1. Linear search
Time:
- Worst: O(n)
- Average:
- Best:

Code:
public class LinearSearchExample {
   
    private static int[] a = {1000, 11, 50, 1, 2, 3, 78, 100, 101, 102, 1000, 1000};
   
    public static void main(String[] args) {
        int x = 1000;
        int count = 0;
       
        for (int i = 0; i < a.length; i++) {
            if (a[i] == x) {
                count++;
            }
        }
       
        if (count == 0) {
            System.out.println("not found");
        } else {
            System.out.println("Match results: " + count);
        }
    }
}

2. Binary search
Time:
- Worst:O (log n)
- Average:
- Best:

Code:
public class BinarySearchExample {

    // sorted array
    private static int[] a = { 10, 21, 23, 54, 61, 72 };

    public static void main(String[] args) {
        int x = 10;

        // sort array
        int left = 0;
        int right = a.length;
        int mid;
        while (left <= right) {
            mid = (left + right - 1) / 2;

            if (a[mid] == x) {
                System.out.println("Found " + x + " in array.");
                break;
            } else if (a[mid] < x) {
                left = mid + 1;
            } else {
                right = mid - 1;
            }
        }

    }
}

Read more…

AJAX/JQUERY

I. AJAX Basic
function loadDoc() {
    var xhttp = new XMLHttpRequest();

    xhttp.onreadyStatechange = function() {
        if (this.readyState==4 && this.status == 200) {
          
            // this.responseText: response data from url.
            // $.parseJSON(data): parse string to JSON data.
      
            var jsonData = $.parseJSON(this.responseText);
          
            // $('.class'): addressed class in HTML.
            // $('#id'): addressed id in HTML.
            // $('#id').text(data): set data to this ID or class.
            // $('#id').append(data): append data to this ID or class.
            $('.full-name').text(jsonData.full_name);
            $('#full-name').text(jsonData.full_name);
            $('.full-name').append(jsonData.full_name);
          
            //clear data
            top.$('.full-name').text("");
        }
    };
  
    xhttp.open("GET", "url", true);
    xhttp.setRequestHeader("Content-type", "application/json");
    xhttp.send;  
}

AJAX-Jquery:

$.ajax({
 url: <url>,
 type: <GET, POST, PUT,...>,
 async: <true, false>,
 dataType: <text, json,xml,...>,
 data: { //paramter for sending....
 },
 success:function(data, status, xhr) {
  //get data here
 }
 
 
});

Ex:
var test = ajax(); // test maybe doesn't have value because of async = true,
funtion ajax(){
 .....
 async: true,
 .....
 success:function(data, status, xhr) {
  return data;
 }
}
var test = ajax(); //test will be have data.
funtion ajax(){
 var result;
 var response = $.ajax({
  url: <url>,
  type: <GET, POST, PUT,...>,
  async: false,
  dataType: <text, json,xml,...>,
  data: { //paramter for sending....
  },
  success:function(data, status, xhr) {
   //get data here
   result = data;
  }
 });
 
 if (result) {
  return result;
 }
}

=========================================================================
JQUERY
#clear elements
$("elements").remove();

#remove value on form
$("#form_ID").trigger('reset');

// set checked of Selection box
#(element).prop("checked", true/false);

Read more…

Oracle-PL/SQL
1. Check Oracle performance

-- check defragment of table

select table_name,avg_row_len,round(((blocks*16/1024)),2)||'MB' "TOTAL_SIZE",

round((num_rows*avg_row_len/1024/1024),2)||'Mb' "ACTUAL_SIZE",

round(((blocks*16/1024)-(num_rows*avg_row_len/1024/1024)),2) ||'MB' "FRAGMENTED_SPACE",

(round(((blocks*16/1024)-(num_rows*avg_row_len/1024/1024)),2)/round(((blocks*16/1024)),2))*100 "percentage"

from all_tables WHERE table_name='SC_QST_MAIN';

-- analyze index

analyze index SCC_I18N.IDX_QST_MAIN validate structure;

--gather statstic

exec DBMS_STATS.GATHER_TABLE_STATS (ownname => '"SCC_I18N"', tabname => '"SC_QST_MAIN"', estimate_percent => 1);

-- check response time of database

SET LINESIZE 200 PAGESIZE 50000

    COL BEGIN_TIME FORMAT A17

    COL END_TIME FORMAT A17

    COL INST_ID FORMAT 999

    COL "Response Time (msecs)" FORMAT 999,999,999,999.99

    SELECT TO_CHAR (BEGIN_TIME, 'DD-MON-YYYY HH24:MI') BEGIN_TIME,

         TO_CHAR (END_TIME, 'DD-MON-YYYY HH24:MI') END_TIME,

         INST_ID,

         ROUND (VALUE * 10, 2) "Response Time (msecs)"

    FROM GV$SYSMETRIC

    WHERE     1 = 1

         AND METRIC_NAME = 'SQL Service Response Time'

    ORDER BY INST_ID;

-- explain sql statement

explain plan for select count(user_key) from sc_qst_main where  reg_dt >= TO_DATE('20190421000000', 'YYYYMMDDHH24MISS') AND user_key = '2803115653';

select * from table(dbms_xplan.display);

Read more…

 I) How to remove something from right click context menu windows 10
 Sometime, Context menu in right  click has many items and long line. we should remove some item that you not use long time or frequently.
1. Go to Run or Window + R
2. Type 'regedit'
3. Go to Computer\HKEY_CLASSES_ROOT\*\shellex\ContextMenuHandlers
4. Choose programs which you want to disable menu.
5. Change default value to -{hex code}.
Ex:
Before: {A94757A0-0226-426F-B4F1-4DF381C630D3}
After: -{A94757A0-0226-426F-B4F1-4DF381C630D3}

Video ref:

 

 2. Folder/File not refresh

Ref: https://superuser.com/questions/390030/explorer-does-not-auto-refresh

Refer to Windows 7 does not refresh folder views on Microsoft Answers and Windows Explorer doesn't refresh when moving/deleting. Several posters stated that the issue pertains to the Windows UI Shell and several solutions exist:

• Remove a "Network Connection" that points to a server that is not currently available.
• Disable the Client for Microsoft Networks in properties of Local Area Connection.
• Or simply going through Control Panel -> Folder Options -> View, then click 'Reset Folders'.

