shell中的if 和case两个条件语句
1. if的语法1：
if
条件
then
commands
else
commands
fi
if的语法2：
语法：if 条件
then
commands
elif 条件
then
commands
elif 条件
then
commands
....
else
commands
fi
说明：read 是读取用户输入的一行
if语句的一些说明
首先：if 条件1
then
commands
elif 条件2
then
commands
elif 条件3
then
commands
....
else
commands
fi
中的条件1,2,3等，都是同等地位的，也就是相当于条件1不满足，则看条件2，条件1、2都不满足，则看条件3 以此往下推(-a 为and的意思，但是不能用&&替代，否则会报错，-o 为or意思)。
case条件语句：
语法： case 条件 in
xxx)
commands;;
xxx)
commands;;
xxx)
commands;;
esac
说明：这个esac 就是case的结束，想if...fi 一样的，
注意commands;; 中的“；；”不能少掉。
例子
1、 写一个脚本 /root/bin/createuser.sh ，实现如下功能：使用一个用户名做为参数，如果指定参数的用户存在，就显示其存在，否则添加之；显示添加的用户的 id 号等信息
[root@localhost bin]# cat createuser.sh
#!/bin/bash
# Date: 2016.8.12
# Author: cyh
# Description: Determine whether a user exists
user=`cut -d: -f1 /etc/passwd |grep -o "$1"`
[ $# -ne 1 ] && exit 1
if [[ -n $user ]]; then
echo "User already exists!"
echo "The user information: `grep "^$1\>" /etc/passwd |cut -d: -f 1,3`"
else
useradd $1 > /dev/null
echo "The user has been created!"
echo "The user information: `grep $1 /etc/passwd |cut -d: -f 1,3`"
fi
脚本测试
[root@localhost bin]# getent passwd tom
[root@localhost bin]# createuser.sh tom
The user has been created!
The user information: tom:2028
[root@localhost bin]# getent passwd tom
tom:x:2028:2033::/home/tom:/bin/bash
[root@localhost bin]#
[root@localhost bin]# getent passwd zhangsan
zhangsan:x:2022:2024::/home/zhangsan:/bin/bash
[root@localhost bin]# createuser.sh zhangsan
User already exists!
The user information: zhangsan:2022
2 、写一个脚本 /root/bin/yesorno.sh ，提示用户输入 yes 或 no, 并判断用户输入的是 yes 还是 no, 或是其它信息
[root@localhost bin]# cat yesorno.sh
#!/bin/bash
# Description: Determine the user input characters(yes or no).
read -t5 -p "Please input yes or no: " ans
[ -z $ans ] && echo "You have no input, script has quit" && exit 10
case $ans in
[yY][eE][sS]|y|Y)
echo "Input is yes!"
;;
[nN][oO]|n|N)
echo "Input is no!"
;;
*)
echo "Input error, please enter again."
;;
esac
脚本测试
[root@localhost bin]# yesorno.sh
Please input yes or no: y
Input is yes!
[root@localhost bin]# yesorno.sh
Please input yes or no: yEs
Input is yes!
[root@localhost bin]# yesorno.sh
Please input yes or no: nO
Input is no!
[root@localhost bin]# yesorno.sh
Please input yes or no: n
Input is no!
[root@localhost bin]# yesorno.sh
Please input yes or no: tt
Input error, please enter again.
3 、写一个脚本 /root/bin/filetype.sh, 判断用户输入文件路径，显示其文件类型（普通，目录，链接，其它文件类型）
[root@localhost bin]# cat filetype.sh
#!/bin/bash
# Author: cyh
# Date: 2016.8.15
# Description: Input a file, and determine the type of the file
if [ $# -ge 1 ]; then
if [ -z $1 ]; then
echo "Your input is empty"
elif [ -d $1 ]; then
echo "this is a directory."
elif [ -h $1 ]; then
echo "this is a link file."
elif [ -f $1 ]; then
echo "this is a common file."
elif [ -c $1 ]; then
echo "this is a character device file."
elif [ -b $1 ]; then
echo "this is a block device file."
elif [ -s $1 ]; then
echo "this is a socket file."
elif [ -p $1 ]; then
echo "this is a pipe file."
else
echo "The input character is empty or file does not exist,please specify again."
fi
else
read -t 10 -p "Please input file name: " file
[[ ! -e $file || -z $file ]] && echo "The input is empty or file does not exist,please specify again." && exit
type=`ls -ld $file 2> /dev/null | cut -c1`
case $type in
d)
echo "this is a directory."
;;
-)
echo "this is a common file."
;;
l)
echo "this is a link file."
;;
s)
echo "this is a socket file."
;;
c)
echo "this is a character device file."
;;
b)
echo "this is a block device file."
;;
p)
echo "this is a pipe file."
;;
*)
echo "this not is file."
esac
fi
脚本测试
[root@localhost bin]# filetype.sh dfdfd
The input character is empty or file does not exist,please specify again.
[root@localhost bin]# filetype.sh /var
this is a directory.
[root@localhost bin]# filetype.sh /var/log/
this is a directory.
[root@localhost bin]# filetype.sh /var/log/messages
this is a common file.
[root@localhost bin]# filetype.sh /tmp/cat
this is a link file.
[root@localhost bin]# filetype.sh /tmp/hogsuspend
this is a pipe file.
[root@localhost bin]# filetype.sh /var/log/messages21
The input character is empty or file does not exist,please specify again.
[root@localhost bin]# filetype.sh
Please input file name: dfsdfsfq2
The input character is empty or file does not exist,please specify again.
[root@localhost bin]# filetype.sh
Please input file name: /etc/fstab
this is a common file.
[root@localhost bin]# filetype.sh
Please input file name: /tmp/cat
this is a link file.
[root@localhost bin]# filetype.sh
Please input file name: /tmp/hogsuspend
this is a pipe file.
4 、写一个脚本 /root/bin/checkint.sh, 判断用户输入的参数是否为正整数
[root@localhost bin]# cat checkint.sh
#!/bin/bash
#
read -p "Please input an integer:" integer
#[ $integer -lt 0 ] && echo "Please enter at least one value" && exit 10
if [[ $integer =~ ^0*[0-9][0-9]*$ ]]; then
echo "this is an integer"
else
echo "this not an integer"
fi
脚本测试
[root@localhost bin]# checkint.sh
Please input an integer:12
this is an integer
[root@localhost bin]# checkint.sh
Please input an integer:3.14
this not an integer
[root@localhost bin]# checkint.sh
Please input an integer:df
this not an integer
[root@localhost bin]# checkint.sh
Please input an integer:09
this is an integer
[root@localhost bin]# checkint.sh
Please input an integer:9f
this not an integer