Conditional test, if and case conditional test statements of shell script

---

Preface

One 、 Conditions for testing

Test whether a specific expression holds , When conditions hold , The return value of the test statement is 0, Otherwise, other values

 Format ：
test  Conditional expression 
 or 
[  Conditional expression  ]  （ Spaces are required on both sides of the conditional expression , And this method is more commonly used ）

File test

File testing refers to testing based on a given path name , Determine whether the corresponding file or directory , Or judge whether the file is readable 、 Can write 、 Executable, etc .

Common options for file testing

After performing the conditional test operation , Through predefined variables " $ ? " You can get the return status value of the test command , So as to judge whether the condition is true . for example , Do the following to test the directory /media/ Whether there is , If the return value " $ ? " by 0, Indicates that this directory exists , Otherwise, it means that it does not exist or although it exists, it is not a directory

[[email protected] ~]# test  -d /media/;echo $?
0      ###  The return value is 0

Example ：

[[email protected] ~]# test  -d /etc/;echo $?  ###  Test whether it's a directory 
0      ###  Return value  0    Is a directory 

[[email protected] ~]# test  -e /etc/abc;echo $?       ###  Test whether the directory or file exists 
1      ###  Return value  1  There is no directory file 

[[email protected] ~]# test -f /etc/yum.conf ;echo $?      ###  Whether the test is a file 
0      ###  Return value  0   It's a document 

[[email protected] ~]# test -r /etc/yum.conf ;echo $?      ###  Test whether the current user has permission to read 
0      ###  Return value  0    The current user has read permission 

[[email protected] ~]# test -w /etc/yum.conf ;echo $?      ###  Test whether the current user has permission to write 
0      ###  Return value  0   The current user has write permission 

[[email protected] ~]# test -x /etc/yum.conf ;echo $?      ###  Test whether the current user has permission to execute 
1      ###  Return value  1    The current user does not have permission to execute 

have access to “&&” and “echo” Together with

[[email protected] ~]# test  -d /etc/ && echo "yes"
yes

Integer comparison

Integer value comparison refers to the comparison between two given integer values , Judge the relationship between the first number and the second number , If it is greater than 、 be equal to 、 Less than the second number

 Format ：
[  Integers 1  The operator   Integers 2 ]

Common test operators

 The operator 	 meaning 
-eq	 be equal to （Equal）
-ne	 It's not equal to （Not Equal）
-lt	 Less than （Leser Than）
-gt	 Greater than （Greater Than）
-le	 Less than or equal to （Lesser or Equal）
-ge	 Greater than or equal to （Greater or Equal）

Query whether the number of files in the current directory is greater than 10, If it is greater than , Then give a hint

[[email protected] ~]#   ls | wc -l
11
[[email protected] ~]# test ` ls | wc -l ` -gt 10 && echo " Number of files greater than 10"
 Number of files greater than 10

Check whether the system memory is lower than 1300M, If it is lower than, it will prompt

[[email protected] ~]#  free -m
              total        used        free      shared  buff/cache   available
Mem:           3771         724        1244          17        1801        2712
Swap:          2999           0        2999
[[email protected] ~]# free -m | grep "Mem" 
Mem:           3771         724        1244          17        1801        2712
[[email protected] ~]# free -m | grep "Mem" | awk '{print $4}'
1244
[[email protected] ~]# free=`free -m | grep "Mem" | awk '{print $4}'`
[[email protected] ~]# echo $free
1244
[[email protected] ~]# test `echo $free` -lt 1300 && echo " Out of memory 1300M"
 Out of memory 1300M

String comparison

String comparisons are often used to check user input 、 Whether the system environment meets the conditions , In providing interactive operation shell Script , It can also be used to judge whether the position parameters entered by the user meet the requirements

 Format ：
[  character string 1 =  character string 2 ] 			### Judge whether it is equal 
[  character string 1 ！=  character string 2 ] 			### Judge whether it is not equal 
[ -z  character string  ]                    ### Determine whether the string content is empty 

Common test operators

 The operator 	 meaning 
=	 String content is the same 
!=	 String content is different ,！ The sign means the opposite 
-z	 The string content is empty 

Judge whether the current system language environment is “us.en”, If not, it will prompt

[[email protected] ~]# echo $LANG
zh_CN.UTF-8
[[email protected] ~]# test ` echo $LANG `="us.en" && echo  The current language is not us.en, The current language is  `echo $LANG` 
 The current language is not us.en, The current language is  zh_CN.UTF-8

