ $string1 !=@H_502_14@@H_502_14@$string2 ]@H_502_14@@H_502_14@@H_502_14@如果string1不等于string2@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@if[ -n $string]如果string 非空(非0),返回0(true)@H_502_14@@H_502_14@
@H_502_14@if@H_502_14@@H_502_14@[ -z $string@H_502_14@@H_502_14@]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果string 为空
@H_502_14@if[ $sting ]如果string 非空,返回0 (和-n类似) @H_502_14@@H_502_14@


@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@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
@H_502_14@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)

@H_502_14@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


@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@@H_502_14@@H_502_14@-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
@H_502_14@test.sh: line 3: test: y: integer expression_r_r_r expected
@H_502_14@期望整数形式,即-eq不支持字符串




@H_502_14@@H_502_14@@H_502_14@=放在别的地方是赋值,放在if [ ] 里就是字符串等于,@H_502_14@shell里面没有==的,那是c语言的等于


@H_502_14@@H_502_14@@H_502_14@@H_502_14@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



@H_502_14@@H_502_14@= 作为等于时,其两边都必须加空格,否则失效
@H_502_14@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 [@H_502_14@$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 [@H_502_14@$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?
@H_502_14@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



@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@If[$ANS]等价于if [ -n $ANS ]
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@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@H_502_14@说明“回车”就是空串
[macg@machome ~]$ sh test.sh
input your choice:
34
no empty


@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@整数条件表达式,大于,小于@H_502_14@,s@H_502_14@hell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



@H_502_14@@H_502_14@整数操作符号-ge,-lt,别忘了加-

iftest $a@H_502_14@ge100 ; then

[macg@machome ~]$ sh test.sh
test.sh: line 4: test: ge: binary operator expected
iftest $a -ge 100 ; then

[macg@machome ~]$ sh test.sh
input a:
123
3bit



@H_502_14@============================逻辑表达式=========================================
@H_502_14@
@H_502_14@逻辑非 !条件表达式的相反
@H_502_14@if [ ! 表达式 ]
@H_502_14@if [ ! -d $num ]@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果不存在目录$num


@H_502_14@逻辑与 �Ca@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@条件表达式的并列
@H_502_14@if [ 表达式1@H_502_14@@H_502_14@�Ca@H_502_14@@H_502_14@表达式2 ]


@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@逻辑或 -o@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@条件表达式的或
@H_502_14@if [ 表达式1@H_502_14@@H_502_14@�Co 表达式2 ]

@H_502_14@@H_502_14@@H_502_14@@H_502_14@
逻辑表达式

  • @H_301_410@表达式与前面的=!= -d �Cf �Cx -ne -eq -lt等合用

  • @H_301_410@逻辑符号就正常的接其他表达式,没有任何括号( ),就是并列

@H_502_14@if [ -z "$JHHOME" -a -d $HOME/$num ]

  • @H_301_410@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



@H_502_14@@H_502_14@@H_502_14@最常见的赋值形式,赋值前对=两边的变量都进行评测
@H_502_14@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"

@H_502_14@if [ -z "$JHHOME" -a -d $HOME/$num ]如果变量$JHHOME为空,且$HOME/$num目录存在
@H_502_14@then
@H_502_14@JHHOME=$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@H_502_14@-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



@H_502_14@@H_502_14@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 [@H_502_14@$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@H_502_14@$ANS="Yes",而变量又为空,所以走else了

[macg@machome ~]$ vi test.sh

echo "input your choice:"
read ANSecho "input your choice:"
read ANS

if [@H_502_14@$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



@H_502_14@===================以@H_502_14@@H_502_14@test 条件表达式 作为if条件===================================

@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@if test $num -eq 0等价于if [ $num �Ceq 0 ]

@H_502_14@@H_502_14@@H_502_14@test表达式,没有 []
@H_502_14@if test $num -eq 0@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@
@H_502_14@then
@H_502_14@echo "try again"
@H_502_14@else
@H_502_14@echo "good"
@H_502_14@fi

@H_502_14@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



@H_502_14@======================if简化语句=================================

@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@最常用的简化if语句

@H_502_14@&& 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
@H_502_14@||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

@H_502_14@
@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@用简化 if 和$1,$2,$3来检测参数,不合理就调用help
@H_502_14@[ -z "$1" ] && help如果第一个参数不存在(-z字符串长度为0 )
@H_502_14@[ "$1" = "-h" ] && help@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@@H_502_14@如果第一个参数是-h,就显示help


@H_502_14@例子
@H_502_14@#!/bin/sh

@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0
@H_502_14@cp ifcfg-eth0.bridge /etc/sysconfig/network-scripts/ifcfg-eth0

@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth1
@H_502_14@cp ifcfg-eth1.bridge /etc/sysconfig/network-scripts/ifcfg-eth1

@H_502_14@[ -f "/etc/sysconfig/network-scripts/ifcfg-eth0:1" ] && rm -f /etc/sysconfig/network-scripts/ifcfg-eth0:1

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