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Posts tagged “Filesystem permissions

Understanding /etc/passwd

Q. Can you explain /etc/passwd file format for Linux and UNIX operating systems?

A. /etc/passwd file stores essential information, which is required during login i.e. user account information. /etc/passwd is a text file, that contains a list of the system’s accounts, giving for each account some useful information like user ID, group ID, home directory, shell, etc. It should have general read permission as many utilities, like ls use it to map user IDs to user names, but write access only for the superuser (root).

Understanding fields in /etc/passwd

The /etc/passwd contains one entry per line for each user (or user account) of the system. All fields are separated by a colon (:) symbol. Total seven fields as follows.

Generally, passwd file entry looks as follows (click to enlarge image):

passwd-file

Fig.01: /etc/passwd file format

  1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
  2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
  3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
  4. Group ID (GID): The primary group ID (stored in /etc/group file)
  5. User ID Info: The comment field. It allow you to add extra information about the users such as user’s full name, phone number etc. This field use by finger command.
  6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
  7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

Task: See User List

/etc/passwd is only used for local users only. To see list of all users, enter:
$ cat /etc/passwd
To search for a username called tom, enter:
$ grep tom /etc/passwd

/etc/passwd file permission

The permission on the /etc/passwd file should be read only to users (-rw-r–r–) and the owner must be root:
$ ls -l /etc/passwd
Output:

-rw-r--r-- 1 root root 2659 Sep 17 01:46 /etc/passwd

Reading /etc/passwd file

You can read /etc/passwd file using the while loop and IFS separator as follows:

#!/bin/bash
# seven fields from /etc/passwd stored in $f1,f2...,$f7
# 
while IFS=: read -r f1 f2 f3 f4 f5 f6 f7
do
 echo "User $f1 use $f7 shell and stores files in $f6 directory."
done < /etc/passwd

Your password is stored in /etc/shadow file

Your encrypted password is not stored in /etc/passwd file. It is stored in /etc/shadow file. In the good old days there was no great problem with this general read permission. Everybody could read the encrypted passwords, but the hardware was too slow to crack a well-chosen password, and moreover, the basic assumption used to be that of a friendly user-community.

Almost, all modern Linux / UNIX line operating systems use some sort of the shadow password suite, where /etc/passwd has asterisks (*) instead of encrypted passwords, and the encrypted passwords are in /etc/shadow which is readable by the superuser only.

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Giving WordPress Its Own Directory

 

Many people want WordPress to power their site’s root (e.g.http://example.com) but they don’t want all of the WordPress files cluttering up their root directory. WordPress allows you to install it into a subdirectory, but have your blog exist in the site root.

WARNING: Multisite Users – Please Read

This process is not applicable to and does not work if you have enabled MultiSite.

Moving a Root install to its own directory

The process to move WordPress into its own directory is as follows:

  1. Create the new location for the core WordPress files to be stored (we will use /wordpress in our examples). (On linux, use mkdir wordpress from your www directory. You’ll probably want to use “chown apache:apache” on the wordpress directory you created.)
  2. Go to the General panel.
  3. In the box for WordPress address (URL): change the address to the new location of your main WordPress core files. Example:http://example.com/wordpress
  4. In the box for Site address (URL): change the address to the root directory’s URL. Example: http://example.com
  5. Click Save Changes. (Do not worry about the error message and do not try to see your blog at this point! You will probably get a message about file not found.)
  6. Move your WordPress core files to the new location (WordPress address).
  7. Copy (NOT MOVE!) the index.php and .htaccess files from the WordPress directory into the root directory of your site (Blog address). The .htaccess file is invisible, so you may have to set your FTP client to show hidden files. If you are not using pretty permalinks, then you may not have a .htaccess file. If you are running WordPress on a Windows (IIS) server and are using pretty permalinks, you’ll have a web.config rather than a .htaccess file in your WordPress directory. As stated above, copy (don’t move) the index.php file to your root directory, but MOVE (DON’T COPY) the web.config file to your root directory.
  8. Open your root directory’s index.php file in a text editor
  9. Change the following and save the file. Change the line that says:
    require('./wp-blog-header.php');
    to the following, using your directory name for the WordPress core files:
    require('./wordpress/wp-blog-header.php');
  10. Login to the new location. It might now be http://example.com/wordpress/wp-admin/
  11. If you have set up Permalinks, go to the Permalinks panel and update your Permalink structure. WordPress will automatically update your .htaccess file if it has the appropriate file permissions. If WordPress can’t write to your .htaccess file, it will display the new rewrite rules to you, which you should manually copy into your .htaccess file (in the same directory as the main index.php file.)

Using a pre-existing subdirectory install

If you already have WordPress installed in its own folder (i.e. http://example.com/wordpress) then the steps are as follows:

  1. Go to the General panel.
  2. In the box for Site address (URL): change the address to the root directory’s URL. Example: http://example.com
  3. Click Save Changes. (Do not worry about the error message and do not try to see your blog at this point! You will probably get a message about file not found.)
  4. Copy (NOT MOVE!) the index.php and .htaccess files from the WordPress directory into the root directory of your site (Blog address). The .htaccess file is invisible, so you may have to set your FTP client to show hidden files. If you are not using pretty permalinks, then you may not have a .htaccess file. If you are running WordPress on a Windows (IIS) server and are using pretty permalinks, you’ll have a web.config rather than a .htaccess file in your WordPress directory. As stated above, copy (don’t move) the index.php file to your root directory, but MOVE (DON’T COPY) the web.config file to your root directory.
  5. Open your root directory’s index.php file in a text editor
  6. Change the following and save the file. Change the line that says:
    require('./wp-blog-header.php');
    to the following, using your directory name for the WordPress core files:
    require('./wordpress/wp-blog-header.php');
  7. Login to your site. It should still be http://example.com/wordpress/wp-admin/
  8. If you have set up Permalinks, go to the Permalinks panel and update your Permalink structure. WordPress will automatically update your .htaccess file if it has the appropriate file permissions. If WordPress can’t write to your .htaccess file, it will display the new rewrite rules to you, which you should manually copy into your .htaccess file (in the same directory as the main index.php file.)

Pointing your home site’s URL to a subdirectory

In some cases, you may have a WordPress site that changes significantly every year, such as with a conference website. If you want to install each year’s version of the site in a subdirectory, such as /2010, /2011, and /2012, but have the root domain (yoursite.com) automatically redirect to a particular subdirectory (usually the latest), follow this technique:

  1. Install WordPress in a subdirectory, such as /2012.
  2. In your root folder (not the subdirectory folder), download and open your .htaccess file.
  3. Add the following to your .htaccess file:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www.)?YourDomain.com$
RewriteRule ^(/)?$ blog [L]
  1. In the above code, change the “YourDomain.com” value to your root domain.
  2. In the above code, change the “blog” value to the subdirectory.
  3. Save and upload the .htacess file back to your root directory.

Now when users to go your root domain (yoursite.com), it will automatically redirect to the subdirectory you specified. When you want to redirect to a new subdirectory, such as the conference site for next year, just update the .htaccess redirect code.