Let me know if these links help you out!

Read more…

Rút gọn mệnh đề quan hệ

1. Mệnh đề quan hệ đóng chức năng là tân ngữ trong mệnh đề quan hệ.
Ex:
It's the best movie that I have even seen.
=>It's the best movie I have even seen.

2.Rút gọn bằng cách dùng V_ing
- Lượt bỏ mệnh đề quan hệ khi động từ của mệnh đề quan hệ đang ở thể chủ động. Khi đó ta lượt bỏ mệnh đề quan hệ + to be (nếu có).
Ex:
The woman who teaches English at his school is Ms. Smith.
=> The woman teaching English at his school is Ms. Smith.

The girl who was smilling at you is John's daughter.
=>The girl smilling at you is John's daughter.

The man who was in charge of this department has just resigned.
=> The man being in charge of this department has just resigned.
=> The man in charge of this department has just resigned.

3. Rút gọn bằng cách dùng V-ed/V3
- Lượt bỏ mệnh đề quan hệ trong trường hợp động từ của mệnh đề quan hệ đang ở thể bị động.
Ex:
Some of the phones which were sold last month are broken.
=> Some of the phones sold last month are broken.


Nguồn: https://tienganhmoingay.com/ngu-phap-tieng-anh/rut-gon-menh-de-quan-he/

Read more…

Update the registry to remove the Office 365 activation prompt

1.Close the activation window and all Office apps.
2.Right-click the Start button  Windows Start button in Windows 8 and Windows 10 on the lower-left corner of your screen, and select Run.
3.Type regedit, and then press Enter. Select Yes when prompted to open the Registry Editor.
On the left side of the Registry Editor, under Computer, navigate to the following key in the registry:
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\Office\16.0\Common\OEM
4.Right click the OEM value and click File>Export.
5.Save the key
6.After the key is backed up, select Edit>Delete.
7.Repeat steps 3-6 for the following key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\Common\OEM
8.Close the Registry Editor and start Office again.

Read more…

Documents:

1. https://www.mediafire.com/folder/pes5te8e9hpct/linux

1. "pwd" command line

- Type "passwd" : change current password.

- Type "passwd <user_name>: change password of user name.

- Type "passwd  -S <user_name>": check account lock or unlock.

- Type "passwd  -l <user_name>": lock user.

- Type "passwd  -u <user_name>": unlock user.

 or

- Type "cat /etc/shadow | grep -i <username"


Result:

username:$1$....      :unlocked.

username:!!$1$.....   : locked.


2. "Chage" command line.


- Type "chage -l <username>": view account information.

Output:

Last password change     : Jan 10, 2012

Password expires     : never

Password inactive     : never

Account expires      : never

Minimum number of days between password change  : 0

Maximum number of days between password change  : 99999

Number of days of warning before password expires : 7

-- change expire password, expire account...

#chage <account>

=> this command will be ask one by one for setting

or

#chage -I -1 -m 0 -M 99999 -E -1 <account>

=> this command will auto setting by parameter

3. "ping" command line.


- ping <destination> : sent packet to destination server with endless times.

- ping -i  <interval_count> <destination>: sent packet to destination server with interval time.

- ping -s <size_packet>  <destination> : sent packet to destination server with size packet (bytes).

- ping -c <number_packet>  <destination> : sent packet to destination server with number packet.

- ping -D  <destination> : show timeslap each line.


4. SVN command line


Before type command below, please cd to svn working space.


-- start svn service

#svnserve -d -r <repository_location>


-- kill svnserve

#ps auxww | fgrep svnserve

#kill <process id>


--Check out

#svn checout --username <username> --password <password>


--Update

#svn update --username <username> --password <password>


--check info

#svn info

-- Create repoz

#svnadmin create /repos_path

-- Change repoz information

/respo/passwd: set password for user

/respo/authz: add/remove/edit user

Note:

If you got when sync or update,... with "svn: Can't find a temporary directory: Internal error" . Check write permission in  "/tmp" dir of your server.

Ref: http://svnbook.red-bean.com/en/1.8/svn.ref.svn.c.checkout.html


5. Set JAVA_HOME variable


export JAVA_HOME=jdk-install-dir

export PATH=$JAVA_HOME/bin:$PATH


6. Start/stop oracle db


 // check ORACLE_HOME


#echo $ORACLE_HOME

// if ORACLE_HOME variable not set, please set it


#export ORACLE_BASE=/home/oracle/app/oracle

#export ORACLE_HOME=$ORACLE_BASE/product/11.2.0/dbhome_1

#export PATH=$PATH:$ORACLE_HOME/bin

#export ORACLE_SID= 'SID NAME';

# go to oracle install dicrectory


cd  /home/oracle/app/oracle/product/11.2.0/dbhome_1/bin


-- start listener

# lsnrctl start


--start db

#dbstart


-- stop listener

# lsnrctl stop


--stop db

#dbshut


--check status

#lsnrctl status

#sqlplus / AS SYSDBA

-- change the maximum dump file size

Ref: https://docs.oracle.com/cd/B19306_01/server.102/b14237/initparams116.htm#REFRN10103

SQL > alter system set MAX_DUMP_FILE_SIZE='200m';

7. head command line

"head" is print a number of data of given output.


-- print first 10 line

#head test.txt


--print the first N lines.

#head -n 100 test.txt


--print the first N bytes.
#head -c 256 test.txt


-- print line from 2 files
#head -q test1.txt test2.txt


 --print line from N to M.

head -n 20 | tail -10


8. Grep

#grep "keywork" "file"


-- don't distinct UPPER CASE and lower case

#grep -i "keyword" "file"


-- find with case sensitive

#grep -w "keyword" "file"


-- find and show line before/after your search.

#grep - <A,B or C> n "keyword" "file"

A: after lines

B: before lines

C: around of the match line.

n: number of the line is show.


-- find a keyword on sub folder.

