Author Archives: curiouscat

Cleaning the Screen of Your Mac Air or Mac Powerbook

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My Mac Air screen can get dirty and I am a bit worried about cleaning it due to the warnings that using various cleaning products of methods can damage your screen. From what I have been able to determine the best strategy is just using water and a microfiber fabric. I use one microfiber which I get wet with water and clean the screen. Then I dry it off with another microfiber. Using chemicals can damage the screen. And using even paper towels can result in scratches on the screen.

I have had very good success with just water and the microfiber cloth. I imagine water and microfiber cloth is the best way to clean an iPad (when kids and cats play with it) but I haven’t tried it myself.

From Apple’s web site:

The materials used to make Apple products vary; in some cases each product may have specific cleaning requirements, which may vary by the part you are cleaning. Here are some tips that apply to all products to get you started:

  • Use only a soft, lint-free cloth. Abrasive cloths, towels, paper towels, and similar items may cause damage to the item.
  • Disconnect your Apple product from any external power sources.
  • Disconnect any external devices and other cabling from the product.
  • Keep liquids away from the product.
  • Don’t get moisture into any openings, and don’t use aerosol sprays, solvents, or abrasives.
  • Do not spray cleaners directly onto the item.

Disconnect the display from power, from its connection to a computer, and from any external devices. Next use the cloth that came with your display or another soft, dry cloth to wipe dust from the screen. If additional cleaning of the display panel or case is required use a soft, slightly damp, lint-free cloth. Avoid getting moisture in openings. Don’t use window cleaners, household cleaners, aerosol sprays, solvents, alcohol, ammonia, or abrasives to clean the display.

Warning: Don’t clean the screen with a cleaner containing alcohol or acetone. Use a cleaner intended for use with a screen or display. Never spray cleaner directly on the screen. It may drip inside the display and cause damage.

When cleaning the outside of your Mac mini, first shut down your Mac mini and unplug the power cord along with other connected devices. Then use a damp, soft, lint-free cloth to clean the computer’s exterior. Avoid getting moisture in any openings. Do not spray liquid directly on the computer. Do not use aerosol sprays, solvents, or abrasives that might damage the finish.

Related: Curious Cat Gadgets Blog – Amazon Kindle DX (the big one) – Amazon’s Android Tablet, Kindle Fire, is Very Attractively Priced

Checklist: Setting Up a New Domain on VPS

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Two great hosts for Ruby on Rails are Slicehost and Linode. With these hosts you fully manager your virtual private server, installing the operating system, modifying apache (on Ning…), etc.. I use Ubuntu as the operating system and Apache as the web server.

If you are moving a domain from elsewhere it can be wise to reduce the TTL time to say 5 minutes a few days before you make the switch. This is make the change propagate across the internet more quickly.

  1. And DNS entry on your profile (login to your, for example, Linode account)
  2. Add a new file for /etc/apache2/sites-available/your_site_name.com

    3. [bash]sudo nano /etc/apache2/sites-available/example.com[/bash]

    The file would look something like this: with your ip in place of 8.8.8.8

    ServerAdmin is the address Apache will use to send error messages to. Using gmail and the + option lets you use one gmail account and just use rules to filter all your sites.

  3. create the directories needed on your server
  4. enable the site (for apache)

    [bash]sudo a2ensite example.com[/bash]

    5. You should see the file you created /etc/apache2/sites-available/example.com now also at /etc/apache2/sites-enabled/example.com

  5. Test the site out to make sure the setup is working properly. Create a index.html page and just verify the page is displayed. Change your local hosts file to point to your server IP address for the new domain you created. If not, take steps to get this to work, before continuing with the rest of the checklist.
  6. copy over the site – if you are moving the site from elsewhere
  7. remember to move the database over, if the site relies on a database
  8. restart apache
    [bash]sudo /etc/init.d/apache2 restart[/bash]

    9. You can test the site out, before updating the DNS, by changing your local hosts file to point to your server IP address for the new domain you created.

    Resources: Install the Apache 2 Web Server on Ubuntu 10.04 LTS (Lucid)

Replace Text in Files Recursively (Linux)

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To replace text in all the files in a directory, recursively, you can use grep.

sudo grep -rl texttoreplace /somedirectory/ | sudo xargs sed -i ‘s/repacethis/withthis/g’

[bash]sudo grep -rl oldtext /example/directory/ | sudo xargs sed -i ‘s/oldtext/newtext/g'[/bash]

The g on the end, lets it replace all the instances of the text in each file.

