六、流程控制语句

选择结构if 只有条件判断结果为真时才执行相应的操作

循环结构
for、while/until 反复执行相同操作时&＃xff0c;使用循环结构

分支结构case 根据变量值的匹配结果执行相应的操作

当&＃39;条件成立&＃39;时执行相应的操作
否则&＃xff0c;不执行任何操作

if 条件判断表达式;then条件判断成立时&＃xff0c;要执行的操作fi

if 条件判断表达式
then条件判断成立时&＃xff0c;要执行的操作
fi

#!/bin/bash
ping -c 3 localhost &> /dev/null
if [ $? -eq 0 ];thenecho aaaaafi

#!/bin/bash
read -p "please input ip address " addr
if [ -z $addr ];thenecho "你没有输入要ping的ip地址"exit
fi
ping -c 3 $addr &> /dev/null
if [ $? -eq 0 ];thenecho "$addr is onlines"
fi
echo "script over!!!"

特殊写法&＃xff1a;if [ -f "$file1" ];then echo 1;fi相当于[ -f "$file1" ] && echo 1

[root&＃64;localhost ~]#cat if_01.sha&＃61;$1b&＃61;$2if [ $a -gt $b ]thenecho "yes,$a >&＃61; $b"fiif [ $a -le $b ]thenecho "yes,$a<&＃61;$b"fi[root&＃64;localhost ~]# sh if_01.sh 4 5yes,4<&＃61;5

read -p "pls input two numbers:" a bif [ $a -gt $b ]thenecho "yes,$a >&＃61; $b"fiif [ $a -le $b ]thenecho "yes,$a<&＃61;$b"fi

pls input two numbers:100 99yes,100 >&＃61; 99

[root&＃64;localhost scripts]# cat if4.sh#!/bin/bashFILEPATH&＃61;"server/scripts"if [ -e "$FILEPATH/if3.sh" ]thenecho "$FILEPATH/if3.sh is exist."fiif [ ! -e "$FILEPATH/if3.sh" ]then[ ! -d $FILEPATH ] && mkdir -p $FILEPATH[ -d $FILEPATH ] && {cd $FILEPATHtouch if3.shecho "if3.sh is touched."}fi

&＃43; FILEPATH&＃61;server/scripts&＃43; &＃39;[&＃39; -e server/scripts/if3.sh &＃39;]&＃39;&＃43; &＃39;[&＃39; &＃39;!&＃39; -e server/scripts/if3.sh &＃39;]&＃39;&＃43; &＃39;[&＃39; &＃39;!&＃39; -d server/scripts &＃39;]&＃39;&＃43; mkdir -p server/scripts&＃43; &＃39;[&＃39; -d server/scripts &＃39;]&＃39;&＃43; cd server/scripts&＃43; touch if3.sh&＃43; echo &＃39;if3.sh is touched.&＃39;if3.sh is touched.

total used free shared buffers cachedMem: 988 769 218 0 97 313-/&＃43; buffers/cache: 358 629Swap: 2047 0 2047

629

cur_free&＃61;&＃96;free -m|awk &＃39;/buffers\// {print $NF}&＃39;&＃96;chars&＃61;"current memory is $cur_free."if [ $cur_free -lt 650 ]thenecho $charsecho $chars|mail -s "chars" 12345678&＃64;qq.comfi

current memory is 629.

[root&＃64;localhost scripts]# cat judge_system.shchars&＃61;"current memory is $cur_free."if [ $cur_free -lt 650 ]thenecho $chars|mail -s "chars" 12345678&＃64;qq.comfi

#judge.....* * * * * /bin/bash /server/scripts/judge_system.sh >&/dev/null

当&＃39;条件成立&＃39;时执行命令序列1
否则&＃xff0c;执行命令序列2

if 条件判断表达式;then条件判断成立时&＃xff0c;要执行的操作
else条件判断不成立时&＃xff0c;要执行的操作
fi

if 条件判断表达式
then条件判断成立时&＃xff0c;要执行的操作
else条件判断不成立时&＃xff0c;要执行的操作
fi

if [ -f “$file1” ];then echo 1;else echo 0;fi相当于&＃xff1a;[ -f “$file1” ] && echo 1 || echo 0

#!/bin/bash
service httpd status &> /dev/null
if [ $? -eq 0 ];thenecho "网站服务正在运行"
elseservice httpd start
fi

#!/bin/bash
service sshd status &> /dev/null
if [ $? -eq 0 ];thenecho "sshd in running"
elseservice sshd start
fi

#!/bin/bash
service $1 status &> /dev/null
if [ $? -eq 0 ];thenecho "$1 is running!!!!"
elseservice $1 start
fi

