My notebook


September 2013

send test mail linux-sendmail


echo "This is a test" | /usr/sbin/sendmail


Checking your Ubuntu Version

command lsb_release -a

TimeZone Linux

ln -sf /usr/share/zoneinfo/Europe/Athens /etc/localtime
 date -s 14:47:00




mongodb new db and user

1. Show all database

Issue “show dbs” to display all available databases.

MongoDB shell version: 1.8.1
connecting to: test
> show dbs
admin   0.03125GB
local   (empty)

Currently, only two databases are available – “admin” and “local“.

2. Define a database name

Issue “use new-databasename” to switch from default database to define database (even non-exists database name will work). However, MangoDB doesn’t create any database yet, until you save something inside.

> use mkyongdb
switched to db mkyongdb
> show dbs
admin   0.03125GB
local   (empty)

P.S Database “mkyongdb” is not created yet.

3. Save It

Define a collection named “users“, and save a dummy document(value) inside.

> {username:"mkyong"} )
> db.users.find()
{ "_id" : ObjectId("4dbac7bfea37068bd0987573"), "username" : "mkyong" }
> show dbs
admin   0.03125GB
local   (empty)
mkyongdb        0.03125GB

This says, “save this document (value) ‘{username:"mkyong"}‘ into the ‘user’ collection”. When this “save” operation is performed, MangoDB will create the “user” collection, and “mkyongdb” database automatically.


  1. SQL to MongoDB Mapping chart

Ubuntu iSCSI Initiator

iSCSI Initiator

iSCSI (Internet Small Computer System Interface) is a protocol that allows SCSI commands to be transmitted over a network. Typically iSCSI is implemented in a SAN (Storage Area Network) to allow servers to access a large store of hard drive space. The iSCSI protocol refers to clients as initiators and iSCSI servers as targets.

Ubuntu Server can be configured as both an iSCSI iniator and a target. This guide provides commands and configuration options to setup an iSCSI initiator. It is assumed that you already have an iSCSI target on your local network and have the appropriate rights to connect to it. The instructions for setting up a target vary greatly between hardware providers, so consult your vendor documentation to configure your specific iSCSI target.

iSCSI Initiator Install

To configure Ubuntu Server as an iSCSI initiator install the open-iscsi package. In a terminal enter:

sudo apt-get install open-iscsi

iSCSI Initiator Configuration

Once the open-iscsi package is installed, edit /etc/iscsi/iscsid.conf changing the following:

node.startup = automatic

You can check which targets are available by using the iscsiadm utility. Enter the following in a terminal:

sudo iscsiadm -m discovery -t st -p
  • -m: determines the mode that iscsiadm executes in.
  • -t: specifies the type of discovery.
  • -p: option indicates the target IP address.

Change example to the target IP address on your network.

If the target is available you should see output similar to the following:,1

The iqn number and IP address above will vary depending on your hardware.

You should now be able to connect to the iSCSI target, and depending on your target setup you may have to enter user credentials. Login to the iSCSI node:

sudo iscsiadm -m node --login

Check to make sure that the new disk has been detected using dmesg:

dmesg | grep sd

[    4.322384] sd 2:0:0:0: Attached scsi generic sg1 type 0
[    4.322797] sd 2:0:0:0: [sda] 41943040 512-byte logical blocks: (21.4 GB/20.0 GiB)
[    4.322843] sd 2:0:0:0: [sda] Write Protect is off
[    4.322846] sd 2:0:0:0: [sda] Mode Sense: 03 00 00 00
[    4.322896] sd 2:0:0:0: [sda] Cache data unavailable
[    4.322899] sd 2:0:0:0: [sda] Assuming drive cache: write through
[    4.323230] sd 2:0:0:0: [sda] Cache data unavailable
[    4.323233] sd 2:0:0:0: [sda] Assuming drive cache: write through
[    4.325312]  sda: sda1 sda2 < sda5 >
[    4.325729] sd 2:0:0:0: [sda] Cache data unavailable
[    4.325732] sd 2:0:0:0: [sda] Assuming drive cache: write through
[    4.325735] sd 2:0:0:0: [sda] Attached SCSI disk
[ 2486.941805] sd 4:0:0:3: Attached scsi generic sg3 type 0
[ 2486.952093] sd 4:0:0:3: [sdb] 1126400000 512-byte logical blocks: (576 GB/537 GiB)
[ 2486.954195] sd 4:0:0:3: [sdb] Write Protect is off
[ 2486.954200] sd 4:0:0:3: [sdb] Mode Sense: 8f 00 00 08
[ 2486.954692] sd 4:0:0:3: [sdb] Write cache: disabled, read cache: enabled, doesn't
 support DPO or FUA
[ 2486.960577]  sdb: sdb1
[ 2486.964862] sd 4:0:0:3: [sdb] Attached SCSI disk

In the output above sdb is the new iSCSI disk. Remember this is just an example; the output you see on your screen will vary.

Next, create a partition, format the file system, and mount the new iSCSI disk. In a terminal enter:

sudo fdisk /dev/sdb

The above commands are from inside the fdisk utility; see man fdisk for more detailed instructions. Also, the cfdisk utility is sometimes more user friendly.

Now format the file system and mount it to /srv as an example:

sudo mkfs.ext4 /dev/sdb1
sudo mount /dev/sdb1 /srv

Finally, add an entry to /etc/fstab to mount the iSCSI drive during boot:

/dev/sdb1       /srv        ext4    defaults,auto,_netdev 0 0

It is a good idea to make sure everything is working as expected by rebooting the server.

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