Shell script-conditional judgment

Conditional judgment grammar format

• Format 1: test conditional expression
• Format 2: [Conditional expression]
• Format 3: [[Conditional expression]] supports regular=~

note
[] And [[ ]] must have spaces on both sides

Conditional judgment related parameters

Determine file type

Judgment parameter	meaning
-e	Determine whether the file exists (any type of file)
-f	Determine whether the file existsandIs a normal file
-d	Determine whether the file exists and is a directory
-L	Determine whether the file exists and is a soft link file
-b	Determine whether the file exists and is a block device file
-S	Determine whether the file exists and is a socket file
-c	Determine whether the file exists and is a character device file
-p	Determine whether the file exists and is a named pipe file
-s	Determine whether the file exists and is a non-empty file (with content)

Instance

# 先判断zhu目录是否存在 存在则echo 不存在则创建
[root@maomao shell]# [ -d zhu ] && echo '存在' || mkdir zhu

# 前面加! 是非的意思
! -d 如果不存在 则创建
#!/bin/bash
back_dir=/usr/local/mysql_bcak	
if [ ! -d $back_dir ];then
       mkdir -p $back_dir
fi
       echo "开始备份..."

# -s 可以判断文件是否非空
[root@maomao shell]# [ -s free.sh ] && echo '非空' || echo "空"
非空
[root@maomao shell]# touch 123.txt
[root@maomao shell]# [ -s 123.txt ] && echo '非空' || echo "空"
空

test -e file                    只要文件存在条件为真
[ -d /shell/dir1 ]             判断目录是否存在，存在条件为真
[ ! -d /shell/dir1 ]        判断目录是否存在,不存在条件为真
[[ -f /shell/1.sh ]]        判断文件是否存在，并且是一个普通的文件

Determine file permissions

Judgment parameter	meaning
-r	Whether it is readable by the current user
-w	Whether the current user can write to it
-x	Is it executable by the current user
-u	Is there a suid, high-level risk bit
-g	Whether sgid, advanced authority mandatory bit
-k	Is there a t bit, high-level permission sticky bit

Determine the new and old files

The old and new here refer toFile modification time

Judgment parameter	meaning
file1 -nt file2	Compare whether file1 is newer than file2
file1 -from file2	Compare whether file1 is older than file2
file1 -ef file2	Compare whether it is the same file, or used to determine whether the hard link points to the same inode

# 创建一个硬链接
[root@maomao shell]# ln free.sh free2.sh 

[root@maomao shell]# [ free.sh -ef free2.sh ]
[root@maomao shell]# echo $?
0

Judge integer

Judgment parameter	meaning
-eq	equal
-born	Unequal
-gt	more than the
-lt	Less than
-give	greater or equal to
-the	Less than or equal to

Instance

#!/bin/bash
# 磁盘空间警报脚本
disk_use=`df -Th |grep '/$' |awk '{print $(NF-1)}' |awk -F"%" '{print $1}'` 
mail_user=root
if [ $disk_use -gt 90 ];then
		echo "`date +%F-%H` disk：${disk_use}%" |mail -s "disk warning..." $mail_user	
fi

# 内存警报脚本
#!/bin/bash
mem_used=`free -m |grep Mem |awk '{print $3}'`
mem_total=`free -m |grep Mem |awk '{print $2}'`
mem_percent=$((mem_used*100/$mem_total))
warning_file=/shell/mem_war.txt
	rm -rf $warning_file
	#if [ $mem_percent -ge 80 ];then
	if (($mem_percent>60));then
			echo "`date +%F-%H` memory:${mem_percent}%" > $warning_file
	fi

	if [ -f $warning_file ];then
			mail -s "mem warning..." root < $warning_file
			rm -rf $warning_file
	fi

Judgment string

Judgment parameter	meaning
-from	Judge whether it isairString, it is true if the string length is 0
-n	Judge whether it isnon emptyString, the string length is not 0 is true
string1 = string2	Determine whether the strings are equal
string1 != string2	Determine whether the strings are not equal

