Fanatical Solutions

Basically, screen allows you to create virtual terminals which are not connected to your actual xterms or console screens. You can then disconnect from a screen session and reconnect from somewhere else while preserving your shell or other running processes.

1. Set the screen binary (/usr/bin/screen) setuid root. By default, screen is installed with the setuid bit turned off, as this is a potential security hole.
2. The teacher starts screen in a local xterm, for example via screen -S SessionName. The -S switch gives the session a name, which makes multiple screen sessions easier to manage.
3. The student uses SSH to connect to the teacher’s computer.
4. The teacher then has to allow multiuser access in the screen session via the command Ctrl-a :multiuser on (all screen commands start with the screen escape sequence, Ctrl-a).
5. Next the teacher grants permission to the student user to access the screen session with Ctrl-a :acladd student where student is the student login ID.
6. The student can now connect to the teacher’s screen session. The syntax to connect to another user’s screen session is screen -x username/session.

alias ls='ls --color=none'

With that, all the pieces are in place: multiple users can share a screen session for any sort of command-line-based instruction. The teacher can at any time take control of the session by switching all other users to read-only access. Finally, you can turn on the logging facilities in screen to get an accurate and usable log of the entire session (or just portions of the session if you desire).

To make vi display line numbers, you need to set the number flag. To do so:

• Press the Esc key if you are currently in insert or append mode. 
   
• Press  :  (the colon). The cursor should reappear at the lower left corner of the screen next to a  :  prompt. 
   
• Enter the following command: set number
• A column of sequential line numbers will then appear at the left side of the screen. Each line number references the text located directly to the right.

To turn off line numbering, again follow the preceding instructions, except this time enter the following line at the  :  prompt: set nonumber

Normally, vi will forget the setting you’ve chosen once you’ve left the editor. You can, however, make either setting take effect automatically whenever you use vi on a particular account. To do this, place the appropriate set statement as a line in a file in your home directory named.exrc .

For a list of all current settings, at the  :  prompt, enter:set allFor a list of everything that you have set, at the  :  prompt, enter: set

Linux files are setup so access to them is controlled. There are three types of access:

• read
• write
• execute

Each file belongs to a specific user and group. Access to the files is controlled by user, group, and what is called other. The term, other, is used to refer to someone who is not the user (owner) of the file, nor is the person a member of the group the file belongs to. When talking about setting permissions for "other" users to use, it is commonly referred to as setting the world execute, read, or write bit since anyone in the world will be able to perform the operation if the permission is set in the other category.

File names and permission characters

File names can be up to 256 characters long with "-", "_", and "." characters along with letters and numbers.
When a long file listing is done, there are 10 characters that are shown on the left that indicate type and permissions of the file. File permissions are shown according to the following syntax example: drwerwerwe
There are a total of 10 characters in this example, as in all Linux files. The first character indicates the type of file, and the next three indicate read, write, and execute permission for each of the three user types, user, group and other. Since there are three types of permission for three users, there are a total of nine permission bits. The table below shows the syntax:

• Character 1 is the type of file: – is ordinary, d is directory, l is link.
• Characters 2-4 show owner permissions. Character 2 indicates read permission, character 3 indicates write permission, and character 4 indicates execute permission.
• Characters 5-7 show group permissions. Character 5=read, 6=write, 7=execute
• Characters 8-10 show permissions for all other users. Character 8=read, 9=write, 10=execute
• There are 5 possible characters in the permission fields. They are:
• r = read – This is only found in the read field.
• w = write – This is only found in the write field.
• x = execute – This is only found in the execute field.
• s = setuid – This is only found in the execute field.
• If there is a "-" in a particular location, there is no permission. This may be found in any field whether read, write, or execute field.

Examples

Type "ls -l" and a listing like the following is displayed:

Which means the following:

The fields are as follows:

• Type field: The first character in the field indicates a file type of one of the following:

1. d = directory
2. l = symbolic link
3. s = socket
4. p = named pipe
5. - = regular file
6. c= character (unbuffered) device file special
7. b=block (buffered) device file special

• Permissions are explained above.
• Links: The number of directory entries that refer to the file. In our example, there are four.
• The file’s owner in our example is George.
• The group the file belongs to. In our example, the group is team1.
• The size of the file in bytes
• The last modification date. If the file is recent, the date and time is shown. If the file is not in the current year, the year is shown rather than time.
• The name of the file.