#grep -r "keywordd" "file"


--find all result not contain keyword.

#grep -v "keyword" "file"


--count results.

#grep -c "keyword" "file"


--just show file name in result.

#grep -l "keyword" "file"


--show the  line number in result

#grep -n "keyword" "file"


--find PID of a program

#grep -ef | grep java

#grep -ef | egrep 'keyword1|keyword2|keyword3'

--Find and delete N files 

$find . -name '*.aud' -print0 | xargs -n1000 -0 rm -f

9. Check Java Memory

-- go to JDK folder

cd <jdk>/bin


-- find PID of Java

ps -ef | grep java


-- check JAVA heap

jmap -heap <PID>


10. Check phycial memory, swap

 -- check memory

# grep MemTotal /proc/meminfo


 -- check swap

# grep SwapTotal /proc/swaps



11. Check file system information (df - disk filesystem)

-- display all disk filesystem

#df


-- display all file system

#df -a


-- display stored used of system.

#df -h


--display stored used of a folder.

#df -h /tmp

-h: show Gb.

-m: show Mb

-k: show Kb:


12.Check/config IP address

On Redhat 7

-- show IP info

#ip addr show


-- show network interface

#nmcli -p dev


-- set IP address

#vi /etc/sysconfig/network-scripts/ifcfg-<interface_name>

TYPE=Ethernet

PROXY_METHOD=none

BROWSER_ONLY=no

BOOTTPROTO=DHCP

DEFROUTE=yes

IPV4_FAILURE_FATAL=no

IPV6_AUTOCONF=yes

IPV6_DEFROUTE=yes

IPV6_FAILURE_FATAL=no

IPV6_ADDR_GEN_MODE=stable-privacy

NAME=enp0s3

ONBOOT=yes //yes if you want to connect Internet.

IPADDR=x.x.x.x

PREFIX=32

GATEWAY=x.x.x.x

DNS1=x.x.x.x

DNS2=x.x.x.x


//CentOS 6

DEVICE="p4p1"
HWADDR="00:22:19:09:4D:3C"
NM_CONTROLLED="yes"
ONBOOT="yes"    # Interface enabled
BOOTPROTO="static"  #Assigning in static mode 
IPADDR=192.168.1.11 #IP address
NETMASK=255.255.255.0 # Subnet Mask
GATEWAY=192.168.1.1 # Default Gateway
DNS1=8.8.8.8 #Primary DNS
DNS2=4.2.2.2 #Secondary DNS

 and then restart network

//on Redhat

#systemctl restart network

//on Centos

#systemctl start NetworkManager.service
#systemctl stop NetworkManager.service

#systemctl restart NetworkManager.service

//using nmcli

#nmcli networking off
#nmcli networking on

Note:

Oracle VMware
Map network interface
1. Device > Setting > Network:
Adapter 1 > Attach to: Host only adapter. > select VM Network Interface.

13. Enable SSH
on REDHAT7

-- check SSH

#systemctl status sshd

#systemctl enable sshd

#firewall-cmd --zone=public --permanent --add-service=ssh

-- restart ssh service
#service sshd restart

14. Remove file/directory
--remove dir with confirm prompt

#rm -r <directory_name>


-- remove dir without confirm prompt

#rm -rf <directory_name>


15. Create/Modify for user, group.

 /etc/shadow: show the password of user, user group

 /etc/group: show all group

 # ls -l file

-rw-r--r-- 1 root root 0 Nov 19 23:49 file

type of file | owner(rw-) | group (r--) | other (r--)


Option:

+ : add more permission

- : remove permission

= : change permisson


-- change file permission

#chmod 640 <file_name>;


--change owner file

#chown <user> <file_name>;
options: -r : change owner for files and sub folder.


-- add execute permission to file owner.

#chmod u+x file_name;

--add read/write to group

chmod g+rw file_name


--remove write permission to other.

chmod o-w file_name


-- we change usr owner, group and other.

chmod u=rwx,g=rx,o= file_name


--create user

#useradd <user_name> <options>

// Create user with default home/<username>
# useradd -m USERNAME
// Create user with specific home
#useradd -m USERNAME -d /Path

Option:

-p <password>

-- Modify user
-add group
#usermod -a -G <group> <user> 

-- modify primary group
#usermod -g <user> <group>

-- delete user

#userdel -r <user_name>

Option:

-r : remove also user's home directory

-f:  removal in case the user is logged


-- change group owner ship of file.

#chgrp <group_name> <file_name>

-- add User Group, Group Id, Change Group, Del Group

#groupadd <group_name>

#groupmod -g 601 <group_name>

#usermod -G <group1, group2,...> <user_id>

#groupdel <group>

-- Change home directory

#usermod -d /home/<user_home> <user_id>

-- Change Shell

#usermod -s /bin/bash <user_id>

#usermod -s /bin/nologin <user_id>

-- Make a user to ADMIN user

$usermod -aG wheel <user_name>

-- Change script of a user (User Login)

$ usermod -s /sbin/nologin <username>

--Make a use
16. Check System information

====Redhat 7=======

 --show kernel

#uname -a


-- show name and version of distribution
#head -n1 /etc/issue

-- check centos version

#hostnamectl

--show all partition that registered on system

#cat /proc/partitions


--show Total memory

#grep MemTotal /proc/meminfo


--show CPU

#grep "model name" /proc/cpuinfo


-- show info about disk

#hdparm -i /dev/sda


====Redhat 7=======
17. Set session timeout

/etc/ssh/ssh_config

 ClientAliveInterval 120

ClientAliveCountMax 720


18. Secure log

-- these log for accessing, changed password of users.

