This is my first tutorial so I hope you enjoy it.
Linux Basics
by dingus
dingus@linuxmail.org www.rootslash.com 
July 21, 2001

Table of Contents:
1) Introduction
2) Privileged Users
3) Installation
4) Directory Structure
5) Command Line
6) Text Editors
7) Pipes and Redirection
8) Shortcuts
9) Compiling and Installing Packages
10) Network Tools
11) Ownership
12) Permissions
13) Administrative Commands
14) Users and Groups
15) Conclusion


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1) Introduction
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This tutorial is intended for those who are new to Linux or want to review and improve their basic skills. I, dingus, will attempt to cover the general aspects of installation to being able to understand the basics of your Linux machine. This tutorial is also best utilized when the reader, you, are at your computer trying these commands yourself. Although I do believe I give enough information to just simply read through it, you will definitely get more out of it by doing it as I go along. If you have questions, comments or complaints please feel free to go to my website and find thee most appropriate way to contact me. If you did not spot the URL above, the address is www.rootslash.com 


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2) Privileged Users
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Before diving into everything we need to know what “root” is. “root” is a user and can do anything to any file even it if is not owned by him or her. The user has no limitations. “root” is often referred to as the super user account as it really is a “super” account. All other users only have permission to edit files they own. For information on permissions you should skip to the “Permissions” section of this tutorial. There are also privileged accounts, which may have some super user abilities as well. 

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3) Installation
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I'm not going to tell you how to install Linux in this section simply because it varies from distribution to distribution. Before installing Linux make sure to read all documents which came with it. If you downloaded Linux then make sure that you go your distribution's web page and read all the installation material. It would also be a good idea to print the installation instructions out as well so you are able to look at it during the installation.

Most Linux distributions give you a choice of how you want to install. The options are usually something along the lines of "Default Install" which installs all available packages and "Custom" which allows you to choose which packages to install. Make sure to choose "Custom" install because this will allow you to choose what is installed into your system. By having less installed, it lowers the likely hood of being cracked. The reason is because if you have fewer applications and services running there are less vulnerabilities in which you can be cracked. Another reason to install only what is needed is that your system will have less to run and handle, resulting in higher system performance.


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3.2) fdisk
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“fdisk” is a partition tool that comes in the default installation of all Linux distributions. It must be run as root. At the very minimum you should at least make a root partition as well as a swap partition. Although the swap partition is not required it is highly recommended. The swap partition is used for virtual memory and the root partition is used for all the other files that go into your system. Some users like to make partitions for /home, /var and other big directories if they are running a server. This is for security reasons so that someone does not try to launch a Denial of Service attack by overflowing the amount of logs which is held in the /var directory. So if you are planning on running a server you might consider making a separate partition for /var as well as other directories. Now that that is out of the way, we will get on to creating partitions. When first booting up from either a boot disk or CD-ROM you should get a prompt. Type:

fdisk /dev/hda

substituting /dev/hda for the drive you want to partition. All drives including CD-ROM, floppy and any other kind of drives you can think of are in the /dev directory. It might happen that your hard drive is not labeled "hda" and could be something like "hdb" or "hdc." In that case substitute "hda" with the proper drive. If you have SCSI hard drives they will be labeled "sdx" where "x" is a letter a-z. So for example if you were a SCSI hard drive user you would execute this command:

fdisk /dev/sda

After you have specified which drive you want to partition it's a good idea to take a look at how that disk is setup by using the "p" command. It will "print" the current partitions within that disk. I'm going to assume you want to delete all partitions and install Linux on a fresh hard drive. So to delete them type "d" and then "1" and then "d" again then "2" and so on until all partitions are deleted. The "d" stands for delete obviously and the numbers such as "1" and "2" are the partition numbers. Now type "p" to make sure nothing is listed other then the headings. If everything is gone you are all set to start creating your new partitions, if there are still partitions listed then delete them using the method described previously. We are now going to make the swap partition by typing "n" which means "new" partition. When you type that it will ask for either extended or primary. Choose primary by typing "p." fdisk will proceed to ask which partition number and you will respond with "1" since this is your first partition that you are creating. Then it will ask for the first cylinder, just choose the default by either entering the number or pressing enter. Then it asks for the last cylinder or size or size in megabytes or size in kilobytes. You will want to use the size in megabytes option by typing: +128M. That is if you have 64 megabytes of ram. It is a good rule of thumb to use double the amount of ram you have for your swap space. So in our case we have 64 megabytes of ram so we chose 128 megabytes for our swap space. So if we had 128 megabytes of ram we would put 256 megabytes for our swap. Although you do not have to always use double, it is just a good idea. We are now done creating our swap partition.

