2read, if, test, wildcard

1read keyboard command to read the values ​​of variables

Usually variable values read from a keyboard in a shell script and user interaction where
the value of the order of several variables can be read once, the value of the input variables need separated by spaces

[root@meditation ~]# read a b
hello world
[root@meditation ~]# echo $a
hello
[root@meditation ~]# echo $b
world

[root@meditation ~]# read -p "input your passwd:" -s  -t 20 passwd

• -p: display a message
• -s: Hide Password
• -t: set the time, off 20 seconds
• -n: length limit 12-n:. maximum of 12 characters
• -r: allows you to enter special characters

You can also mix and echo, to achieve the effect of a password

[root@meditation ~]# echo -n "input your passwd:";read passw
input your passwd:123456

Simple interactive input information

[root@meditation ~]# cat read.sh 
#!/bin/bash
read -p "请输入姓名:" name
read -p "请输入年龄:" age
read -p "请输入性别:" sex

cat<<eof
******************
你的基本信息如下:
姓名: $name
年龄: $age
性别: $sex
******************
eof

[root@meditation ~]# bash ./read.sh 
请输入姓名:苍老师
请输入年龄:34
请输入性别:女
******************
你的基本信息如下:
姓名: 苍老师
年龄: 34
性别: 女
******************
[root@meditation ~]# 

2 flow control statements if

2.1 single-branch if statement

[root@meditation ~]# cat if1.sh 
#!/bin/bash

if ls /mnt
then
    echo "it is ok"
fi
[root@meditation ~]# bash if1.sh 
it is ok

2.2 pairs of branch if statement

[root@meditation ~]# cat if2.sh 
#!/bin/bash

if grep sss /etc/passwd
then
    echo "it's ok"
else
    echo "it's err"
fi
[root@meditation ~]# bash if2.sh 
it's err

More than 2.3 branch if statement

Determining whether the user is present in the system, whether the home directory

[root@meditation ~]# cat if3.sh 
#!/bin/bash

read -p "input a user:" users
if grep $users /etc/passwd
then
    echo "the user $users exists on this system"
elif ls -d /home/$users
then
    echo "$users has a home direcotry"
    echo "the user $users not exists on this system"
else
    echo "$users nor has a home direcotry"
    echo "the user $users not exists on this system"
fi

[root@meditation ~]# bash if3.sh 
input a user:sunlizhao31
sunlizhao31:x:1003:1003::/home/sunlizhao31:/bin/bash
the user sunlizhao31 exists on this system

3test test command

The test shell command is used to check if a condition is established, it can be numeric, character, file test three aspects

• If the result is right, also called the result is true, with $? = 0 indicates the contrary is false, non-0 means

3.1 numerical comparison

parameter	Explanation	Examples
-eq	True equals	"$a" -eq "$b"
-born	It is not equal to True	"$a" -ne "$b"
-gt	True greater than	"$a" -gt "$b"
-give	True or greater	"$a" -ge "$b"
-lt	Less than True	"$a" -lt "$b"
-the	True or less	"$a" -le "$b"

Both versions: test and []

[root@meditation ~]# cat test1.sh 
#!/bin/bash

if test 2 -eq 1
then
    echo "ok"
else
    echo "err"
fi
[root@meditation ~]# bash test1.sh 
err

[root@meditation ~]# cat test2.sh 
#!/bin/bash

if [ 2 -le 10 ]
then
    echo "ok"
else
    echo "err"
fi
[root@meditation ~]# bash test2.sh 
ok

The user inputs two numbers, to determine the size

Only use integer

[root@meditation ~]# cat test3.sh 
#!/bin/bash

read -p "input var1: " var1
read -p "input var2: " var2

if [ $var1 -gt $var2 ]
then
    echo "$var1 > $var2"
elif [ $var1 -lt $var2 ]
then
    echo "$var1 < $var2"
else
    echo "$var1 = $var2"
fi

[root@meditation ~]# bash test3.sh 
input var1: 12
input var2: 21
12 < 21
[root@meditation ~]# bash test3.sh 
input var1: 12
input var2: 12
12 = 12

3.2 String comparison

symbol	description
==	Equal to true
!=	Not equal true
-z string	0 string length True
-n string	The length of the string is not empty True
str1>str2	It is larger than the true str1 str2
str1<str2	less than true str1 str2

When comparing strings

• The greater than and less than the number must escape, or else it will shell redirection symbols as
• Above and below, and their order is not the same sort test sort test, a sequence of an ascii, less than uppercase lowercase

File Compare 3.3

symbol	description
-e filename	If the file or directory exists True
-r filename	If the file exists and is readable True
-w file name	If the file exists and is writable True
-x file name	If the file exists and is executable True
-s filename	If the file exists and there is at least one character True
-d filename	True if file exists and is a directory was
-f filename	True if file exists and is a regular file
-c filename	If the file exists and the file was true for the character
-b filename	If the file exists and is a block special file True
file1 file2 -nt	Check if file1 is newer than file2
file1 file2 -ot	Check if file1 is newer than file2