Logic test

Logical testing refers to judging the dependency between two or more conditions . When the system task depends on several different conditions , According to whether these conditions are established at the same time or as long as one of them is established , There needs to be a testing process

 Format ：
[  expression 1 ]  The operator  [  expression 2 ]

 command 1  The operator   command 2

Common test operators

 The operator 	 meaning 
-a or &&	 Logic and , And it means 
-o or ▕▕	 Logic or , Or it means 
！	 Logical not 
[[email protected] ~]# [ -d /etc ] && [ -r /etc ] && echo "you can open it "
you can open it 
[[email protected] ~]# [ -d /etc ] || [ -d /home  ] && echo " ok "
 ok 

if sentence

Single branch structure

For single branch selection structure , Only in " Conditions established " The corresponding code will be executed , Otherwise, nothing will be done

 Format ：
if [  Conditional judgment  ]; than
	 When the condition is right , Execute one or more commands 
fi

Using a single branch if Sentence judgment test Does the file exist

[[email protected] ~]# if test -e /etc ; then echo Directory exists ; fi
Directory exists

[[email protected] ~]# if test -e /etc/yum.conf ; then echo  File exists ; fi
 File exists 

example 2.
Determine the mount point , If it doesn't exist, it will be created automatically

#!/bin/bash
MOUNT_DIR="/media/cdrom/ "
if [ ! -d $MOUNT_DIR ];
then
mkdir -p $MOUNT_DIF
fi

example 3
Determine whether the end of the input is .sh

#!/bin/bash
read -p " Please enter filename :" file
if [[$file == *.sh ]] ;then
echo " This is a shell Script "
fi

Two branch structure

Double branch if The statement is only on a single branch basis for " Conditions not established " To perform another operation , instead of " Sit idly by " Without doing anything

if [  Conditional judgment  ]; then
	 When the condition is right , You can execute one or more commands 
else	
	 When the conditional judgment doesn't hold , You can execute one or more commands 
fi 		

In the judgment of the same data , If the data requires two different judgments , We need two branches if Statement ：

Example ：
Judge whether the current login user is an administrator

[[email protected] ~]# vim a.sh

#!/bin/bash
if [ $UID -eq 0 ];then
echo " The current login user is administrator root"
else
echo " The current login user is not an administrator "
fi

[[email protected] ~]# sh a.sh
 The current login user is administrator root

Check that the directory exists

[[email protected] ~]# vim b.sh

#!/bin/bash 
read -p " Check that the directory exists , Please enter the directory ：" aaa
if ls $aaa &> /dev/null
then
echo " Directory exists "
else
echo " Please enter the correct path "
fi

[[email protected] ~]# sh b.sh
 Check that the directory exists , Please enter the directory ：/etc
 Directory exists 

Multi branch structure

And single branch 、 Double branch if Statement than , Multiple branches if The structure of the statement can perform different operations according to multiple mutually exclusive conditions

if [  Conditional judgment  ]; then
	 When the condition is right , You can execute one or more commands 
elif [  Conditional judgment 2  ]; then
	 When the condition is right , You can execute one or more commands 
else	
	 When none of the above conditions are true , You can execute one or more commands 
fi 		

Example
Excellent students are distinguished according to the test scores entered 、 qualified 、 Three unqualified grades

[[email protected] ~]# vim c.sh

#!/bin/bash
read -p " Please enter your score (0-100) : " GE
if [ $GE -ge 85 ] && [ $GE -le 100 ]
then
echo "$GE  branch , good !"
elif [ $GE -ge 70 ] && [ $GE -le 84 ]
then
echo "$GE  branch , qualified !"
else
echo "$GE  branch , unqualified !"
fi

[[email protected] ~]# sh c.sh
 Please enter your score (0-100) : 99
99  branch , good !
[[email protected] ~]# sh c.sh
 Please enter your score (0-100) : 79
79  branch , qualified !
[[email protected] ~]# sh c.sh
 Please enter your score (0-100) : 59
59  branch , unqualified !

