shell流程控制语句

一.单分支结构

if [ 条件表达式 ];then
　　command
fi

或

if [ 条件表达式 ]

then
　　command
fi

或

[ 条件表达式 ] && command

执行逻辑如下图

二.双分支结构

if [ 条件表达式 ];then
　　command1
else
　　command2
fi

或

if [ 条件表达式 ]

then
　　command1
else
　　command2
fi

或

[ 条件 ] && 命令1 || 命令2

执行逻辑如下图

三.多分支结构

if [ 条件表达式1 ];then
　　command1
elif [ 条件表达式2 ];then
　　command2
else
　　command3
fi

或

if [ 条件表达式1 ]

then
　　command1
elif [ 条件表达式2 ]

then
　　command2
else
　　command3
fi

执行逻辑：

如果条件1满足，执行命令1后结束；
如果条件1不满足，再看条件2，如果条件2满足执行命令2后结束；
如果条件1和条件2都不满足执行命令3结束.

图示如下

四.if的嵌套使用

if [ 条件1 ];then
　　命令1
　　if [ 条件2 ];then
　　　　命令2
　　fi
else
　　if [ 条件3 ];then
　　　　命令3
　　elif [ 条件4 ];then
　　　　命令4
　　else
　　　　命令5
　　fi
fi

执行逻辑如下：

如果条件1满足，执行命令1；如果条件2也满足执行命令2，如果不满足就只执行命令1结束；
如果条件1不满足，不看条件2；直接看条件3，如果条件3满足执行命令3；如果不满足则看条件4，如果条件4满足执行命令4；否则执行命令5

五.相关练习

1.当内存剩余空间小于900M时，发出提示信息

[root@server shell02]# free -m
             total       used       free     shared    buffers     cached
Mem:           980        136        844          0         11         43
-/+ buffers/cache:         81        899
Swap:         1983          0       1983
[root@server shell02]# cat 7.sh
#!/bin/bash

free=$(free -m |grep ^Mem|tr -s " "|cut -d " " -f 4)

if [ $free -le 900 ]
then
        echo "system mem less than 900M now."
fi
[root@server shell02]# sh 7.sh
system mem less than 900M now.

2.判断vsftp软件是否已经安装了

[root@server shell02]# cat 11.sh
#!bin/bash

rpm -q vsftpd &> /dev/null

if [ $? -eq 0 ]
then
        echo "vsftpd software has installed."
else
        yum -y install vsftpd &> /dev/null
fi
[root@server shell02]# sh 11.sh
vsftpd software has installed.

3.输入一个文件路径，判断该文件是什么类型的？

[root@server shell02]# cat 10.sh
#!bin/bash

read -p "pls input the file path:" file_path

if [ -e $file_path ]
then
        if [ -L $file_path ]
        then
                echo "$file_path is a link file."
        elif [ -d $file_path ]
        then
                echo "$file_path is a directory."
        elif [ -f $file_path ]
        then
                echo "$file_path is a text file."
        elif [ -b $file_path ]
        then
                echo "$file_path is a block file."
        elif [ -c $file_path ]
        then
                echo "$file_path is a character file."
        elif [ -p $file_path ]
        then
                echo "$file_path is a pipe file."
        elif [ -S $file_path ]
        then
                echo "$file_path is a socket file."
        fi
else
        echo "$file_path is not exist."
fi
[root@server shell02]# sh 10.sh
pls input the file path:/tmp
/tmp is a directory.
[root@server shell02]# sh 10.sh
pls input the file path:/etc/inittab
/etc/inittab is a text file.
[root@server shell02]# sh 10.sh
pls input the file path:/dev/sda
/dev/sda is a block file.

[root@server shell02]# cat 9.sh
#!bin /bash

read -p "pls input the file path:" file_path

if [ -e $file_path ]
then
        file_type=$(ls -ld $file_path|cut -c1)
        if [ $file_type = - ]
        then
                echo "$file_path is a text file."
        elif [ $file_type = d ]
        then
                echo "$file_path is a directory."
        elif [ $file_type = b ]
        then
                echo "$file_path is a black file."
        elif [ $file_type = c ]
        then
                echo "$file_path is a character file."
        elif [ $file_type = p ]
        then
                echo "$file_path is a pipe file."
        elif [ $file_type = l ]
        then
                echo "$file_path is a link file."
        elif [ $file_type = s ]
        then
                echo "$file_path is a socket file."
        fi
else
        echo "$file_path is not exist"
fi