Conditional statements of shell programming

One . Conditions for testing

1. File test and integer test

2. String testing and logic testing

Two .if sentence

1.if Single branch statements

2.if Two branch statements

3.if Multi branch statement

3、 ... and .case Branch statement

One . Conditions for testing

（ One ）. Conditional test operation

1.test command

Test whether the expression holds , Return if established 0, Otherwise, other values are returned

Format 1: test Conditional expression

Format 2:【 Conditional expression 】

To make shell The script program has certain " intelligence ", The first problem is how to distinguish between different situations to determine what to do . for example , When disk utilization exceeds 95% when , Send alarm information ; When the backup directory does not exist , Can automatically create : When the source code compiles the program , If the configuration fails, the installation will not continue Shell The environment returns the status value according to the command execution （$?〉 To judge whether the execution is successful , When the return value is o Time means success , otherwise （ Not О value ） Indicates failure or exception .

Use special testing tools —test command , You can test specific conditions , And judge whether the condition is true according to the return value （ The return value is 0 Indicates that the condition is true ).

Grammar format :

Format l test Conditional expression

Format 2〔 Conditional expression ]# Note that you need to separate a space or... Between the brackets and the expression

[[ Conditional expression ]]

2. File test

[ Operator file or directory ]

Common test operators

File testing refers to testing based on a given path name , Determine whether the corresponding file or directory , Or judge whether the file is

Can be read 、 Can write 、 Executable, etc . Common operation options for file testing are as follows , When using, put the test object after the operation option .

-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 if executable （Excute) jurisdiction .

-b: Test whether it is a device file

-c: Test whether it is a character device file

-s: The test exists and the file size is empty

-L: Test whether it is a linked 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

example 1

test -d letcl sysconfig/

test -f letc/ sysconfig/

test -e letcl sysconfig/

example 2

[ -f / home / humajun/ ]

[ -d / home / humajun/ ]  & & echo "YES"

[ !  -e  /opt/ kgc ]  & & mkdir /opt/ kgc

3. Integer comparison

[ Integers 1 Operator integer 2]

Common test operators

-eq: be equal to (Equal)

-ne: It's not equal to (Not Equal)

-gt: Greater than (Greater Than)

-lt: Less than (Lesser Than)

-le: Less than or equal to (Lesser or Equal)

-ge: Greater than or equal to (Greater or Equal)

1.2 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 . Common operation options for integer value comparison are as follows , When using, put the operation options to

Compare between two integers .

Common test operators

-eq: The first number equals (Equal) The second number .

-ne: The first number is not equal to (Not Equal) The second number .-gt: The first number is greater than (Greater Than) The second number .-lt: The first number is less than (Lesser Than） The second number .

-le: The first number is less than or equal to (Lesser or Equal） The second number .-ge: The first number is greater than or equal to ( Greater or Equal) The second number

example 1

who lwc -l

[ $ (who | wc -l) -le 5 ] &&echo " Too few users "

[ $(who / wc -l) -ge 10 ] && echo "> = 10. "

example 2

FreecC=$ (free -m l grep "Mem: " | awk '{print $6} ')

[$Freecc -lt 4096 ]&& echo ${FreecC}MB

mem_size=$ (free -m | grep "Mem: " | awk '{print $4} ')

[$mem_size -ge 1024 ]&& echo

" because 192.168.10.17 The server system has insufficient memory resources , Please handle the fault as soon as possible ."

1.3 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 . The common operation options for string comparison are as follows .

-: The first Ningfu string is the same as the second string .

!=: The first string is not the same as the second string , among "!" The sign means reverse .

-z Check if the string is empty (zero), Variables that are undefined or given null values are treated as empty strings .

example 1

echo $LANGzh_CN.UTF-8

[ $LANG!="en. us" ]& & echo "Not en.us"

example 2

read -p " Overwrite existing file ( yes/ no ) ?"

ACK Overwrite existing file

( yes/no ) ?yes

[$ACK = "yes" ]&& echo" Cover "

1.4 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 these

Whether the conditions are true at the same time or as long as one of them is true , There needs to be a testing process .

Common logic test operations, such as ” Next , Put it between different test statements or commands .

&&: Logic and , Express " and ”, Only when the last two conditions are true , The return value of the entire test command is by

0 ( It turns out that ). Use test Command test ,“&&” Can be changed to "-a”.