Determine the file category

[[email protected] ~]# vim d.sh

#!/bin/bash
read -p " Enter the file to identify :" name
if [ -d $name ] ;then
echo " This is a directory "
elif [ -f $name ];then
echo " This is a document "
elif [ -b $name ];then
echo" This is a device file "
else
echo " Unable to determine file type "
fi

[[email protected] ~]# sh d.sh
 Enter the file to identify :/etc
 This is a directory 
[[email protected] ~]# sh d.sh
 Enter the file to identify :/etc/yum.conf
 This is a document 
[[email protected] ~]# sh d.sh
 Enter the file to identify :root
 Unable to determine file type 

nesting if sentence

example 1. Judge httpd Did the service start

Judge whether to start

If you start ------ Output started

If it doesn't start ------ Determine whether to install ------ If installed ------ start-up

If not installed ------ install ------ If the installation is successful ------ start-up

If the installation is not successful ------ Report errors

[email protected] ~]# vim e.sh

#!/bin/bash

ps -aux | grep httpd | grep -v grep
if [ $? -ne 0 ];then
       if [ "$(rpm -q httpd)" == " Package not installed httpd" ];then
       yum install httpd -y
       systemctl start httpd
       else
       systemctl start httpd
       fi
else
    echo "httpd Service started " 
fi
```c
[[email protected] ~]# sh e.sh 
root      11870  0.6  0.1 224072  5044 ?        Ss   15:05   0:00 /usr/sbin/httpd -DFOREGROUND
apache    11874  0.0  0.0 226156  3112 ?        S    15:05   0:00 /usr/sbin/httpd -DFOREGROUND
apache    11875  0.0  0.0 226156  3112 ?        S    15:05   0:00 /usr/sbin/httpd -DFOREGROUND
apache    11876  0.0  0.0 226156  3112 ?        S    15:05   0:00 /usr/sbin/httpd -DFOREGROUND
apache    11877  0.0  0.0 226156  3112 ?        S    15:05   0:00 /usr/sbin/httpd -DFOREGROUND
apache    11878  0.0  0.0 226156  3112 ?        S    15:05   0:00 /usr/sbin/httpd -DFOREGROUND
httpd  Service started 

case sentence

case Statement can make the structure of the script program clearer 、 distinct , Commonly used for service startup 、 restart 、 Stopped script , Some services do not provide such control scripts , Need to use case Statement writing

case Statement is mainly applicable to the following situations : There are many values for a variable , You need to execute different command sequences for each value . This is the case with multi - branched if The sentences are very similar , It's just if Statements need to judge multiple different conditions , and case The statement just judges the different values of a variable

 Format ：
case  Variable name  in
 Pattern 1）
	 Command sequence 
    ;;
 Pattern 2）
	 Procedures section 
	;;
*）
	 Other program execution segments that do not contain the contents of the first variable and the second variable 
	 Default segment 
	;;
esac

Be careful ：

case The line must begin with a word “in”, The first mock exam is a single bracket. ")" end

Double a semicolon ";;" Indicates the end of the command sequence

In pattern string , You can use square brackets to indicate a continuous range , Such as "[0-9]"; You can also use vertical bars | Represents or , such as a|b

final ) Represents the default mode , Where is equivalent to wildcard

Example
Check the character type entered by the user

[[email protected] ~]# vim f.sh

#!/bin/bas

read -p " Please enter a character , And press Enter Key confirmation :" KEY
case "$KEY" in
[a-z] | [A-Z])
echo " You're typing in letters "
;;
[0-9])
echo " You're typing in numbers "
;;
*)
echo " What you enter is a space or special character "
esac

[[email protected] ~]# sh f.sh k
 Please enter a character , And press Enter Key confirmation :k
 You're typing in letters 
[[email protected] ~]# sh f.sh 
 Please enter a character , And press Enter Key confirmation :5
 You're typing in numbers 
[[email protected] ~]# sh f.sh  
 Please enter a character , And press Enter Key confirmation : 
 What you enter is a space or special character 

Example 2

[[email protected] ~]# vim g.sh

#!/bin/bash
read -p " Please enter your score :" score
case $score in
100 )
echo " Full marks ！"
;;
9[0-9])
echo "$score branch "
;;
[78][0-9])
echo "$score  branch "
;;
6[0-9])
echo "$score branch "
;;
[0-9]|[1-5][0-9])
echo "$score  branch "
;;
*)
echo " Incorrect input !"
esac

[[email protected] ~]# sh g.sh 
 Please enter your score :100
 Full marks ！!
[[email protected] ~]# sh g.sh 
 Please enter your score :91
91 branch 
[[email protected] ~]# sh g.sh 
 Please enter your score :78
78  branch 
[[email protected] ~]# sh g.sh 
 Please enter your score :65
65 branch 
[[email protected] ~]# sh g.sh 
 Please enter your score :50
50  branch 
[[email protected] ~]# sh g.sh 
 Please enter your score :1000
 Incorrect input !

summary

• The syntax of conditional testing ： File test 、 Integer comparison 、 String comparison 、 Logic test .
• if The syntax of conditional statements ： Single branch 、 Double branch 、 Multiple branches .
• case The syntax of a multi branch statement