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Conditional test, if, case conditional test statements of shell script
2022-07-04 01:07:00 【Wozkid】
One 、 Conditions for testing
1.1 test command
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 ）
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1.2 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
Common test operators meaning
-d Test whether it's a directory （directory）
-e Test whether the directory or file exists （Exist）
-f Whether the test is a file （File）
-r Test whether the current user has permission to read （Read）
-w Test whether the current user has permission to write （Write）
-x Test whether the current user has permission to execute （eXcute）
-L Test whether it is a soft connection file
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
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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
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1.3 Integer comparison
Format ：
[ Integers 1 The operator Integers 2 ]
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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
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1.4 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
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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
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1.5 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
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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
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Two 、if sentence
2.1 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
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# 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
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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 $MOUNT_DIF
fi
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example 3
Determine whether the end of the input is .sh
#!/bin/bash
read -p " Please enter filename :" file
if [[$file == *.sh ]] ;then
echo " This is a shell Script "
fi
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2.2 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
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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
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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
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2.3 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
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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 !
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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
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2.4 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
[[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
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3、 ... and 、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
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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
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Example 2
[[email protected] ~]# vim g.sh
#!/bin/bash
read -p " Please enter your score :" score
case $score in
100 )
echo " Xiuer !"
;;
9[0-9])
echo "$score branch , copy 10 All over !"
;;
[78][0-9])
echo "$score branch , copy 20 All over !"
;;
6[0-9])
echo "$score branch , copy 30 All over !"
;;
[0-9]|[1-5][0-9])
echo "$score branch , Copy all 30 All over !"
;;
*)
echo " Incorrect input !"
esac
[[email protected] ~]# sh g.sh
Please enter your score :100
Xiuer !
[[email protected] ~]# sh g.sh
Please enter your score :91
91 branch , copy 10 All over !
[[email protected] ~]# sh g.sh
Please enter your score :78
78 branch , copy 20 All over !
[[email protected] ~]# sh g.sh
Please enter your score :65
65 branch , copy 30 All over !
[[email protected] ~]# sh g.sh
Please enter your score :50
50 branch , Copy all 30 All over !
[[email protected] ~]# sh g.sh
Please enter your score :1000
Incorrect input !
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Four 、 summary
1. The syntax of conditional testing ： File test 、 Integer comparison 、 String comparison 、 Logic test .
2.if The syntax of conditional statements ： Single branch 、 Double branch 、 Multiple branches .
3.case The syntax of a multi branch statement
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