#!/bin/bash
read -p "请输入服务名" servername
if [ $ servername&＃61;&＃61;"vsftpd" -o $servername&＃61;&＃61;"httpd" -o $servername&＃61;"sshd" -o $servername&＃61;"crond" -o $servername&＃61;mysql];thenservice $servicename status &> /dev/nullif [ $? -eq 0 ];thenecho $ servicename is runningelse service $servicename startfi
fi

read -p "请输入学生成绩&＃xff1a;" chengji
if [ -z $chengji ];thenexit
fi
if [ $chengji -ge 1 ]&& [ $chengji -le 100 ]
thenif [ $cengji -lt 60 ]then echo "不及格"else echo "及格"fi
elseecho "输入格式错误"
fi

#!/bin/bash
#if语句用于判断输入的文件是否不存在&＃xff0c;不存在则执行then和else间的命令
if [ ! -e "$1" ]
then echo "file $1 do not exist."exit 1
#输入的文件存在时&＃xff0c;执行else和fi间的命令
elseecho "file $1 exist."
fi

#!/bin/bash
#提示用户输入要删除的文件
echo "Please input the file which you want to delete:" read file
#通过if/else结构判断文件是否被删除
if rm -f "$file"
then echo "Delete the file $file successfully!"
elseecho "Delete the file $file failed!"
fi

#!/bin/bash
mysql&＃61;"/var/lib/mysql"
date&＃61;(date &＃43;\%Y\%m\%d)
cd $mysql
if [ -e /binlogdir ];thentar -zcf /binlogdir/binlog-$date.tar.gz data-bin.* &> /dev/null
elsemkdir /binlogdirtar -zcf /binlogdir/binlog-$date.tar.gz data-bin.* &> /dev/null
fi

相当于if语句嵌套
针对多个条件分别执行不同的操作

if 条件判断表达式1;then条件1判断成立时&＃xff0c;要执行的操作

elif 条件判断表达式2;then条件2判断成立时&＃xff0c;要执行的操作
if 条件判断表达式N;then条件N判断成立时&＃xff0c;要执行的操作
else以上所有的条件判断都不成立时&＃xff0c;要执行的操作
fi

#!/bin/bash
read -p "输入成绩" cj
if [ -z $cj ];thenecho "你没有输入成绩分数"exit
elseif [ $cj -eq 0 ];thenecho "补考"elif [ $cj -lt 0 ];thenecho "输入的成绩超过范围&＃xff0c;有效范围在1-100"

elif [ $cj -ge 1 ]&&[ $cj -lt 60 ];thenecho "不及格"

elif [ $cj -ge 60 ]&&[ $cj -lt 80 ];thenecho "良好"

elif [ $cj -ge 80 ]&&[ $cj -le 100 ];thenecho "优秀"

elif [ $cj -gt 1 ];thenecho "输入的成绩超过范围&＃xff0c;有效范围在1-100"fi
fi

#!/bin/bash
#
if [ $1 &＃61; &＃39;q&＃39; ];thenecho "Quiting..."exit 1
elif [ $1 &＃61; &＃39;Q&＃39; ];thenecho "Quiting..."exit 2
elif [ $1 &＃61; &＃39;quit&＃39; ];thenecho "Quiting..."exit 3
elif [ $1 &＃61; &＃39;Quit&＃39; ];thenecho "Quiting..."exit 4
elseecho $1
fi

#!/bin/bash
#提示用户输入分数&＃xff08;0&＃xff5e;100&＃xff09;
echo "Please Input a integer(0-100): "
read score
#判断学生的分数类别
if [ "$score" -lt 0 -o "$score" -gt 100 ]
thenecho "The score what you input is not integer or score is not in (0-100)"
elif [ "$score" -ge 90 ]
thenecho "The grade is A!"
elif [ "$score" -ge 80 ]
then echo "The grade is B! "
elif [ "$score" -ge 70 ]
thenecho "The grade is C!"
elif [ "$score" -ge 60 ]
thenecho "The grade is D! "
elseecho "The grade is E! "
fi

#ifelifelse-exam2.sh脚本的执行结果
#!/bin/bash
#提示输入年份
echo "Please input a year: "
read year
#设置取余参数
let "n1&＃61;$year % 4"
let "n2&＃61;$year % 100"
let "n3&＃61;$year % 400"
#判断输入的年份是否是闰年
if [ ! "$n1" -eq 0 ]
then leap&＃61;0
elif [ ! "$n2" -eq 0 ]
then leap&＃61;1
elif [ ! "$n3" -eq 0 ]
then leap&＃61;0
elseleap&＃61;1
fi
#输出用户输入的年份是否是闰年
if [ "$leap" -eq 1 ]
then echo "$year is a leap year!"
elseecho "$year is not a leap year!"
fi