Probably in /var/log/auth.log on Debian, or /var/log/secure on Redhat

19. Set time/date

Set date from the command line

date +%Y%m%d -s "20200202"

Set time from the command line

date +%T -s "12:00:00"

Set time and date from the command line

date -s "19 APR 2012 11:14:00" 

View Real-time date time

watch -n 1 date

View Real-time with command

watch <command>

20. Set HostName
#hostnamectl set-hostname <new-host-name>

21. set sFTP for only users

On CentOS
--key gen
#ssh-keygen
Generating public/private rsa key pair.
Enter file in which to save the key (/root/.ssh/id_rsa): /home/gis/.ssh/id_rsa
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/gis/.ssh/id_rsa.
Your public key has been saved in /home/gis/.ssh/id_rsa.pub.
The key fingerprint is:
SHA256:Wtrl4sVFLG10qpNUWcSB8C7BZswkD7C2XKs+WMug9Xk root@centos-test
The key's randomart image is:
+---[RSA 2048]----+
|      ..o o.o*=. |
|       . O *oo.  |
|      o . @ B    |
|     o o = O     |
|      o S * o    |
|    o .* + +     |
|   o *+oo +      |
|  . ..*.Eo       |
|      .o.        |
+----[SHA256]-----+

#Copy and past public key to /home/<user>/.ssh/authorized_keys file. (you also add many public keys)

# Get private key input to Firezilla > Edit > Setting > SFTP > add private key to there.
Ref:
1. https://blog.tinned-software.net/setup-sftp-only-account-using-openssh-and-ssh-key/
2. https://blog.tinned-software.net/ssh-passwordless-login-with-ssh-key/

Note: when you ssh key for connecting but failed, please check permission of user like that:

Ref: https://unix.stackexchange.com/questions/311408/passwordless-ssh-is-not-working-in-centos-7/388649

/home/username

drwxr----x. 18 username     groupname  4096 May 11 11:52 username

/home/username/.ssh

268823097 drwx------   2 username groupname     29 May 11 11:53 .ssh

/home/username/.ssh/authorized_keys

-rw-r----- 1 username groupname 402 May 11 11:53 authorized_keys

22. Override and append

ll -al /folder > text.txt

ll -al /folder >> text.txt

23. Yum, NPM

Install, check software in:

-- Install DNS 

$yum install bind

24. Config host deny and allow

// deny host and protocol

vi /etc/host.deny

//allow host and protocol

vi /etc/host.allow

Ex:

telnetd:ALL:deny

sshd:ALL:deny

ftpd:ALL:deny

Note:
ALL: ALL
This denies all service to all hosts, unless they are permitted access by entries in the allow file.

25. SNPM

Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. Devices that typically support SNMP include cable modems, routers, switches, servers, workstations, printers, and more

#show running snmp

#redhat

ps -ef| egrep snmp

lsof -i :199

#stop snmp

kill -9 <PID>

service snmpd stop

#start snmp

service snmpd start

#check config

chkconfig snmpd off (<= redhat 6)

systemctl disable snmpd.service  (>= redhat 7)

#Config file

vi /etc/snmp/snmpd.conf

option:

-- allow all ip income.

rocommunity !snmp_shb*nms 

26. Find world-writable files

#find / -perm -2 -ls

27. automount service

--find automount service

#ps -ef | grep automount

or

#ps -ef | grep autofs

-- turn off/on automount service

#chkconfig autofs off (for Rhat 6)

#systemctl disable autofs.service (for Rhat 7)

27. finger service

Desc:

A finger service is running on this host. The finger protocol is used to find out information about users on a remote system. Finger servers can usually provide either a list of logged-in users or detailed information on a single user.
Finger information has been frequently used by crackers as a way to initiate a social engineering attack on a company's computer security system. By using a finger client to get a list of a company's employee names, email addresses, phone numbers, and so on, a cracker can telephone or email someone at a company requesting information while posing as another employee.

--find finger service

#ps -ef | grep inetd

--kill process

#kill -HUP [PID]

--config finger

#vi /etc/inetd.conf

 finger stream tcp nowait bin /usr/lbin/inetd -s   (we can comment this as disable finger)

LINUX for xinetd

#vi /etc/xinetd.d/finger

--restart xinetd service

#service xinetd restart

#systemctl restart xinetd.service

28. Check file, directory has owner.

#find / -nouser -o -nogroup -xdev -ls 2> /dev/null

29. Make Home dir for exist user.

You want to make home dir for existing user in Linux.

#mkhomedir_helper <username>

30. Disable telnet

# vi /etc/xinetd.d/telnet
service telnet
{
    flags       = REUSE
    socket_type = stream       
    wait        = no
    user        = root
    server      = /usr/sbin/in.telnetd
    log_on_failure  += USERID
    disable     = yes // change there
}

#service xinetd restart

31. init process in Linux

// parent of each process

/etc/inittab

Meaning in the inittab

id:runlevels:action:process

// the script has prefix "S", the system call and run:

/etc/rc.d/init.d/S<command>

// the script has prefix "K", the system call and run:

/etc/rc.d/init.d/K<command>

Run level in /etc/rc.d/

0 : Halt system

1 : Take system to single-user mode (good for Linux system maintenance)

2 : User defined or distribution like Debian use it

3 : Full multi-user mode (text mode login)

4 : Not used/user-defined

5 : Full multi-user GUI mode login

6 : Reboot system

32. rsync (Đồng bộ file, thư mục)

#rsync options source destination

option:

-v : hiển thị kết quả

-r : copy dữ liệu recursively, not sure

-a: chp phép copy dữ liệu recursively, đồng thời giữ nguyên dược tất cả các thông số thư mục và file.

-z: nén dữ liệu trước khi truyền.

--delete :  xoá dữ liệu ở destination nếu source không tốn tại dữ liệu đó.

--exclude : loại trừ dữ liệu không muốn truyền đi.

Ex:

#rsync -zvh backup.tar /tmp/backups/

#rsync -avzh /a /b

sync between servers

#rsync -avz backup/ root@192.168.88.165:/home

#rsync -zvzh root@192.168.88.165:/root/data backup/data1

Sync together by SSH

#rsync -avzhe ssh backup.tar root@192.168.0.100/backups/   -- local remote

#rsync -avzhe ssh root@192.168.0.100:/root/install.log /tmp/ -- servers remote

//copy directory

#cp -r <dir_source> <target>

 

*copy overide without confirm.

