Linux shell编程——if条件判断
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thenif 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if0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 [ -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 和< ,会被当作尖括号，只有-ge,-gt,-le,lt
$ vi test.sh echo "input a:"
read a
if[ $a -ge 100 ] ; then
echo 3bit
else
echo 2bit
fi
$ sh test.sh input a:
123
3bit
$ sh test.sh
input a:
20
2bit
整数操作符号-ge,-gt,-le,-lt, 别忘了加-
iftest $age 100 ; then $ sh test.sh
test.sh: line 4: test: ge: binary operator expected
iftest $a -ge 100 ; then $ 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）
$ 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"
----------------------- $ 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
$ mkdir ppp $ 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
$ sh test.sh input your choice:
n
y
$ 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
$ 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
$ sh test.sh input your choice:
no
n
$ sh test.sh
input your choice:
yes
y
$ 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
$ 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> -O FILE
FILE exists and is owned by the effective user> -p FILE
FILE exists and is a named pipe
-s FILE
FILE exists and has a> -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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