Tech Trivia

Recursively set permissions on directories

Brad — Sun, 28 Feb 2010 04:43:57 +0000

The following command will execute a command on a directory and all its sub-directories:

$ find plugins -type d -exec chmod g+w {} \;

In this case, we’re changing the group permissions on the plugins directory and all sub-directories.

Update your SELinux policy

Brad — Mon, 22 Feb 2010 14:15:02 +0000

If you’ve recently installed an application or applied some patches that begin to generate a lot of SELinux audit entries, you can update your local policy to accommodate your recent changes. Please note that this isn’t a substitute for ensuring your files have the appropriate selinux roles and types.

# grep avc /var/log/audit/audit.log | audit2allow -M mypol
# semodule -i mypol.pp

Multiple select list in rails

Brad — Wed, 28 Oct 2009 13:58:21 +0000

I have recently been tinkering with ruby on rails again and was banging my head against the wall with a multiple select list for a “has and belongs to many” relationship between models. After a lot of Googling and experimenting I finally got it running. The resulting code is deceptively simple!

This example illustrates the relationship between users and their roles. Each user can have multiple roles, and each role can have multiple users.

The User Model

# models/admin/user.rb
class Admin::User < ActiveRecord::Base
  has_one :division, :class_name => "Admin::Division"
  has_and_belongs_to_many :role, :class_name => "Admin::Role"
  validates_presence_of :first_name, :last_name, :user_name, :password
end

The Role Model

# models/admin/role.rb
class Admin::Role < ActiveRecord::Base
  has_and_belongs_to_many :user, :class_name => "Admin::User"
  validates_presence_of :role
end

The User View (partial)

# views/admin/users/_form.html.erb
<% form_for(@user) do |f| %>
  <%= f.error_messages %>
  
    <%= f.label :first_name %>

    <%= f.text_field :first_name %>
  
  
    <%= f.label :last_name %>

    <%= f.text_field :last_name %>
  
  
    <%= f.label :user_name %>

    <%= f.text_field :user_name %>
  
  
    <%= f.label :password %>

    <%= f.password_field :password %>
  
  
    <%= f.label :role %>

	<%= collection_select(:admin_user, :role_ids, @roles, :id, :role,
		{ :selected => @user.role_ids },
		{:multiple => true, :role => 'admin_user[role_ids][]' }) %>
  
  
    <%= f.submit 'Save' %>
  
<% end %>

The User Controller

# excerpt from controllers/admin/users_controller.rb
# This is just the update method
#
  def update
    # if the multiple select list is empty,
    # make sure the role_ids array exists
    params[:admin_user][:role_ids] ||= []

    respond_to do |format|
      if @user.update_attributes(params[:admin_user])
        flash[:notice] = 'User was successfully updated.'
        format.html { redirect_to(@user) }
        format.xml  { head :ok }
      else
        format.html { render :action => "edit" }
        format.xml  { render :xml => @user.errors, :status => :unprocessable_entity }
      end
    end
  end

Display Comma Delimited Roles

Once I got the select list working, I wanted a simple way to display a comma delimited list of a user’s roles. I messed about with looping over all roles and just displaying the ones that matched the appropriate values, but it turns out there is a much simpler and elegant way to do this. The following snippet of code leverages the HMABT mapping between the user model and role model to access all the role names associated with a particular user. It then joins them all together delimiting the values with commas.

# excerpt from views/admin/users/index.html.erb
#
<%=h user.role.map(&:role).join(', ') %>

The Admin Name Space

Please note that my example is complicated a little by the fact that the models, views and controllers are all within a “/admin” name space. Here’s an excerpt from my routes.rb file which shows the necessary code to make it work.

# excerpt from config/routes.rb
map.namespace :admin do |admin|
    admin.resources :roles, :users
  end

Additional Resources

The following resources were quite helpful to me in getting this working:

Update text in multiple files in Linux

Brad — Thu, 25 Jun 2009 21:34:09 +0000

I recently came across a command that allows me to search and replace a string of text in all files within a directory structure. This is very useful for updating static information in HTML files and the like.

find ./ -iname \*.htm\* -exec sed -i 's/www\.example\.com/www2\.example\.com/g' {} \;

Note that you must denote special characters with the backslash character (e.g., the dots in www.example.com).

Find world writable directories

Brad — Tue, 12 May 2009 18:47:56 +0000

I recently found myself needing to locate all world writable files in a directory tree and came across the following useful command:

find /var/ -type d -perm -o+w -exec ls -ld {} \;

The syntax may vary depending on the flavour of Unix/Linux you’re using, but this worked on Ubuntu for me.