●11: Logic or , Express " perhaps ”, As long as one of the two conditions “ One set up , The return value of the entire test command is 0 ( It turns out that ). Use test Command test ,“11” Can be changed to "-o”.

●!: Logical not , Express " No ”, Only when the specified conditions are not true , The return value of the entire test command is 0 ( It turns out that ).

example 1

[ -d  /etc ] && [ -r /etc ] && echo "You can open it"

[ -d  /etc ] || [ -d /home ]  &&  echo "ok"

In the operation options of the above logic test ,"&&" and “11" It is also commonly used for command operations with different intervals , Its function is similar . In fact, I have been in contact with “&&" Application of operation , Such as “make && make install" Compilation of Installation operation .

for example , To judge the current Linux Whether the kernel version of the system is greater than 3.4, You can do the following . among ,

The kernel version number passes uname and awk Command to get

example 2

uname - r

// check Look at the kernel version information 3.10.0-514.e17.x86_ 64 I

Mnum=$ (uname -r I awk -F. ' {print $1}') // Take the major version number

Snum=$ (uname -r | awk -F. '{print $2}') // Take the secondary version number

[ $Mnum-ge3 ] && [ $Snum-gt4 ] && echo" Meet the requirements "

Two .if Statement single branch

（ One ）. Single branch if sentence

Actually use "&&" and “11" Logic test can complete simple judgment and perform corresponding operations , But when you need to choose the life to execute

Make more sentences , This approach will make the execution of the code very complex , Hard to understand . And use a dedicated if

Conditional statements , You can better organize the script structure , Make layers clear ,

Clear and easy to understand

（ Two ）. Single branch if

sentence : For single branch selection structure , Only in “ Conditions established ” The corresponding code will be executed , Otherwise, nothing will be done .

Grammar format :

1)

if Conditional test operation

then

Command sequence

fi

# Notice that there's an end , Make a pair at the beginning and end, otherwise you will report a syntax error

2)

if Conditional test operation ; then

Command sequence

fi

if[3 -gt 2]; then .

echo "ok""

fi

[3-gt2] & &echo"ok"

example 1 Determine whether the directory exists

#! /bin/bash

if ls /mnt

then

echo "it's ok"

fi

example 2

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

#! /bin/bash

MOUNT_ DIR= "/media/ cdrom/”

#2. If you don't have this directory, create a directory

if[ !-d$MOUNTDIR ]

then

mkdir  -P  $MOUNT_DIR

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 Foot wood "

fi

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

example 1 Determine whether the target host is alive or not , Print if you live is up, If it doesn't survive, print is down

C ping The number of times

-1 How long each time ping- once , The unit is seconds

-W Time for feedback , If it doesn't work, you can speed up the feedback time , The unit is seconds

#! /bin/bash

ip=192.168.10.10

ping -c 2 -i 0.2 -W 3 $ip &> /dev/null

if[$?-eq0];then

echo "$ip is up "

else

echo "$ip is down"

fi

Judge by . sh The script at the end is a shell Script ( Use of brackets : Can pattern match )

[abc==abc]

[[ abc == abc* ]]

example 4

#! /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

example 5

#! /bin/bash

netstat -natup I grep ":80" &> /dev/nu1l

if[$?-eq0];then

echo " The website service is already running !”

else

echo " start-up httpd service " .

yum install -y httpd > /dev/null

systemctl start httpd

fi

（ 3、 ... and ）. 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

ps aux l grep httpd l grep -v grep

if [ $? -ne o ] ;then

if [ "$ (rpm -q httpd) " == " Package not installed httpd " ];then

yum -y install httpd

systemctl start httpd

else

systemctl start httpd

fi

else

echo "httpd is running"

fi

example 2. Determine whether a user has a home directory , Printing is normal , Did not ask if you need to delete , If yes, delete it, and judge that the system has incomparable users ---- Yes --- Determine whether there is a home directory ---- Yes ---- Output normal user

nothing ----- Ask if you want to delete this user ---- yes ---- Delete

no ---- Exit script

nothing ---- Prompt that there is no such user

example 1 Users of the system :