# /cp -r <dir> <target>

*copy and not override exist file

#cp -Rn source target

* copy and not override exist file, print message per files (option = v)

#cp -Rvn source target

33. Find cmd

#find /data -name test.txt

#find/data -name test*

34. zip//unzip by gzip/gunzip, tar

#gzip /data/test1.txt

#gunzip /data/test1.txt.gz

--create file tar

#tar -cvf test.tar test.txt

--create tar file for folder "test"

#tar -cvf /home/test.tar /home/test

-- unzip

#tar -vxf test.tar -C /data/

-- create tar.gz

#tar -czvf test.tar.gz test

--List files in tar

#tar -ztvf test.tar.gz

-- Extract tar.gz

#tar -xzvf test.tar.gz

35. lsof

Sometime, you deleted a large file and it not descrease size of folder. please check deleted files.

Because when you deleted a large but the running process is holding it, the file will be not deleted actually.

show delete files

#lsof <path_deleted_file>

#lsof | grep <path_deleted_file>

solution:

#kill -9 <PID>

$ /usr/sbin/lsof | grep deleted
ora    25575(PID) data   33u   REG      65,65  4294983680   31014933 /oradata/DATAPRE/file.dbf (deleted)

Resolved: https://access.redhat.com/solutions/2316

36. Kill command

#kill <option> <PID>

Option:

1 (HUP) - Reload a process.
9 (KILL) - Kill a process. (Kill process ngay lập tức)
15 (TERM) - Gracefully stop a process. (Gửi tín hiệu đến tiến trình để kill)

37. top cmd
Top cmd check resources of linux as memory, cpu,...

CPU(s):
1.6%us
0.6% sys
95.5%id: free % of CPU.

//Get percent of CPU
top -bn2 | grep '%Cpu' | tail -l | grep -P '(....|...) id, '|awk '{print "CPU Usage:" 100-$8 "%"}'

top -bn2 | grep '%Cpu' | tail -l | grep -P '(....|...) id, '|awk '{print "CPU Usage:" 100-($8/4) "%"}'

38. crontab

crontab -l  : list of crontab

crontab -e : edit crontab

5 * * * * /home/admin/admin_crontab.sh

//edit for another user

crontab -u <user> -e

/etc/cron.allow  : allow crontab for users.

/etc/cron.deny : deny crontab of users.

-- Check the user is using the cron tab

$ll /var/spool/cron

 

39. tcpdump

Capture package in a network interface.

#tcpdump dst <IP_ADDRESS> 

 

-- capture packages and write files.

#tcpdump dst <IP_ADDRESS> and src port 22 -w FILE_NAME.pcap

 

-- Capture package within range port.

#tcpdump portrange 21-25 -c 9

40. netstat

show all connection on server.

# netstat -nat

# netstart -nat | grep LISTEN

# netstat -nltp

check port and pid using

#netstat -nlp | grep <port> 

41. iptable cmd

--list

$ iptable -L

 

--allow all ip access via port 22

$ iptable -A INPUT -p tcp --dport 22 -j ACCEPT

 

How to use module extension: -m <extension name> --<properties> 

x:

$ iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

 

42. Setup vsftpd

--install vsftpd

#yum install vsftpd

--edit configuration

#vi /etc/vsftpd/vsftpd.conf

-- restart service

#service vsftpd restart

43. NTP client/ server

#vi /etc/ntp.conf

#service ntpd restart

-- check status of NTP

#ntpq -pn

NTP Settings

# vi /etc/ntp.conf

restrict 192.168.100.0 mask 255.255.255.0 nomodify notrap # Release access restrictions so that local servers can get time information.

#server 0.rhel.pool.ntp.org # All existing servers are not used.

#server 1.rhel.pool.ntp.org

#server 2.rhel.pool.ntp.org

server [NTP server_ip] # Specifies the time-based server in the private network.

Service Registration

 # service ntpd start

# chkconfig ntpd on

Check service operation

# ps -ef | grep ntp

Check if time sync

 # ntpq -pn

Check for errors

# ntptime | grep returns"

44. du

check size of directory

du -sh path-file

45. Check format of disk

#lsblk -f

Result:

[admin@localhost ~]$ lsblk -f

NAME        FSTYPE      LABEL UUID                                   MOUNTPOINT

sda

├─sda1      ext4              7b0e08ee-f797-4861-a572-173542fe7740   /boot

└─sda2      LVM2_member       01ZNuF-dhgH-5gQh-wvgO-lWff-3gHH-gSshRM

  ├─cl-root xfs               35522e98-5618-48a6-a2af-35048a5f3d00   /

  └─cl-swap swap              9dbaf491-f737-40b5-a22e-4fd3653d2d98   [SWAP]

sr0

46. Count files

Ref: https://devconnected.com/how-to-count-files-in-directory-on-linux/

$ find <directory> -type f | wc -l

$ ls -l | wc -l OR $ ls | wc -l

47. Run process in background

Ref: https://linuxize.com/post/how-to-run-linux-commands-in-background/

When you make a process to do long time, you can put it background process so that do another process

 

 //make background process

$ <command_line> &

 

//list of background process

$ jobs 

$jobs -l

48. Firewall in Centos

Ref: https://www.digitalocean.com/community/tutorials/how-to-set-up-a-firewall-using-firewalld-on-centos-8

When you make a process to do long time, you can put it background process so that do another process

* Zone: least trusted to most trusted

drop > block >  public > external > internal > dmz > work > home > trusted

* Rule Performance
runtime-to-permanent : apply for test and lost after reload

permanent: apply permanent and efftect after reload

//Start/stop firewall

#systemctl enable firewalld

#systemctl start firewalld

//Check status 

#firewalld-cmd --state

-- Get default zone

#firewall-cmd --get-default-zone

--Get active zone

#firewall-cmd --get-active-zones

--Get all rule

#firewall-cmd --list-all

--Get a list of the available zones

#firewall-cmd --get-zones

-- Get a list of specific zone

#firewall-cmd --zone=home --list-all

--Get a list of the available zone less

#firewall-cmd --list-all-zones | less

-- Changeing the Zone of an Interface (--get-active-zones for check result again)