[root&＃64;localhost scripts]# cat if_02.shread -p "pls input two numbers:" a bif [ $a -gt $b ]thenecho "yes,$a > $b"elif [ $a -eq $b ]thenecho "yes,$a&＃61;$b"elseecho "yes,$a<$b"fi

a&＃61;$1b&＃61;$2if [ $# -ne 2 ]thenecho "Usage:sh $0 num1 num2"exit 1fiif [ $a -gt $b ]thenecho "yes,$a > $b"elif [ -eq $b ]thenecho "yes,$a&＃61;$b"elseecho "yes,$a<$b"fi

[root&＃64;localhost scripts]# cat if_02.sha&＃61;$1b&＃61;$2if [ $# -ne 2 ]thenecho "Usage:sh $0 num1 num2"exit 1fi[ -n "&＃96;echo $1|sed &＃39; s/[0-9]//g&＃39;&＃96;" ]&& echo "第一个参数必须为数字" && exit 1[ -n "&＃96;echo $2|sed &＃39; s/[0-9]//g&＃39;&＃96;" ]&& echo "第二个参数必须为数字" && exit 1if [ $a -gt $b ]thenecho "yes,$a > $b"elif [ -eq $b ]thenecho "yes,$a&＃61;$b"elseecho "yes,$a<$b"fi

[ -n "&＃96;echo $num|sed &＃39; s/[0-9]//g&＃39;&＃96;" -a -n "&＃96;echo $2|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] &&\

[ -n "&＃96;echo $num|sed &＃39; s/[0-9]//g&＃39;&＃96;" -a -n "&＃96;echo $2|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] &&{echo "两个参数都必须为数字

exit 1}

[ -n "&＃96;echo $1|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] && echo "第一个参数必须为数字" && exit 1[ -n "&＃96;echo $2|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] && echo "第二个参数必须为数字" && exit 1

[ -n "&＃96;echo $num|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] && echo "第二个参数必须为数字" && exit 1

[ -n "&＃96;echo $num|sed &＃39; s/[0-9]//g&＃39;&＃96;" ] && {echo "第二个参数必须为数字"exit 1}

0

0 #这个说明前面的结果不为0&＃xff0c;即有非数字字符

1 #这个说明前面的结果去掉数字后为0&＃xff0c;即没有非数字字符

-z “$num” #如果num长度不为0表达式“$num” &＃61;"${num//[^0-9]/}" #把num中的非数字部分删除&＃xff0c;然后结果是不是等于num本身&＃xff0c;如果两者都成立就是数字。

[root&＃64;localhost scripts]# [ -n "$num" -a "$num" &＃61; "$num//[^0-9]/" ] && echo "it is num"

expr $1 &＃43; 0>/dev/null 2>&1 #把变量和整数相加看是否成功执行[ $? -eq 0 ] && eccho init

[root&＃64;localhost scripts]#cat if-multi.sh#!/bin/bash#this scripts is created by oldboy#fun:compare two numread -t 10 -p "pls input two num:" a bif [ $a -gt $b ];thenecho "yes,$a > $b"elif [ -eq $b ];thenecho "yes,$a&＃61;$b"elseecho "yes,$a<$b"fi

[root&＃64;localhost scripts]#cat if-multi.sh#!/bin/bash#this scripts is created by oldboy#fun:compare two numread -t 10 -p "pls input two num:" a bif (( $a > $b ));thenecho "yes,$a > $b"elif [ -eq $b ];thenecho "yes,$a&＃61;$b"elseecho "yes,$a<$b"fi

[root&＃64;localhost script]# cat check_db.sh#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|awk -F &＃39;[ :]&＃43;&＃39; &＃39;{print $5}&＃39;&＃96; #这里的思路不是最佳的if [ #portNum -eq 3306 ];then #这里的判断最好用字符串判断的语法&＃xff1b;假如端口不存在&＃xff0c;显示[: -eq: unary operator expectedecho "db is running"else/data/3306/mysql restart #这是多实例MySQL启动脚本fi

#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|awk -F &＃39;[ :]&＃43;&＃39; &＃39;{print $5}&＃39;&＃96;if [ $portNum -eq 3306 ];thenecho "db is running"else/etc/init.d/mysqld restart #普通MySQL服务启动fi