Create an ISO image from a DVD in Mac OS X

Brad — Sun, 10 May 2009 00:26:12 +0000

1. Insert CD/DVD

2. Open a terminal window and use the following command to determine your DVD device:
$ drutil status Vendor Product Rev HL-DT-ST DVDRW GSA-S10N AP09
Type: DVD-ROM Name: /dev/disk1

3. Umount the disk with the following command:
$ diskutil unmountDisk /dev/disk1 Disk /dev/disk1 unmounted

4. Use the dd command to create the ISO file:
$ dd if=/dev/disk1 of=file.iso bs=2048

5. Mount the ISO file to verify that it is readable:
$ hdid file.iso

If you’re looking for a utility to allow you to read ISO files on the Windows platform, try Virtual Clone drive from SlySoft.

Using rsync over ssh

Brad — Fri, 08 May 2009 00:58:46 +0000

You can securely synchronize directory structures between two Linux computers by using the following command:

rsync -ave ssh --delete myhost.example.com:Documents/ Documents/

The −−delete option ensures that any files or directories that no longer exist on the source are also removed from the destination.

Reset TYPO3 password

Brad — Mon, 13 Apr 2009 14:06:27 +0000

If you forget your typo3 password, you can reset it using the mysql command line tool as follows:

mysql> use typo3; mysql> update be_users set password='bacb98acf97e0b6112b1d1b650b84971' where username = 'admin';

This resets the admin user password to “joh316″.

Date format

Brad — Wed, 30 Apr 2008 01:57:34 +0000

Format a date in ruby using the string from time method (strftime).

Format options:
  %a - The abbreviated weekday name (``Sun'')
  %A - The  full  weekday  name (``Sunday'')
  %b - The abbreviated month name (``Jan'')
  %B - The  full  month  name (``January'')
  %c - The preferred local date and time representation
  %d - Day of the month (01..31)
  %H - Hour of the day, 24-hour clock (00..23)
  %I - Hour of the day, 12-hour clock (01..12)
  %j - Day of the year (001..366)
  %m - Month of the year (01..12)
  %M - Minute of the hour (00..59)
  %p - Meridian indicator (``AM''  or  ``PM'')
  %S - Second of the minute (00..60)
  %U - Week  number  of the current year,
          starting with the first Sunday as the first
          day of the first week (00..53)
  %W - Week  number  of the current year,
          starting with the first Monday as the first
          day of the first week (00..53)
  %w - Day of the week (Sunday is 0, 0..6)
  %x - Preferred representation for the date alone, no time
  %X - Preferred representation for the time alone, no date
  %y - Year without a century (00..99)
  %Y - Year with century
  %Z - Time zone name
  %% - Literal ``%'' character

   t = Time.now
   t.strftime("Today is %m/%d/%Y")   #=> "Today is 04/29/2008"
   t.strftime("at %I:%M %p")            #=> "at 10:04 PM"

Ruby on Rails for RHEL 5

Brad — Mon, 28 Apr 2008 02:06:49 +0000

Here is the quick and dirty Ruby on Rails setup for Redhat Enterprise Linux v5.0 with MySQL.

(2009-12-04 Update: This does not work with RHEL 5.4. The mysql ruby gem requires ruby 1.8.6 and RHEL 5.4 only ships with ruby 1.8.5)

Using yum install ruby and mysql. The command below does not list all the required packages, but due to dependencies, the additional packages will be installed (or should be – this is all from memory).

[bm@app3 ~]$ sudo su - [root@app3 ~]# yum install ruby ruby-devel ruby-libs ruby-irb \ > ruby-rdoc subversion-ruby mysql-server mysql-devel [....]

Complete the basic configuration of MySQL.
[root@app3 ~]# /sbin/chkconfig mysqld on [root@app3 ~]# service mysqld start [root@app3 ~]# mysql_secure_installation

Download and install RubyGems. As of the date this post was written, the latest version is 1.1.1.
[root@app3 ~]# wget http://rubyforge.org/frs/download.php/35284/rubygems-1.1.1.zip [root@app3 ~]# unzip rubygems-1.1.1.zip [root@app3 ~]# cd rubygems-1.1.1 [root@app3 rubygems-1.1.1]# ruby setup.rb [root@app3 ~]# cd .. [root@app3 ~]# rm -rf rubygems-1.1.1

Now install Rails, Mongrel, the Ruby MySQL interface, Capistrano
[root@app3 ~]# gem install rails --include-dependencies --no-rdoc --no-ri [root@app3 ~]# gem install mongrel --include-dependencies --no-rdoc --no-ri [root@app3 ~]# gem install mysql -- --with-mysql-config=/usr/bin/mysql_config [root@app3 ~]# gem install capistrano --include-dependencies --no-rdoc --no-ri

Phusion’s passenger gem provides a “mod_rails” module for Apache. The installation is very straight forward.
[root@app3 ~]# gem install passenger [root@app3 ~]# passenger-install-apache2-module

Just follow the on-screen instructions to finish the installation.