#firewall-cmd --zone=home --change-interface=eth0

-- Adjusting the default zone

#firewall-cmd --set-default-zone=home

* Setting Rules for Application

-- Get a list of the available service

#firewall-cmd --set-default-zone=home

-- Enable service

#firewall-cmd --zone=public --add-service=http

-- Opening a port for zone

#firewall-cmd --zone=public --add-port=5000/tcp

-- Opening a range port for zone

#firewall-cmd --zone=public --add-port=4990-4999/udp

-- Opening a port for zone

#firewall-cmd --zone=public --list-ports

-- Remove Allow port

#firewall-cmd --zone=public --remove-port=22/tcp

// Create new zone

firewall-cmd --new-zone=mariadb-access --permanent

firewall-cmd --reload

firewall-cmd --get-zones

//Allow specifics ip

firewall-cmd --zone=mariadb-access --add-source=10.24.96.5/20 --permanent

// Allow port

firewall-cmd --zone=mariadb-access --add-port=3306/tcp  --permanent

firewall-cmd --reload

// Remove IP

# firewall-cmd --zone=mariadb-access --remove-source=10.24.96.5/20 --permanent

// Remove port

firewall-cmd --zone=mariadb-access --remove-port=3306/tcp --permanent

//Remove Zone

firewall-cmd --permanent --delete-zone=mariadb-access

// Reload

# firewall-cmd --reload

49. Check Java Heap memory

Ref:

1. jstat cmd: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html

2. jstatd cmd: https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstatd.html#BABHHDIB

3. jps cmd:  https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jps.html#CHDGHCGB

$ jps -l

$jcmd <PID> GC.heap_info

$jmap -heap <PID>

50. mysql check

$ SHOW ENGINE INNODB STATUS\G;

$ SHOW FULL PROCESSLIST;

$  SHOW TABLES;

$ SHOW FULL COLUMNS FROM table_name; //show column info

$ DESCRIBE table_name;  

# find query slow

$ show global variables like 'slow%log%';

+---------------------+-------------------------------+

 

| Variable_name       | Value           |

 

+---------------------+-------------------------------+

 

| slow_query_log      | ON           |

 

| slow_query_log_file | /var/log/mysql/mysql-slow.log |

 

+---------------------+-------------------------------+

51. Delete trace file in oracle , GRID Infragstructure

Ref: https://logic.edchen.org/how-to-delete-trace-files-in-oracle/

$ adrci 

$ show homes // get all home path
$ set homepath <home_path in command above>

$ show control

ADR Home = /u01/app/grid/diag/crs/node2/crs:
*************************************************************************
ADRID                SHORTP_POLICY        LONGP_POLICY         LAST_MOD_TIME                            LAST_AUTOPRG_TIME                        LAST_MANUPRG_TIME                        ADRDIR_VERSION       ADRSCHM_VERSION      ADRSCHMV_SUMMARY     ADRALERT_VERSION     CREATE_TIME
-------------------- -------------------- -------------------- ---------------------------------------- ---------------------------------------- ---------------------------------------- -------------------- -------------------- -------------------- -------------------- ----------------------------------------
3841422557           720                  8760                 2015-08-21 16:03:18.127927 +08:00                                                 2018-03-07 10:43:49.850300 +08:00        1                    2                    82                   1                    2015-08-21 16:03:18.127927 +08:00
1 rows fetched

$ set control (SHORTP_POLICY = 168)

$ set control (LONGP_POLICY = 672)

$ purge -age 10080 // minus

after *.trc, *.trm will delete on /oracle/diag/rdbms/<INSTANCE>/<SSID>/trace

// Delete log in GRIG infragstructure

Ref:

 https://docs.oracle.com/en/engineered-systems/health-diagnostics/autonomous-health-framework/ahfug/tfactl-managelogs.html#GUID-07EFE012-6954-4C77-9B8E-E83CA8A7E80A

-- Show access user

$ tfactl access lsusers

-- Add access user

$ tfactl access add -user oracle -local

Syntax:

tfactl managelogs 

[-purge [[-older nm|h|d] | [-gi] | [-database all|d1,d2,...]]] 

[-show [usage|variation] [[-older nd] | [-gi] | [-database all|d1,d2,...]]]

Example:

-- Show log before purge

$tfactl managelogs -show usage -older 30d -gi

--Purge log

$tfactl managelogs -purge -older 30d -gi

TIPs

1. connection close when use sftp although you input correct password

- Root cause: because the sftp account is expired.

- Check: grep -i <account> /var/log/*

- Solution: change the password of account

2. Files in Linux

/bin, /sbin : chứa các tập tin nhi phân hỗ trợ cho việc boot, thực thi các lệnh

/boot : chứa kernel, file hỗ trợ cho việc load hệ điều hành

/dev "  chưa các tập tin thiết bị như CDRom, HDD

/etc:  chứa các tập tin cấu hình hệ thống

/home : home directory của user.

/lib :  chứa các kernel module, các thư viện chia sẻ cho các tập tin nhị phân trong /bin và sbin

/mnt: chứa các mount point cửa các thiết bị mount vào hệ thống.

/proc : lưu trữ thông tin về kernel, các tiến trình

Ex: Nếu có một tiến trình là PID = 001, bạn có thể check trong /proc/<PID>/

/root: lưu trữ home directory cho user root.

/tmp: chứa các file tmp.

/usr: chứa các chương trình đã được cài đặt

/var: chứa các log file, hàng đợi các chương trình, mailbox của users.

3. Not allow root user access by SSH

#vi /etc/ssh/sshd_config

uncomment PermitRootLogin line

PermitRootLogin yes

4. Grant "su" for another user

Ref: https://www.digitalocean.com/community/tutorials/how-to-create-a-sudo-user-on-centos-quickstart

#usermod -aG wheel username

(By default, on CentOS, members of the wheel group have sudo privileges.)

#su - username

#su <command-line>

5. Install and config ISCSI

Description: An iSCSI initiator is the client who wishes to access the storage on a target, or server. The IP address of the target needs to be known for this process. 

