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

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

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
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
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:

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:

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:

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

[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

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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