using Elementary OS with logitech keyboard and mouse

    1. install blueman in AppCenter
    2. For mouse, add “<device oui=”AB:CD:EF:” type=”mouse” name=”Microsoft Mobile Mouse” pin=”0000″/>”to /usr/share/gnome-bluetooth/pin-code-database.xml which is the MAC of mouse is “AB:CD:EF:GH:IJ:KL”.
    3. sudo service bluetooth restart

Optimize Linux running on USB

1. Use memory instead of disk

This tip can also lead to data loss. If you do it, you will have to always shut down your computer properly from now on, because unexpected power failures will lead to data loss.

Linux usually ensures that all changes are written to disk every few seconds. Since disk writes are so slow, you can change your system to keep things in memory longer. All changes will be written to memory, and the excruciatingly slow writes to happen in the background while you continue working. This has an instant, noticeable effect, but it can lead to data loss.

Add these lines to /etc/sysctl.conf, and reboot.

vm.swappiness = 0
vm.dirty_background_ratio = 20
vm.dirty_expire_centisecs = 0
vm.dirty_ratio = 80
vm.dirty_writeback_centisecs = 0

The problem: using this tip means that your system stops writing changes to disk until you shut down or type “sync” at a command line. If your system loses power unexpectedly, you will get bad blocks. I did. You can limit the amount of data loss in the event of a power failure to one minute by setting vm.dirty_writeback_centisecs = 6000.

A side effect is that shutting down your computer will may take several minutes where it appears to be doing nothing. Don’t cut the power until it’s done, because it is busy writing all those changes to disk.



2. Using ramdisks (tmpfs) for temporary files

It is quite easy to move directories where temporary files are stored (primarily /tmp) to a ramdisk, and it can make a significant difference in performance. The following lines added to /etc/fstab cause /tmp and /var/tmp to be stored in ramdisks:

    tmpfs    /tmp       tmpfs    defaults    0 0
    tmpfs    /var/tmp   tmpfs    defaults    0 0


3. noatime

By default, the ext3/ext4 filesystem updates the access time attribute on a file system object whenever it’s read. This results in even the most trivial file reads result in a write operation. Keeping the access time updated is only important for some very specific tasks which we’re not likely to come across.

Open /etc/fstab and for each mount that is on your USB storage, add a noatime parameter. For example:

/dev/mapper/vg_root-lv_root /               ext4    errors=remount-ro 0       1


/dev/mapper/vg_root-lv_root /               ext4    noatime,errors=remount-ro 0       1



4. IO Scheduler



The default IO Scheduler used by Ubuntu is cfq. Changing this to deadline will see improved performance during times where multiple processes are trying to read/write to disk. You can change the default scheduler for all devices by adding a kernel parameter into grub.

Open /etc/default/grub and find the following line:


Add the elevator option:


Or, if you added the verbose option earlier:

GRUB_CMDLINE_LINUX="elevator=deadline verbose"

Now, use update-grub to apply your changed setting to grub.cfg and restart for the change to take effect.

sudo update-grub
sudo reboot

To see that your new scheduler has taken effect you can query which scheduler is currently in use for a specific device with this command:

cat /sys/block/<device>/queue/scheduler

delete all email in exim

To print a list of the messages in the queue, enter:

exim -bp

To remove a message from the queue, enter:

exim -Mrm {message-id}

To remove all messages from the queue, enter:

exim -bp | awk '/^ *[0-9]+[mhd]/{print "exim -Mrm " $3}' | bash

To remove all messages from the queue with specific word, enter:

exim -bp|grep “username”| awk {‘print $3’}| xargs exim -Mrm

ssh without password using certificate

Generate a pass-phraseless SSH key:

$ ssh-keygen -t rsa -b 2048
Generating public/private rsa key pair.
Enter file in which to save the key (/home/username/.ssh/id_rsa): 
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/username/.ssh/id_rsa.
Your public key has been saved in /home/username/.ssh/

Copy your keys to the target server:

$ ssh-copy-id id@server
id@server's password: 

Now try logging into the machine, with ssh 'id@server', and check in:


to make sure we haven’t added extra keys that you weren’t expecting.