[root&＃64;localhost script]# cat check_db01.sh#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|awk -F &＃39;[ :]&＃43;&＃39; &＃39;{print $5}&＃39;&＃96;if [ $portNum &＃61;&＃61; “3306” ];thenecho "db is running"else/data/3306/mysql restartfi

[root&＃64;localhost script]# cat check_db02.sh#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|wc -l&＃96;if [ #portNum -eq 1 ];thenecho "db is running"else/data/3306/mysql restartfi

[root&＃64;localhost script]# /data/3306/mysql start正在启动 mysqld&＃xff1a;

tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN

2 #通过把进程转换成为行数&＃xff0c;即进程个数&＃xff0c;方便做判断。

1 #通过把端口转换为行数&＃xff0c;即端口个数&＃xff0c;方便做判断

[root&＃64;localhost script]# cat check_db03.sh#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|wc -l&＃96;mysqlProcessNum&＃61;&＃96;ps -ef|grep mysql|grep -v grep|wc -l&＃96;if [ #portNum -eq 1 -a $mysqlProcessNum -eq 2 ];thenecho "db is running"else/data/3306/mysql restartfi

[root&＃64;localhost script]# cat check_db03.sh#!/bin/bash###portNum&＃61;&＃96;netstat -lnt | grep 3306|wc -l&＃96;mysqlProcessNum&＃61;&＃96;ps -ef|grep mysql|grep -v grep|wc -l&＃96;if [ #portNum -eq 1 ] && [ $mysqlProcessNum -eq 2 ];thenecho "db is running"else/data/3306/mysql restartfi

[root&＃64;localhost script]# cat if-judge-db0.sh#!/bin/bash###MYSQL&＃61;/data/3306/mysqlLogPath&＃61;/tmp/mysql.logportNum&＃61;&＃96;netstat -lnt | grep 3306|wc -l&＃96;mysqlProcessNum&＃61;&＃96;ps -ef|grep mysql|grep -v grep|wc -l&＃96;if [ #portNum -eq 1 ] && [ $mysqlProcessNum -eq 2 ];thenecho "db is running"else$MYSQL start >$LogPathsleep 10portNum&＃61;&＃96;netstat -lnt | grep 3306|wc -l&＃96;mysqlProcessNum&＃61;&＃96;ps -ef|grep mysql|grep -v grep|wc -l&＃96;if [ #portNum -ne 1 ] && [ $mysqlProcessNum -ne 2 ];thenwhile truedokillall mysqld >/dev/null 2>&1[ $? -ne 0 ] && breaksleep 1done$MYSQL start >>$LogPath && status&＃61;"successful" || status&＃61;"failure"mail -s "mysql startup status is $status" 1092327070&＃64;qq.com < $LogPathfifi

[root&＃64;localhost script]# mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();"

[root&＃64;localhost script]# cat check_db.sh
#!/bin/bash
###
mysqlStatus&＃61;&＃96;mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();" >& /dev/null&＃96;if [ $? -eq 0 ];thenecho "db is running"
else/data/3306/mysql restart
fi

#!/bin/bash
###
MYSQL&＃61;/data/3306/mysql
LogPath&＃61;/tmp/mysql.log
mysqlStatus&＃61;&＃96;mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();" >&/dev/null&＃96;if [$? -eq 0 ];then
echo "db is running"
exit 0
else$MYSQL start >$LogPathsleep 10mysqlStatus&＃61;&＃96;mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();" >&/dev/null&＃96;if [ $? -ne 0 ];thenwhile truedo killall mysqld >/dev/null 2>&1[ $? -ne 0 ] && breaksleep 1done$MYSQL start >>$LogPath && status&＃61;"successful" || status&＃61;"failure"mail -s "mysql startup status is $status" 12345678&＃64;qq.com < $LogPathfi
fi

#!/bin/bash
###
MYSQL&＃61;/data/3306/mysql
LogPath&＃61;/tmp/mysql.log
mysqlStatus&＃61;&＃96;mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();" >&/dev/null&＃96;if [$? -eq 0 ];thenecho "db is running"
else[ -x $MYSQL ] && $MYSQL start >$LogPathsleep 10mysqlStatus&＃61;&＃96;mysql -uroot -p &＃39;oldboy&＃39; -S /data/3306/mysql.sock -e "select version();" >&/dev/null&＃96;if [ $? -ne 0 ];thenwhile truedo killall mysqld >/dev/null 2>&1[ $? -ne 0 ] && breaksleep 1done[ -x $MYSQL ] && $MYSQL start >>$LogPath && status&＃61;"successful" || status&＃61;"failure"mail -s "mysql startup status is $status" 1092327070&＃64;qq.com < $LogPathfi
fi