How to see both Oracle VM Virtual Box 32bit/64bit options?

e.g. If there are only 32 bit Linux distro in the OS list, you should enable virtualization support in BIOS

* BIOS -> CMOS Setup Utility -> Virtualization Technology -> [Enabled]

How to install/use rlwrap?

Installation
e.g.
# rpm -ivh rlwrap-0.42-1.el6.x86_64.rpm  

Usage
Add alias to .bash_profile file (or .bashrc, etc.)
alias sqlplus='rlwrap sqlplus'

$ sqlplus hr/hr
SQL> select 1 from dual;

Key trick is that while in sqlplus command line, use up or down keys to navigate sql history.

How to export/set ORACLE_SID?

Linux
export variable
$ export ORACLE_SID=XE

check variable
$ echo $ORACLE_SID

Windows
set variable
> set ORACLE_SID=XE

check variable
> echo %ORACLE_SID%

How to start/stop/configure samba service?

If you need, uninstall old samba
# yum erase samba samba-common samba-client 

install tool
# yum install samba4

or upgrade tool
# yum upgrade samba4

configure settings
# vi /etc/samba/smb.conf

e.g.
[share_folder]
 comment = Share Folder
 path = /share_folder
 create mode = 0777
 readonly = no
 browseable = yes
 public = yes

Check configuration of samba
# testparm

check samba service and version
# smbstatus

startup options
# chkconfig smb on
# service smb start

connect to share folder
e.g. 
RUN> \\10.10.10.10\share_folder

remove startup option and stop vnc
# chkconfig smb off
# service smb stop

How to find and investigate a process in top command output?

Total connection by machine:
SQL> select machine, count(*) from v$session group by machine order by 2;

Total number of database user who connected:
SQL> select count(1) "NO. Of DB Users Connected", to_char(sysdate,'DD-MON-YYYY:HH24:MI:SS') sys_time from v$session where username is NOT NULL;

Define a process(pid) from top:
17831 ora11g .. ora_m000_ALPERDB  

Get detailed info from database:
SQL> select * from v$process where spid = 17831;
-> M000 means MMON Slave Process
-> e.g. pid -> 16, sid -> 555

SQL>
SELECT s.username, osuser, process machine, p.terminal, type, sid, s.serial#, s.program
FROM v$session s, v$process p
WHERE p.pid = 16
AND s.paddr = p.addr;

SQL> select * from gv$session where sid = 555;

Get sql from below code related to example process if it has SQL:
SQL>
SELECT 
      s.sid
     ,s.CLIENT_INFO
     ,s.MACHINE
     ,s.PROGRAM
     ,s.TYPE
     ,s.logon_time
     ,s.osuser
     ,sq.sorts
     ,sq.DISK_READS
     ,sq.BUFFER_GETS
     ,sq.ROWS_PROCESSED
     ,sq.SQLTYPE
     ,sq.SQL_TEXT
 FROM gv$session s    
    , gv$sql sq
WHERE s.SQL_HASH_VALUE = sq.HASH_VALUE
  --AND s.inst_id= 1
  AND s.sid = 555 
  AND sq.inst_id= s.inst_id

How to kill tty/pts terminal?

Check first your connection to prevent killing yourself
$ tty

Kill other connection
$ skill -9 pts/1
or
# skill -KILL pts/1

Info
Teletypewriter - TTY
pseudo terminal slave - PTS

watch examples

$ watch free -m
-> Display free command output in every two seconds 

$ watch -n 5 free -m
-> Display free command output in every five seconds  (with -n parameter)

screen commands

screen	->	start the tool
ctrl + a	->	control screen
ctrl + a + c	->	define new screen
ctrl + a + n	->	switch between screen
ctrl + a + d	->	detach screen
ctrl + a + n	->	switch between screen
screen -r	->	reattach screen
screen -r [xyz]	->	reattach specific screen
screen -ls	->	list the screens
ctrl + a + x	->	lock screen
ctrl + a + k	->	kill screen

How to start/stop/configure vnc service?

install tool:
yum install tigervnc-server

configure settings:
vi /etc/sysconfig/vncservers

e.g.
VNCSERVERS="2:root 3:oracle 4:alper"
VNCSERVERARGS[2]="-geometry 1280x1024 -nolisten tcp -localhost"
VNCSERVERARGS[3]="-geometry 1280x1024"
VNCSERVERARGS[4]="-geometry 1280x1024"

change password of vnc
$ whoami
-> alper
$ vncpasswd
-> alper123

startup options:
# chkconfig vncserver on
# service vncserver start

login to system with windows vncviewer tool
e.g. 10.10.10.10.:4

remove startup option and stop vnc:
# chkconfig vncserver off
# service vncserver stop

Info: Virtual Network Computing (VNC) is a graphical desktop sharing system

output columns of top command

Columns:
PID     -> Process ID   
USER    -> Process owner   
PR      -> Priority of process 
NI      -> Nice value of process  
VIRT    -> Virtual memory using by process
RES     -> Physical memory using by process  
SHR     -> Shared memory of process
S       -> Status of process (R-> Running, S-> Sleeep, Z-> Zombie, D-> Uninterruptible Sleep, T-> Traced or Stopped)
%CPU    -> CPU usage of process (as percent)
%MEM    -> RAM usage of process (as percent)
TIME+   -> Total time activity for process 
COMMAND -> indicates of process name