* Install ISCSI Initiator (CentOS)

Packages:

iscsi-initiator-utils-6.2.0.874-19.el7.x86_64.rpm

iscsi-initiator-utils-iscsiuio-6.2.0.874-19.el7.x86_64.rpm

Online install

# yum install iscsi-initiator-utils

Offline install

#rpm -i iscsi-initiator-utils-iscsiuio-6.2.0.874-19.el7.x86_64.rpm iscsi-initiator-utils-6.2.0.874-19.el7.x86_64.rpm

* Config ISCSI Initiator

#vi /etc/iscsi/scsid.conf

Uncomment properites:

node.session.auth.authmehtod=CHAP

node.session.auth.username = username

node.session.authen.password=password

* Restart service

#systemctl restart iscsid

* Discover target

#iscsiadm -m discovery -t sendtargets -p 192.168.1.1

* Connect to the target

#iscsiadm -m node -T iqn.2015-06.com.example:target1 --login

* Find the iSCSI disk name. 

#grep "Attached SCSI" /var/log/messages

* Create a file system on that disk. 

# mkfs.ext4 /dev/sdb

* Mount the file system. 

# mkdir /mnt/iscsiTest

# mount /dev/sdb /mnt/iscsiTest

* Make it persistent across reboots by editing the /etc/fstab file. 

# blkid /dev/sdb

*Get information about running sessions:

#iscsiadm -m session -P 3

* Shorter output

# iscsiadm -m session

*Logout session

#iscsiadm -m node -T iqn.2007-06.com.test.geeklab:storage.geeklab --portal 192.168.10.12:3260 -u

* Delete Record ID

When you discover the targets and log into it, the information is stored in the node database found in /var/lib/iscsi. In order to delete the target, we have to delete the target’s record ID out of /var/lib/iscsi:

# iscsiadm -m node -o delete -T iqn.2007-06.com.test.geeklab:storage.geeklab --portal 192.168.10.12:3260

Ref:

1.  https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/storage_administration_guide/iscsi-api

2. https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/storage_administration_guide/ch-iscsi

3. https://www.thegeekdiary.com/centos-rhel-67-how-to-delete-an-iscsi-target-on-the-initiator-iscsi-client/

6. Create app as service in Linux

Ref: 

1. https://unix.stackexchange.com/questions/236084/how-do-i-create-a-service-for-a-shell-script-so-i-can-start-and-stop-it-like-a-d

2. https://stackoverflow.com/questions/4797717/linux-script-start-stop-restart

3. https://www.linuxquestions.org/questions/linux-newbie-8/linking-init-script-to-specific-rc0-d-rc1-d-etc-directories-827379/

4. https://stackoverflow.com/questions/8837680/adding-symlink-to-scripts-into-rcnumber-d-folders-to-start-process-during-syst

5. https://www.cyberciti.biz/tips/linux-write-sys-v-init-script-to-start-stop-service.html

For Relhat 6.5

1. Create service script in /etc/init.d/rc.d/

$ vi <service_name>

Ex: vi netty

2. Create symlink in /etc/init.d/rc5.d/

ls -s <file> <target>/S##<service_name>

ls -s <file> <target>/K##<service_name>

K: Kill

S: Start

##: Order to run

Ex:

$ ls -s /etc/init.d/rc.d/netty /etc/init.d/rc5.d/S90netty   (Start service)

$ ls -s /etc/init.d/rc.d/netty /etc/init.d/rc5.d/K90netty  (Kill service)

3. Verify your service

$ service netty start

$ service netty stop

$ service netty status

7. Setup lsyncd in Centos/Redhat

Ref: https://medium.com/@jpandithas/implementing-a-two-way-sync-with-lsyncd-a82026725e66

Sync two-way between servers. Make sure SSH was enable on both server.

Server 1: 192.168.1.1

Server 2: 192.168.1.2

Server #1: 

Step 1: install EPEL

$ yum install epel-release

$ yum install lsyncd

Step 2: Create account on the server

$ useradd admin

Step 3: Create key on "admin" account.

$ su admin

$ ssh-keygen -t rsa

$ ssh-copy-id admin@192.168.1.2  // copy key from server #1 => server #2

You can check key of server #1 on server #2.

authorized_keys : content public key of server #1.

id_rsa: private key 

id_rsa.pub: public key

Step 4: Make folder to sync

$mkdir /email/repl

$mkdir /email/rsync_temp

Step 5: config lsyncd

$vi /etc/lsyncd.conf

settings {

        logfile="/var/log/lsyncd/lsyncd.log",

        statusFile="/var/log/lsyncd/lsyncd.status",

        statusInterval=1,

        nodaemon=true,

        insist=true

}

sync {

        default.rsync,

        source  ="/email/repl",

        target  ="admin@172.29.77.32:/email/repl",

        delete  ='running',

        rsync   ={

                rsh ="/usr/bin/ssh -l admin -i /home/admin/.ssh/id_rsa",

                update  =true,

                _extra  ={"--temp-dir=/email/rsync_temp"},

                times   =true,

                archive =true,

                compress=true,

                perms   =true,

                acls    =true,

                owner   =true

        }

}

Step 6: Modify watches

$ sudo sysctl fs.inotify

fs.inotify.max_queued_events = 16384

fs.inotify.max_user_instances = 128

fs.inotify.max_user_watches = 500000

$ sudo sysctl -w fs.inotify.max_user_watches=50000  // change max user watch

$ /etc/sysctl.conf  // you can check again in this file

Step 6: Start/Stop service

$ service lsyncd start

$ service lsyncd stop

$  service lsyncd status

$ tail -f /var/log/lsyncd/lsyncd.log 

$ tail -f /var/log/lsyncd/lsyncd.status

Step 7: Repeat from step #1 on server #2.

8. Setting the banner in Centos/Relhat

$ vi /etc/motd

enter your banner in here

when you login to servers by ssh, this will be show

9. how to exit "telnet" in linux, windows

$ quit 

10. check open port without telnet command line.

$ nmap -p <port> <ip>

$ nc -zv <ip> <port>

// Check Port Open

$ nmap -sU -p U:<port> <IP or Host>

11. Change /etc/resolv.conf files

Stumbled on this problem on CentOS8.