Now we will create our root file system partition. This is pretty much the same as creating the swap partition except we just change a few things. We type "n" again for a new partition and then it will ask us if we want a primary or extended partition, we choose primary by typing "p." For partition number we choose "2" since it is our second partition and then choose the default for the first and last cylinders. Of course you can change the default but the default is all right for what we are doing.

We are almost done using fdisk. We just need to change the type of the first partition we made to specify it as the swap partition. To do this we type "t" which stands for "type" and we type 82. The second partition we made which is the root partition does not need to be changed because it is on the default 83, which is "Linux Native." Now that we have done that type "p" one last time to make sure everything is all right. If all is good type "w" to "write" your partition table to your hard drive and you are done. 


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4) Directory Structure
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The / directory stands for the root directory. It is the top of the directory tree. All directories below are held in this directory.

The /bin directory is where all executable files are held. "bin" stands for binaries. Commands such as "ls" and "cd" are contained here.

The /boot directory is self explanitory. It contains the boot files needed to start the system such as LILO (Linux Loader).

The /cdrom directory is a mounting place for your CD-ROM although you may mount it to any directory you want.

The /dev directory is where all your "devices" are held. All your devices are treated like files. Your partitions, floppy, CD-ROM etc. are found here.

The /etc directory contains the majority of the configuration files. Most are in text form and so you get to edit them with your favorite text editor.

The /home directory is self-explanatory. It holds all the users' home directories. So for example, my user name is "dingus" so I would be /home/dingus. It does not hold the root user's home directory however. 

The /lib directory stands for "libraries" which contains shared library files such as the kernal modules and C Library.

The /lost+found directory is where fragments of data are placed when your system does not know where they belong. This is especially true if you shut your system down improperly causing fsck to run. When fsck cannot find a place for a fragment it is placed in this directory.

The /mnt directory is the typical directory for a person to mount file systems to such as your CD-ROM and floppy.

The /proc directory contains information about the system.

The /root directory is the home directory of the root user.

The /sbin directory means "system binaries" and contains binaries for system administration such as monitoring and configuring. Commands such as "mount" and "shutdown" are found here. The programs can only be run by root or during boot time.

The /tmp diretory is for temporary files.

The /var directory contains the log files, mail spools, lock spools, and printer spools.


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5) The Command Line
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In the section I will discuss various useful commands. I cannot write about every command there is because that would be a book in it self. Here is how I will give examples. 

command <options> <option <option of option>>

The "command" part is necessary for the command to work and stuff that is in <> means it is optional and the command will still work without it. Do not include the <> in the command as those only signify they are not absolutely necessary. For some commands I will not show you all the options simply because there are too many and only a few are useful. If you are interested in finding out the other commands do this:

command --help

An extremely useful tool is "man" which stands for manual. Here is how you use it:

man command

where command is the command you are interested in finding more information about. Here is an example, let's say you want to find information about the "ls" command, you would execute this:

man ls

This would bring up a page all about "ls" and its functions. To get out of the man page simply press "q" on your keyboard. This will bring you back to the command line.

Another command, which gives you information on a command, is "info." This is used the same way man is used. For example, if you wanted information on the command "ls" you would do this.

info ls

By doing that, a text file on "ls" would come up. 


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5.1) ls
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The "ls" command lists files and sub directories within a directory. Since this command has so many options I will only be writing about the most useful ones that I use. Below is the usage of the ls command.

ls <options> <dir>

Below are only useful options, which can be used with the ls command:

-l, --format=long, --format=verbose
The above all do the same thing it's just a matter of preference in which you like more. The -l option lists the contents of a directory in long format. This includes fill permissions, owner, size, last date of modification, and whether something is a directory or not.

-a
This option lists all files and directories even if they are hidden. The hidden files and directories have periods in front of them like ".file" or ".directory" and are often referred to as "dot files."

-1, --format=single-column
This displays each entry on a line of it's own.

-R, --recursive
This option lists all files and subdirectories within all the directories in that directory. A lot of information is put out and I recommend that you use the more command with it. The more command will be explained later in this paper.

Here is a common combination:

ls -la 

That will show all files in the current directory that you are located in and list them in long format giving you in depth detail on the files and directories.