Info:
PR = 20 + NI (PR-RT -> Real Time system process)
PR -> (0 is the highest priority and 39 is the lowest priority)
NI; Nice value (−20 is the highest priority and 19 is the lowest priority)
VIRT; Virtual image (kb) -> disk storage + RAM
RES;  Resident size (kb) -> RAM (non-swapped physical memory)

SHR; Shared mem size (kb)-> RAM (shared memory)

Extras:

Set priority of a process before start;
nice -n 19 tar cvzf archive.tgz largefile

Change priority of a process;
renice -n -20 -p process_id

Ref:

https://ozsoyler.blogspot.com/2017/01/how-to-see-load-averages-on-linux.html

https://ozsoyler.blogspot.com/2014/10/how-to-see-process-that-used-memory-or.html

How to learn linux distribution?

See all info
$ lsb_release -a
a - all

See only distro info
$ lsb_release -i -r
i - id, r - release

How to define timeout setting for auto logout users in linux system?

Configure it
# vi /etc/profile
add following parameter at the end of the profile file:
# this parameter runs as seconds and affects all users. e.g. 3600; one hour
TMOUT=3600

Check it
$ echo $TMOUT

Test result
$ timed out waiting for input: auto-logout

How to see file extensions in current directory?

$ find . -type f | perl -ne 'print $1 if m/\.([^.\/]+)$/' | sort -u

bash: /bin/rm: Argument list too long error Solution

Command
rm -f *

Error 
bash: /bin/rm: Argument list too long error

Solution
Use following command line
$ find . -maxdepth 1 -name "*.txt" -print0 | xargs -0 rm

How to gain root access without entering root password on Linux?

Add the line into the end of /etc/sudoers file as root
vi /etc/sudoers
alper ALL=(ALL) NOPASSWD: ALL

Test your connection
ssh alper@10.10.10.10
$ sudo su -
(executes all /etc/profile, .profile and .bashrc files)
# ifconfig
$ sudo su
(executes only .bashrc file)
# ifconfig

References:

• http://askubuntu.com/questions/168461/how-do-i-sudo-without-having-to-enter-my-password
• http://askubuntu.com/questions/376199/sudo-su-vs-sudo-i-vs-sudo-bin-bash-when-does-it-matter-which-is-used

How to mount devices at startup by /etc/fstab and /etc/rc.d/rc.local for RedHat distro?

-> define uuid for connected devices
ls -l /dev/disk/by-uuid/

-> add related info into /etc/fstab
e.g.
UUID=1111 /alper/sdb1 ext3 defaults,nobootwait 0 0

-> if this cannot mount the path, you may deal with network issues. To fix it, you should add following command into /etc/rc.d/rc.local config file.
mount -a

extra info: -> If you use 1 1 instead of 0 0 for priority option, you may encounter "maintenance mode" problem. To fix it, run this command
mount -o rw, remount /

What are the differences between SAN, NAS and DAS?

SAN(Storage Area Network);
SAN is used for applications to access BLOCK storage over an optical FC network using the SCSI protocol. (e.g. FibreChannel, iSCSI, FoE)

NAS(Network Attached System);
NAS is used for access to FILE storage over TCP/IP on an Ethernet network using either the CIFS (for Windows) or NFS (for Unix) protocol. (e.g. NFS, CIFS, AFS)

DAS(Direct Attached Storage);
DAS is a block device from a disk which is physically [directly] attached to the host machine. (e.g. IDE, SCSI, SATA)

Source: 
http://searchstorage.techtarget.com/tip/SAN-vs-NAS-Which-do-you-need
http://serverfault.com/questions/81723/what-is-the-difference-between-san-nas-and-das

How to use "wall" and "write" commands?

$ wall
-> press "ctrl+d" to send the message

$ echo "Hello World!" | wall

$ sudo wall message.txt
-> send content of message.txt file

$ write alper
-> press "crlt+c" to quit

$ write alper pts/0
-> run "who" to see terminal names like pts/0

What are hard link and soft link in Linux?

Hard link; it may used for backup (replica) purpose. (seems to RAID 1)
e.g.
$ ln original_file.txt hd_file.txt
$ ln original_file2.txt /home/alper/hd_file2.txt

Soft link; it may used as shortcut like in Windows OS.
e.g.
$ ln -s original_file soft_file.txt
$ ln -s original_file /home/alper/soft_file2.txt

to list the links;
$ ls -l 
(l; list)
or 
$ ls -i
(i; inode number)
then find the files by same inode number
$ find . -inum <inode_number>

e.g.
$ find . -inum 10000

tcpdump examples

$ tcpdump -D
-> Display all available interfaces for tcpdump

$ tcpdump -i 1 | more
-> Listen (interface 1) with more command

$ tcpdump -i any
-> Listen all actived interfaces

$ tcpdump -i eth1
-> Listen eth1 interface

$ tcpdump -i eth0 port 22
-> Listen eth0 interface on port 22

$ tcpdump -i eth2 port 23 -w example.pcap
-> Listen eth2 interface on port 23 and save as desired file name