There are users and home directories

There are users but no home directories

No users but home directory

read -p " Please enter a user name :"user

if grep $user fetc/passwd &> / dev/null;then

if[ -d f home/ suser ] ;then

echo" The user status is normal "

else

read -p " The user does not have a home directory , Delete this user , Please enter [yes/no]: " ask

if [ $ask == yes ] ; then

echo " Deleting user ..."

userdel $user_6>_ldev/null; sleep 2

echo" The user has been removed "

elif [ $ask == no ] ;then

exit

fi

fi

else

echo" The user does not exist "

fi

（ Four ）.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 , Each of these needs to be Each value executes different command sequences . 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

case A variable's value in

Pattern 1)

Command sequence 1

;;

Pattern 2)

Command sequence 1

;;

*

Default command sequence

esac

case Line ending must be a word "in", Every time - The pattern must be enclosed with a closing parenthesis ")” 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 the vertical bar symbol “|" Represents or , Such as “A | B".

final “*)” Represents the default mode , Among them * It's equivalent to a wildcard .

Case study :

What to eat today ? - - Seven days a week is not the same

case Statement execution flow : use first " A variable's value " With the model 1 Compare , If the values are the same, the execution mode 1

After the command sequence , Until I met a double semicolon “;;“ Jump back to esac, Means to end a branch : If and mode 1 It doesn't match , Then continue with the mode 2

Compare , If the values are the same, the execution mode 2 After the command sequence , Until I met a double semicolon “;;” Jump back to

esac, Means to end a branch ... . And so on , If no matching values are found , Then execute the default mode “*)” After the command sequence , Until I met esac End the branch after

Typical cases : Check the character type entered by the user

#!/bin/bash

read -P " Please enter a character , And press Enter Key confirmation : " KEY

case "SKEY" ir

[a-z]1[A-Z])

echo " You're typing in letters ”

;

[0-9])

echo " You're typing in numbers "

::

*)

echo " You type in spaces ,*,_ , Equal special character "

esac

#!/bin/bash

read -P " Please enter your score (0-100) :”score

[[ Sscore -ge 85 6& Sscore -le 100 ]] && a="great"

[[ $score -ge 70 &6 sscore -1t 85 ]] &6 a="standard"

[[ Sscore -ge 0 66 $score -1t 70 ]] 66 a="false"

case $a in

great)

echo "Sscore branch , good !"

;;

standard)

echo "sscore branch , qualified !"

;;

false )

echo "sscore branch , unqualified !"

;;

*)

echo " Incorrect input !"

esac

The first method

#!/bin/bash

read -P“ Please enter your score :”score

[ Sscore -eq 100 ] && a=0

[ Sscore -ge 90 ] 66[ $score -1t 100 ] 66 a=10

[ Sscore -ge 70 -a $score -le 89 ] 66 a=20

[ Sscore -ge 60 -a $score -le 69 ] && a=30

[ $score -ge 0 -a $score -1t 60 ] && a=40

case $a in

echo " Xiuer !”

;;

10)

echo "sscore branch , copy 10 All over !"

;;

20)

echo "Sscore branch , copy 20 All over !”

;;

30)

echo "sscore branch , copy 30 All over !"

;;

40 )

echo "sscore branch , Copy all 30 All over !"

;;

*)

echo

” Incorrect input !”

esac

The second method

read -P” Please enter your score :”score

case Sscore in

100 )

echo " Xiuer !”

;;

9(0-9])

echo "Sscore branch , copy 10 All over !"

;;

[78](0-9])

echo "sscore branch , copy 20 All over !"

;;

6[0-9])

echo "Sscore branch , copy 30 All over !"

;;

[0-9]1[1-5][0-9] )

echo "Sscore branch , Copy all 30 All over !"

;;

*)

echo” Incorrect input !”

esac

To write apache Start service script

#!/bin/bash

case $1 in

start)

/usr/bin/systemctl $1 httpd

/usr/bin/ps aux I grep httpd

echo "httpd start ”

;;

stop)

/usr/bin/systemct1 $1 httpd

/usr/bin/ps aux I grep httpd

echo "httpd stop"

;;

restart)

echo "httpd STOP..."

/usr/bin/ps aux I grep httpd

/usr/bin/systemct1 $1 httpd

echo "httpd restat..."

/usr/bin/ps aux I grep httpd

;;

status)

/usr/bin/systemctl $1 httpd

;;

*)

echo "plases input start | stop | restat | status"

esac