The file may be marked immutable:

# lsattr /etc/resolv.conf

----i--------------- /etc/resolv.conf

Remove the immutable flag as follows:

# sudo chattr -i /etc/resolv.conf

# lsattr /etc/resolv.conf

-------------------- /etc/resolv.conf

Ref: https://askubuntu.com/questions/604760/can-not-edit-resolv-conf

12. How to change CentOS baseURL from mirror.centos.org to vault.centos.org

# cd /etc/yum.repos.d/

# sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*

# sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-*

13. Fix error install Virtualbox Addition in Centos

# yum groupinstall 'Development Tools'

14. ip route table (how to use route default ip when you have many interface network)

Ref:  https://www.cyberciti.biz/faq/howto-linux-configuring-default-route-with-ipcommand/

# ip route list   // show default ip routing table

default via 192.168.23.1 dev enp0s3 proto static metric 100

default via 192.168.8.1 dev enp0s8 proto static metric 101

192.168.8.0/24 dev enp0s8 proto kernel scope link src 192.168.8.254 metric 101

192.168.23.0/24 dev enp0s3 proto kernel scope link src 192.168.23.3 metric 100

192.168.122.0/24 dev virbr0 proto kernel scope link src 192.168.122.1 linkdown

#ip route delete default  // delete all default

# ip route add default via 192.168.8.1 dev enp0s8   // add defaut route ip

15. Stop nfslock, nfs in redhat 6.9

#/etc/init.d/nfslock stop //process is rpc.statd

#/etc/init.d/nfs stop

16. Error: Terminating sicnce out of inotify watches

Ref: https://pikedom.com/lsyncd-fails-with-out-of-inotify-watches-error/

$ cat /proc/sys/fs/inotify/max_user_watches

$ sysctl fs.inotify.max_user_watches=400000 // stop lsyncd before change

17. Turn off SELlinux

$vi /etc/selinux/config

$ SELINUX=enforcing => disabled

restart servers

18. Download RPM package with dependencies

$ yum install yum-plugin-downloadonly

$ yum install --downloadonly --downloaddir=<directory> <package-name>

19. Kill many process at one time in Linux 

$ pkill -9 crsctl.bin

20. Git command line

Ref: https://www.javatpoint.com/git-commands

$git clone <https_url>

$git add <files|folders

$git commit -m "comment"

$git push --all

$git pull

$git log

$git config --global

$git config -e //edit config

#Discard Local Changes

$ git reset --hard

#Discard Local Changes Specific File Name

$ git checkout <file_name>

21. Use Deploy Keys in Git

- Login to servers

- Make SSH key following the command below:

$ ssh-keygen -t ed25519 -C "USERNAME@EMAIL.com"

or

$ ssh-keygen -t rsa -b 4096 -C "USERNAME@EMAIL.com"

- Key will stored in /home/<user>/.ssh/

1. id_rsa

2. id_rsa.pub

$cat id_rsa.pub // copy and past to Deploy Keys in Git

- Clone project

$ git clone git@****

Note: if you use git clone with http/https, it will prompt enter Username/password.

22. Upgrade CentOS

$ dnf -y install centos-release-stream

$ dnf -y swap centos-{linux,stream}-repos

$ dnf -y distro-sync

23. Change CentOS to Remi Repository

$ yum install epel-release yum-utils -y

$ yum install http://rpms.remirepo.net/enterprise/remi-release-8.rpm

$ dnf module list php //list version of module

$ sudo dnf module enable php:remi-8.0 -y //enble module in redmi module

23. Mount to RHEL DVD to server

Ref: https://access.redhat.com/solutions/1355683

// Mount the RHEL installation ISO to a directory like /mnt/disc, e.g.:

# mkdir -p  /mnt/disc

# mount -o loop RHEL7.9.iso /mnt/disc

// If you use DVD media, you can mount like below.

# mkdir -p  /mnt/disc

#blkid // check disk mount

# mount /dev/sr0  /mnt/disc

// Copy the media.repo file from the root of the mounted directory to /etc/yum.repos.d/ and set the permissions to 0644 or another similar permissions set:

# cp /mnt/disc/media.repo /etc/yum.repos.d/rhel7dvd.repo

# chmod 644 /etc/yum.repos.d/rhel7dvd.repo

#Edit the new repo file, changing the gpgcheck=0 setting to 1 and adding the following 3 lines

vi /etc/yum.repos.d/rhel7dvd.repo

enabled=1

baseurl=file:///mnt/disc/

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release

To be:

[InstallMedia]

name=DVD for Red Hat Enterprise Linux 7.9 Server

mediaid=1359576196.686790

metadata_expire=-1

gpgcheck=1

cost=500

enabled=1

baseurl=file:///mnt/disc/

gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-redhat-release

//Clear the cache and check whether you can get the packages list from the DVD repo

# yum clean all

# yum repolist enabled

It should look like the following if no other repository is enabled.

To avoid any corruption its recommend to disable any non-RedHat repositories.

# yum repolist enabled

Loaded plugins: product-id, search-disabled-repos, subscription-manager

This system is not registered to Red Hat Subscription Management. You can use subscription-manager to register.

repo id                                                                                 repo name                                                                     

InstallMedia                                                                        Red Hat Enterprise Linux 7.9

repolist: 5,229

24. Change Rotate of Logfile (wtmp, btmp)

Ref: https://www.redhat.com/sysadmin/setting-logrotate

Change config in here:

$ /etc/logrotate.conf

25. Split Big File  by Git Bash

- Open Git Bash

$ cd <your_dir> // Aware path of Windows and Linux

$ split <file_name> -b 500m 

26. Edit Network Configuration in RockyOS 

$ nmtui

27. Compile C/C++ Code

-- Compile C

$ gcc -c -fPIC hello.c -o hello.o 

-- Compile to Shared Object (.so)

$ gcc hello.o -shared -o libhello.so

28. Netstat command in RockyOS

In RockyOS 9.5, default not install netstat command

$ dnf install net-tools

29. Install JDK 8 in CentOS/RockyOS

$ yum -y install java-1.8.0-openjdk

30. Install chkconfig in RockyOS

$ yum install chkcofig

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