[root@localhost test]# cat test.sh 
#!/bin/bash
if [ -e /etc/passwd ]
then
    echo ok
else
    echo err
fi

[root@localhost test]# bash test.sh 
ok
[root@localhost test]# test -e /etc/passwd && echo ok || echo err
ok

4 complicated flow control process conditions and wildcards

4.1 Complex Conditions

Both conditions are true, or is true on the implementation of

Determining a first

if [条件判断一] &&(||) [条件判断二];then
    命令一
elif [条件判断三] &&(||) [条件判断四];then
    命令二
else
    其他命令
fi

The second judge

if [[ 条件判断一 &&(||) 条件判断二 ]];then 
    命令一
elif [[ 条件判断三 &&(||) 条件判断四 ]];then
    命令二
else
    其他命令
fi

[] And the difference [[]] of

[[]] Is [] operator expansion; support * <> characters, do not need to escape

[root@localhost ~]# if [[ $USER == r* ]] ; then echo "hello,$USER" ; else echo $USER not; fi
hello,root
[root@localhost ~]# if [ $USER == r* ] ; then echo "hello,$USER" ; else echo $USER not; fi
root not

[] As a string will r *

1. All characters and logical operators need to use "space" to separate, not connected together
2. In [] expression, commonly> <need an escape character, for size comparison
3. Logical operators &&, || comparison, if the [] symbols, with the outside, such as [] && [] || []
   If the comparison logic, or with the in [], then use -a, -o be expressed as [[x -ay -oz]]

4. [[]] Is [] operator expansion; support * <> characters, do not need to escape, which support the logical operators ||, is no longer used && -a, -o

5. [[]] Can be extended arithmetic, and [] can not.

6. [[]] It can be used for advanced string processing, such as fuzzy matching

4.2 Wildcard

symbol	description
*	Matches zero or more characters a * b, a character can have any arbitrary length between a language B, may be not one, as ab, axybdb
?	Matches any character. A? B, between a and b must be only one character can be any character as axb, abb
[list]	Match any single character list, only a match between a [xyz] b, a and b are a character, but only x, y or z are as axb
[!list]	At the match any single character in the list. A [! 0-9] ab between a character must only have, but are not Arabic numerals. As matched axb
[a-z]	A single character to match any of z. [String1, string2, ...] string1 matches string2 or one or more of the string

[root@meditation ~]# ls /etc/*.conf
/etc/asound.conf  /etc/libaudit.conf    /etc/pam_url.conf    /etc/tcsd.conf
...

[root@meditation ~]# ls /etc/???.conf
/etc/ntp.conf  /etc/sos.conf  /etc/yum.conf

[root@meditation ~]# touch /opt/a{1,2,3}.txt
[root@meditation ~]# ls /opt/a[123].txt
/opt/a1.txt  /opt/a2.txt  /opt/a3.txt
[root@meditation ~]# ls /opt/a[1,2,3].txt
/opt/a1.txt  /opt/a2.txt  /opt/a3.txt

5 combat: three shell script combat

5.1 Inspection Service is running

[root@meditation ~]# cat status3.sh 
#!/bin/bash

if [ $# -ge 1 ] ; then
    systemctl status $1 > /dev/null
    if [ $? -eq 0 ] ; then
    echo "服务正在运行"
    else
    systemctl start $1
    echo "正在打开"
    fi
else
    echo "格式错误"
fi


[root@meditation ~]# chmod +x status3.sh 
[root@meditation ~]# ./status3.sh httpd
服务正在运行

5.2 According to the results interpretation grade

[root@meditation ~]# cat cjcx.sh 
#!/bin/bash

read -p "请您输入成绩: " cj
if [ $cj -ge 0 ] && [ $cj -lt 60 ] ; then
    echo "补考!"
elif [ $cj -ge 60 ] && [ $cj -lt 80 ] ; then
    echo "良好" 
elif [ $cj -ge 80 ] && [ $cj -lt 90 ] ; then
    echo "优秀"
elif [ $cj -ge 90 ] && [ $cj -le 100 ] ; then
    echo "你太棒了"
else 
    echo "请输入0-100之内的有效成绩"
fi
[root@meditation ~]# bash cjcx.sh 
请您输入成绩: 1
补考!
[root@meditation ~]# bash cjcx.sh 
请您输入成绩: 70
良好
[root@meditation ~]# bash cjcx.sh 
请您输入成绩: 100
你太棒了

5.3 automatic backup a directory

[root@meditation ~]# cat etcbak.sh 
#!/bin/bash

baknamefile=`date +%F`
bakdir=/etcbak
srcdir=/etc

[ -e $bakdir ] || mkdir $bakdir
tar -zcvf ${bakdir}/${baknamefile}-etc.tar.gz $srcdir
echo "==================="
ls -lh ${bakdir}/${baknamefile}-etc.tar.gz




[root@meditation ~]# bash etcbak.sh 
...
/etc/cloud/templates/resolv.conf.tmpl
/etc/sysctl.conf
/etc/gshadow-
===================
-rw-r--r-- 1 root root 9.5M Aug  4 20:43 /etcbak/2019-08-04-etc.tar.gz