More ubuntu and linux tips and code samples

WordPress error: Image could not be processed. Please go back and try again.

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If you get an error saying

Image could not be processed. Please go back and try again.

when you try to put a new custom header image for WordPress theme 2010 on a server using Ubuntu the following may help:

[bash]apt-get install php5-gd[/bash]
once it installs then
[code]invoke-rc.d apache2 restart[/code]

This will provide php the ability to manipulate images that WordPress is trying to use.

Using Git and Github

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To download a repository hosted on github
[bash]git clone [email protected]:account_name/repo_name.git[/bash]

The [email protected]:account_name/repo_name.git will be shown on the home page for the repository on github.com

To update your code from the master branch of the repo:

[bash]git pull origin master[/bash]

To commit the changes you have made locally:

[bash]git commit -m "the message explaining what these code changes did"[/bash]

To send your changes to the repo at github:
[bash]git push origin master[/bash]

Replacing a Host Key

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Host keys are used to security log into remote servers (such as Virtual Private Servers – VPS). With Ubuntu if you are using host keys to sign into servers securely and have asked for strict checking, if you make a change (such as rebuilding your VPS) the host key will change and you cannot login and will get a message like:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that the RSA host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
…
Please contact your system administrator.
…
RSA host key for 128.0.0.128 has changed and you have requested strict checking.
Host key verification failed.

if that happens you need to remove your local host key. Then you can sign back in and you will be able to save a new copy of the host key. If you don’t know why the key has changed you should figure that out first as it maybe be an indication of an important security problem. To remove you local key, you can use ssh-keygen -R [ip address of server with the bad key] for example: ssh-keygen -R 128.0.0.128

Then when you try to sign in you will get

The authenticity of host '128.0.0.128 (128.0.0.128)' can't be established.

RSA key fingerprint is ed:...:ea.
Are you sure you want to continue connecting (yes/no)?

And if you know why (such as you made changes to the server) you can say yes and connect and save the new known host key.

Managing Users in Ubuntu

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add a user: sudo adduser [newuser]
sudo adduser john
then give password, and setup home folder,when prompted

edit the list of super users
visudo

list users
cat /etc/passwd

change password of a user – sudo passwd [username]
sudo passwd john
to change your password you can just use sudo passwd.

delete user

sudo deluser username

This will not delete the home folder (and subfolder) those must be dealt with separately. A new user created with the old user name would have access to those files.

temporarily lock a user account – Simply locking a user account will not prevent a user from logging into your server remotely if they have previously set up RSA public key authentication.

sudo passwd -l username

To unlock the account

sudo passwd -u username

Groups

Groups are used in to control permissions (see file permissions)
add user to a group
usermod -G [group-name] [username]
usermod -G basketball john
using the -G switch ads the group as a supplemental group. Using -g would make the group that users primary group.

see what groups a user is in
id john

add a new group: groupadd [new_group_name]
groupadd ruby_developers

Basic MySQL Performance Monitoring

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Basic MySQL Performance Monitoring

regular Ubuntu cli tools

  • mysqladmin status – mysqladmin status -uroot -p

MySQL command line interface tools

[bash]mysql -uroot -p[/bash] to open the command line.

[sql]SHOW GLOBAL STATUS;
SHOW ENGINE INNODB STATUS;
SHOW PROCESSLIST;
SHOW GLOBAL STATUS LIKE ‘Qcache_%’;[/sql]

cli tools

  • mytop – top for MySQL. Install using: sudo apt-get install mytop (assuming Ubuntu operating system). There is a very useful setting file that can be used to set parameters instead of having to include them in each command. Save the file as ~/.mytop.
  • MySQLTuner – provides suggestions on performance improvements and my.cnf settings by analyzing data on your mysql database server.

Setting considerations

  • If Open_tables (SHOW GLOBAL STATUS will show this) is equal to your [bash]table_cache size[/bash] (set in /etc/mysql/my.cnf) that means it is being capped by your setting. The more MySQL has to read the table from disk the more IO and slower response, so if you have available RAM increasing the table_cache size may well make a big difference.
  • Key_reads/Key_read_request ratio should normally be < 0.01 (per MySQL manual, this means that nearly all key requests are taken from RAM). You can get both values using SHOW GLOBAL STATUS and then calculate the ratio. If the ratio is too high, consider increasing the key_buffer (in /etc/mysql/my.sql).
  • key_writes/key_writes_request should normally be near 1 (per MySQL manual)