[root@maomao shell]# [ $USER = 'root' ];echo $?
0
[root@maomao shell]# [ $USER = 'roo' ];echo $?
1
[root@maomao shell]# [ $USER = 'root' ];echo $?
0
[root@maomao shell]# [ $USER == 'root' ];echo $?
0
[root@maomao shell]# [[ $USER == 'root' ]];echo $?
0
[root@maomao shell]# [[ "$USER" == 'root' ]];echo $?
0

Multiple condition judgment

Judgment symbol	meaning	For example
-a 和 &&	Logical and	[ 1 -eq 1 -a 1 -ne 0 ] 、[ 1 -eq 1 ] && [ 1 -ne 0 ]
-o sum ||	Logical OR	[1 -eq 1 -o 1 -ne 1] 、 [1 -eq 1] || [1 -ne 1]

Special Note:

&& before the expressionIs true, Will execute the following code

|| The previous expression为假，才会执行后面的代码

; 只用于分割命令或表达式

实例

• 数值比较

[root@maomao ~]# [ $(id -u) -eq 0 ] && echo "the user is root" || echo "the user is not root"

• 类C风格的数值比较

注意：在(( ))中，=表示赋值；==表示判断
[root@maomao ~]# ((1==2));echo $?
[root@maomao ~]# ((1<2));echo $?
[root@maomao ~]# ((2>=1));echo $?
[root@maomao ~]# ((2!=1));echo $?
[root@maomao ~]# ((`id -u`==0));echo $?
[root@maomao ~]# ((a=123));echo $a
[root@maomao ~]# unset a
[root@maomao ~]# ((a==123));echo $?

• 字符串比较

注意：双引号引起来，看作一个整体；= 和 == 在 [ 字符串 ] 比较中都表示判断
[root@maomao ~]# a='hello world';b=world
[root@maomao ~]# [ $a = $b ];echo $?
[root@maomao ~]# [ "$a" = "$b" ];echo $?
[root@maomao ~]# [ "$a" != "$b" ];echo $?
[root@maomao ~]# [ "$a" !== "$b" ];echo $?        错误
[root@maomao ~]# [ "$a" == "$b" ];echo $?
[root@maomao ~]# test "$a" != "$b";echo $?


test  表达式
[ 表达式 ]
[[ 表达式 ]]

思考：[ ] 和 [[ ]] 有什么区别？

[root@maomao ~]# a=
[root@maomao ~]# test -z $a;echo $?
[root@maomao ~]# a=hello
[root@maomao ~]# test -z $a;echo $?
[root@maomao ~]# test -n $a;echo $?
[root@maomao ~]# test -n "$a";echo $?

# [ '' = $a ];echo $?
-bash: [: : unary operator expected
2
# [[ '' = $a ]];echo $?
0


[root@maomao ~]# [ 1 -eq 0 -a 1 -ne 0 ];echo $?
[root@maomao ~]# [ 1 -eq 0 && 1 -ne 0 ];echo $?
[root@maomao ~]# [[ 1 -eq 0 && 1 -ne 0 ]];echo $?

脚本实例：
批量创建用户的脚本

#!/bin/bash
# 创建用户的脚本
# by stan Z
while true
do
        read -p "Please input number：" num
        if [[ ! "$num" =~ ^[0-9]+$ || "$num" =~ ^0+$ ]];then
                echo "error number!"
        else
                break
        fi  
done
while true
do
        read -p "Please input prefix：" prefix
        if [ -z "$prefix" ];then
                echo "error prefix"
        else
                break
        fi  
done
for i in `seq $num`
do
        user=$prefix$i
        useradd $user
        echo "123" |passwd --stdin $user &>/dev/null
        if [ $? -eq 0 ];then
                echo "$user is created."
        fi
done

逻辑运算符总结

1. 符号;和&&和||都可以用来分割命令或者表达式
2. 分号（;）完全不考虑前面的语句是否正确执行，都会执行;号后面的内容
3. &&符号，需要考虑&&前面的语句的正确性，前面语句正确执行才会执行&&后的内容；反之亦然
4. ||符号，需要考虑||前面的语句的非正确性，前面语句执行错误才会执行||后内容；反之亦然
5. 如果&&和||一起出现，从左往右依次看，按照以上原则