Finally check logging in…

$ ssh id@server


You may also want to look into using ssh-agent if you want to try keeping your keys protected with a pass-phrase.


Usage of RAMFS and TMPFS


# mkdir -p /mnt/tmp

# mount -t tmpfs -o size=20m tmpfs /mnt/tmp


# mkdir -p /mnt/ram

# mount -t ramfs -o size=20m ramfs /mnt/ram


Install PPTP server on Centos 6

1. yum install ppp -y

2 wget


3. rpm -Uhv ppp-2.4.5-33.0.rhel6.x86_64.rpm


4. vi /etc/pptpd.conf

– add “localip”

– add “remoteip”


4. vi /etc/ppp/options.pptpd

– add “ms-dns”


5. vi /etc/ppp/chap-secrets

– add “username” pptpd “password” *


6. vi /etc/sysctl.conf

– change “net.ipv4.ip_forward = 1”


7. sysctl -p

– to take effect


8. iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

service iptables save
service iptables restart

9. chkconfig pptpd on
init 6

Disable sending email and set timezone in crontab

1. crontab -e

2. add the following two lines:


Change timezone in centos permanetly


Please identify a location so that time zone rules can be set correctly.
Please select a continent or ocean.
 1) Africa
 2) Americas
 3) Antarctica
 4) Arctic Ocean
 5) Asia
 6) Atlantic Ocean
 7) Australia
 8) Europe
 9) Indian Ocean
10) Pacific Ocean
11) none - I want to specify the time zone using the Posix TZ format.
#? 5
Please select a country.
 1) Afghanistan 18) Israel 35) Palestine
 2) Armenia 19) Japan 36) Philippines
 3) Azerbaijan 20) Jordan 37) Qatar
 4) Bahrain 21) Kazakhstan 38) Russia
 5) Bangladesh 22) Korea (North) 39) Saudi Arabia
 6) Bhutan 23) Korea (South) 40) Singapore
 7) Brunei 24) Kuwait 41) Sri Lanka
 8) Cambodia 25) Kyrgyzstan 42) Syria
 9) China 26) Laos 43) Taiwan
10) Cyprus 27) Lebanon 44) Tajikistan
11) East Timor 28) Macau 45) Thailand
12) Georgia 29) Malaysia 46) Turkmenistan
13) Hong Kong 30) Mongolia 47) United Arab Emirates
14) India 31) Myanmar (Burma) 48) Uzbekistan
15) Indonesia 32) Nepal 49) Vietnam
16) Iran 33) Oman 50) Yemen
17) Iraq 34) Pakistan
#? 13

The following information has been given:

 Hong Kong

Therefore TZ='Asia/Hong_Kong' will be used.
Local time is now: Fri Oct 31 17:10:49 HKT 2014.
Universal Time is now: Fri Oct 31 09:10:49 UTC 2014.
Is the above information OK?
1) Yes
2) No
#? 1

You can make this change permanent for yourself by appending the line
 TZ='Asia/Hong_Kong'; export TZ
to the file '.profile' in your home directory; then log out and log in again.

Here is that TZ value again, this time on standard output so that you
can use the /usr/bin/tzselect command in shell scripts:

Read the rest of this entry

Change time zone in linux

e.g. cp /usr/share/zoneinfo/Asia/Hong_Kong /etc/localtime

Fixing “error while loading shared libraries”

error while loading shared libraries: cannot open shared object file: No such file or directory

1. edit “/etc/” or add “/etc/”

2. run “ldconfig”


If you have no access right to modify /etc/, you may run command “export”.

1. export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib

or put it in ~/.bashrc


For ./configure, you may add option like “./configure –prefix=$HOME/yourlib