The use of the-if flow control statement and the FOR Loop statement for the 1-23-shell script

Outline:

1. Logical Judgment

2. If flow control statement

3. For loop control statement

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Before you begin, let's look at the logical judgment symbol:

&& and,| | Or,! Non -

A && B must be set up at the same time to pass judgment

---> Judgment process: First execute a to determine whether it is established, if it is established, continue to execute B, determine whether it is established

---> If not, it ends directly and no longer executes B

A | | b A, B has a set up, can be judged by

! A if A is not established, then by judging

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when the condition is judged, that side compares the value or the string, which is what we will use some parameters and symbols, as follows :

Compare values:

Equals--->–eq.

Not equal to--->–ne

Less than--->–lt

Greater than--->–gt

Less than or equal to--->–le

Greater than or equal to--->–ge

Some English words: Help memory

Equal equals, not equal, less than, great than

Compare strings:

= equals, = = equals, same =,! = is not equal, > is greater than, < less than,-Z string is empty,-n string is not null

Note: in the [] structure,< and > need to use escape symbols, such as

Combat 1: Judging the current system language environment

[Email protected] 02]# echo $LANG
en_US. UTF-8
[Email protected] 02]# echo $LANG | Awk-f. ' {print '} '
en_US
[Email protected] 02]# vim lang.sh
[Email protected] 02]# sh-x lang.sh
+ + echo en_US. UTF-8
+ + awk-f. ' {print '} '
+ Language=en_us
+ ' [' en_US = en_US '] '
+ Echo ' The Default Language is 中文版. '
The Default Language is 中文版.
[email protected] 02]# cat lang.sh
#!/bin/bash
# judged if the System Default Language is 中文版

language=$ (echo $LANG | awk-f. ' {print $} ')

If
[$language = ' en_US ']
Then echo "The Default Language is 中文版."
else echo "You are wrong!"
Fi

[Email protected] 02]#

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If Process Control statement

There are three ways to use the IF statement:

The format is as follows:

Single branch:

If condition judgment statement

Then execute the statement

Fi

Dual Branch:

If condition judgment statement

Then execute the statement

Else EXECUTE statement

Fi

Multi-branch:

If condition judgment statement

Then execute the statement

Else EXECUTE statement

Fi

##################################
Combat 1:if Single Branch---> Application

The source code is as follows:

#!/bin/bash
# if
# Then
# fi

if [!-d/root/cdrom]
Then echo "/root/cdrom was not exist!"
Fi

##################################
actual combat 2:if Dual Branch Application--->pinghost.sh
Ping-c 3-i 0.2-w 3 192.168.0.1

-C Send the number of packets

-I packet send interval time, default unit: seconds (s)

-W wait Time exceeded return failure

The source code is as follows:

#!/bin/bash
# Ping a given host and print the host status

Ping-c 3-i 0.2-w 3 >>/dev/null #-c count-i-W Wait
If [$?-eq 0]
Then echo "The host was up."
else echo "The host is down."
Fi

##################################

actual combat 3:if Multi-branch Application---> Judging numerical interval num_where.sh

The source code is as follows:

#!/bin/bash
# judged the input score

Read-p "Input you Score (0-100):" num

If [$num-gt] && [$num-lt 100]
Then echo "great! Good good study!! "
else echo "Day Day up!"
Fi

##################################

Actual combat 4:if Nested use

write scripts to monitor service run status ,
After the startup fails, save the log and restart the service.
Fail again, prompt to restart the host

The source code is as follows:

#!/bin/bash

Systemctl Status $ >>/var/log/ser.log

If [$?-eq 0]
Then echo "The $ is running."
else echo "The $ is dead."
Systemctl Start $ >>/dev/null
If [$?-eq 0]
Then echo "Reboot finish!"
Systemctl Restart $
else echo "warnning you to Reboot your server!"
Fi
Fi

##################################
Combat 5: Query kernel version, and output information

The source code is as follows;

#!/bin/bash
# Search the major version of System, and print info.

prime=$ (Uname-r | awk-f. ' {print $} ')

If [$prime-GT 2]
Then echo, "the major version of System is $prime."
elif [$prime-lt 1]
Then echo "The System was too low."
else echo "failed!"
Fi

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For loop control statements

Usage:

For variable name in variable traversal list

Do

Looping statements

Done #循环结束表示

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Combat: User Management (bulk add users)

Method One:

Operating Environment:

Output Result:

To delete a user in bulk:

Method One optimization:

We found that in a batch of methods to add the user is, when the user does not exist, there will be an error message. Review the following procedures:

Sh–x for01_useradd.sh

After entering the loop, we will first look for/etc/passwd, if there is already a user $user,

However, in the process of operation, we found that when the user does not exist when the user_a value is empty, the judgment condition will be less a comparison value, and the result of the judgment condition becomes: [= Xiaogan], this is the system will error!

To avoid system errors, we can use the-Z or-n option to determine if the user_a variable is empty.

The results of the implementation are as follows:

Method Two: We can also take another way to bulk add users

The source code of each method is as follows:

Method One:

#useradd. Sh

#!/bin/bash
userlist=$ (Cat user.list)

For user in $userlist
Do
user_a=$ (grep-w "$user"/etc/passwd | awk-f: ' {print $} ')
If
[$user _a = $user]
Then
echo "The user $user is already exist."
Else
Useradd $user #2 >>/dev/null
echo "User $user is added."
Fi
Done

###########

#userdel. Sh

#!/bin/bash
userlist=$ (Cat user.list)

For user in $userlist
Do
Userdel-r $user 2>>/dev/null
If [$? = 0]
Then
echo "The user $user is deleted!"
Fi
Done

Optimization:

#usera dd.sh

#!/bin/bash
userlist=$ (Cat user.list)

For user in $userlist
Do
user_a=$ (grep-w "$user"/etc/passwd | awk-f: ' {print $} ')
If
# [$user _a = $user]
[! $user _a-z] && [$user _a = $user]
# to determine whether user_a are empty, not empty on to continue
Then
echo "The user $user is already exist."
Else
Useradd $user #2 >>/dev/null
echo "User $user is added."
Fi
Done

Method Two:

#new useradd.sh

#!/bin/bash
# Batch Add user by scripts
# The New method

userlist=$ (Cat user.list)

For user in $userlist
Do
Useradd $user 2>>/dev/null
If
[$?-eq 0]
Then
echo "123456" | passwd--stdin $user >>/dev/null
echo "Useradd success!"
Else
echo "The user $user is already exists."
Fi
Done

##################################

Combat: Ping.sh #批量测试IP地址是否连通

The source code is as follows:

#!/bin/bash

ip_list=$ (Cat ip.list)

For IP in $ip _list
Do
Ping-w 3-c 3-i 0.2 $ip >>/dev/null
If [$?-eq 0]
Then echo "The $ip was up."
else echo "The $ip is down."
Fi
Done

The use of the-if flow control statement and the FOR Loop statement for the 1-23-shell script