Set User Identification Attribute

The file permissions bits include an execute permission bit for file owner, group and other. When the execute bit for the owner is set to "s" the set user ID bit is set. This causes any persons or processes that run the file to have access to system resources as though they are the owner of the file. When the execute bit for the group is set to "s", the set group ID bit is set and the user running the program is given access based on access permission for the group the file belongs to. The following command:

chmod +s myfile

sets the user ID bit on the file "myfile". The command:

chmod g+s myfile

sets the group ID bit on the file "myfile".

The listing below shows a listing of two files that have the group or user ID bit set.

-rws--x--x   1 root    root    14024 Sep  9 1999 chfn
-rwxr-sr-x   1 root   mail    12072 Aug 16 1999 lockfile

The files chfn and lockfile are located in the directory "/usr/bin". The "s" takes the place of the normal location of the execute bit in the file listings above. This special permission mode has no meaning unless the file has execute permission set for either the group or other as well. This means that in the case of the lockfile, if the other users (world execute) bit is not set with permission to execute, then the user ID bit set would be meaningless since only that same group could run the program anyhow. In both files, everyone can execute the binary. The first program, when run is executed as though the program is the root user. The second program is run as though the group "mail" is the user’s group.

For system security reasons it is not a good idea to set many program’s set user or group ID bits any more than necessary, since this can allow an unauthorized user privileges in sensitive system areas. If the program has a flaw that allows the user to break out of the intended use of the program, then the system can be compromised.

Directory Permissions

There are two special bits in the permissions field of directories. They are:

• s – Set group ID
• t – Save text attribute (sticky bit) – The user may delete or modify only those files in the directory that they own or have write permission for.

Save text attribute

The /tmp directory is typically world-writable and looks like this in a listing:

drwxrwxrwt   13 root     root         4096 Apr 15 08:05 tmp

Everyone can read, write, and access the directory. The "t” indicates that only the user (and root, of course) that created a file in this directory can delete that file.

To set the sticky bit in a directory, do the following:

chmod +t data

This option should be used carefully. A possible alternative to this is

• Create a directory in the user’s home directory to which he or she can write temporary files.
• Set the TMPDIR environment variable using each user’s login script.
• Programs using the tempnam(3) function will look for the TMPDIR variable and use it, instead of writing to the /tmp directory.

Directory Set Group ID

If the setgid bit on a directory entry is set, files in that directory will have the group ownership as the directory, instead of than the group of the user that created the file.

This attribute is helpful when several users need access to certain files. If the users work in a directory with the setgid attribute set then any files created in the directory by any of the users will have the permission of the group. For example, the administrator can create a group called spcprj and add the users Kathy and Mark to the group spcprj. The directory spcprjdir can be created with the set GID bit set and Kathy and Mark although in different primary groups can work in the directory and have full access to all files in that directory, but still not be able to access files in each other’s primary group.

The following command will set the GID bit on a directory:

chmod g+s spcprjdir

The directory listing of the directory "spcprjdir":

drwxrwsr-x 2 kathy spcprj 1674 Sep 17 1999 spcprjdir

The "s” in place of the execute bit in the group permissions causes all files written to the directory "spcprjdir" to belong to the group "spcprj" .

Examples

Below are examples of making changes to permissions:

Below are examples of making changes to owner and group:

Note: Linux files were displayed with a default tab value of 8 in older Linux versions. That means that file names longer than 8 may not be displayed fully if you are using an old Linux distribution. There is an option associated with the ls command that solves this problem. It is "-T". Ex: "ls al -T 30" to make the tab length 30.

Umask Settings

The umask command is used to set and determine the default file creation permissions on the system. It is the octal complement of the desired file mode for the specific file type. Default permissions are:

• 777 – Executable files
• 666 – Text files

These defaults are set allowing all users to execute an executable file and not to execute a text file. The defaults allow all users can read and write the file.

The permission for the creation of new executable files is calculated by subtracting the umask value from the default permission value for the file type being created. An example for a text file is shown below with a umask value of 022:

        666 Default Permission for text file
       -022 Minus the umask value
      —–
        644 Allowed Permissions

Therefore the umask value is an expression of the permissions the user, group and world will not have as a default with regard to reading, writing, or executing the file. The umask value here means the group the file belongs to and users other than the owner will not be able to write to the file. In this case, when a new text file is created it will have a file permission value of 644, which means the owner can read and write the file, but members of the group the file belongs to, and all others can only read the file. A long directory listing of a file with these permissions set is shown below.

-rw-r--r--   1 root     workgrp          14233 Apr  24 10:32 textfile.txt

A example command to set the umask is:

umask 022

The most common umask setting is 022. The /etc/profile script is where the umask command is usually set for all users.

Red Hat Linux has a user and group ID creation scheme where there is a group for each user and only that user belongs to that group. If you use this scheme consistently you only need to use 002 for your umask value with normal users.