shell编程——if语句 if -z -n -f -eq -ne -lt

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
macgpts/0May 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 [ -ffile ]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-

iftest $age100 ; 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

============================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 ANSecho "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