Here is an example if you wanted to list all the files including hidden files in /home/dingus which you are currently not in. Let's just say you're in /usr/bin. This is what the command would be:

ls -a /home/dingus


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5.2) cd 
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The "cd" command stands for "change directory" which lets you change from your current directory to another. It has no options so here is the format in which "cd" is used:

cd <directory>

where directory is the desired directory to change to. Here is an example:

cd /lib/modules

This will bring you from your current directory into /lib/modules. If you do not specify a directory it will bring you back to your home directory. In my case if just typed “cd” then I would go back to /home/dingus.


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5.3) mkdir
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The "mkdir" command means "make directory." The usage of the command is:

mkdir <options> directory

Here are some useful options.

-m, --mode=MODE
This chmods the directory being made, chmod will be explained later. Here is an example of this command:

mkdir -m 711 blah

Here you created a folder named "blah" and chmoded it 711. 

-p, --parents
This creates parent directories if they do not exist. So if a directory called /home/dingus exists and you want to make a directory such as /home/dingus/making/directories/is/fun except making/directories/is does not currently exist but you want the directory "fun" to be created in those directories you would do this if you were in the /home/dingus directory:

mkdir -p making/directories/is/fun 

and this would automatically make the directories making/directories/is. This saves time as you can see since you do not have to first make the directory "making" and then have to cd to "making" and then do "mkdir directories" etc.


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5.4) rmdir
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This command stands for "remove directory." It has similar options as mkdir. Here is the format in which to use rmdir:

rmdir <options> directory

Here are common and useful options:

-p, parents
This deletes intervening parent directories.

--verbose
Prints out what exactly rmdir is doing.

Here is an example:

rmdir --verbose blah

Here you are deleting the "blah" directory and "rmdir" will tell you exactly what it is doing.


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5.5) cp 
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The "cp" command stands for "copy." Here is the use of the command:

cp <options> file directory

Here is a basic command and what it does:

cp blah /home/dingus

Here we are copying the file "blah" into the directory "/home/dingus" as you can see. Below are some common options. Of course there are many more just like in all the other commands explained.

-a, --archive
This keeps the attributes of the file the same.

-R, -r, --recursive
This recursively copies directories.


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5.6) more
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This is an extremely useful tool, which lets you view output of a command one screen at a time. Let’s say you wanted to view the /etc/passwd file but not edit it. You could simply type:

more /etc/passwd

That would show the /etc/passwd file one screen at a time. Another example is if you wanted to list the contents of the /usr/bin/ directory there would be too much stuff flying by to catch everything. This is where "more" is useful. Here is a way to use "more" with the "ls" command:

ls -la | more

This will list the contents screen by screen. To go to the next screen simply press enter. You can use "more" with many other commands but there are far too many to list here. The | is a pipe which lets you send the output of "ls" into "more" which lets you view screen by screen. Pipes will be explained later.


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5.7) less
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This command I believe is more useful then the "more" command in that instead of only being able to view one screen at a time going forward, you can also go back. This command is used the same way as the more command is used. Here are some examples:

less /etc/passwd

and

ls -l | less

This will let your casually browse through the contents of the current directory enabling you to view all the files as much as you want. 


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5.8) cat 
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The "cat" command means "concatenate." It has a variety of uses. One of it's useful features is combining files into one using redirection which is explained later. Its main usage is for displaying files like so: Here is an example:

cat /proc/cpuinfo

where file is the file you want to view. “cat” only lets you view the file and does not let you edit it. You could also make use of the "more" and "less" commands you just learned by trying this:

cat file | less

or you could substitute "less" for "more."

cat smallfile1 smallfile2 smallfile3 smallfile4 > onelargefile

The cat command also can be used to copy files such as the below example:

cat blah > /home/dingus

This copied the file blah into my home directory /home/dingus. 


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5.9) rm
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The "rm" stands for "remove." With rm you can delete directories and files. Please be careful using this command as you delete your entire system with just one command. Here is the basic use of "rm":

rm <options> files/directories

Here are some useful options, of course there are more so use the help command explained in the beginning of this section.

-d, --directory
You must run this command as root or a privileged user. This command removes directories even if they are not empty. 

-f, --force
This removes files without that are write-protected without prompting, so be careful with this.

-r, -R, --recursive
This deletes directories recursively. So all subdirectories within a directory are deleted as well as well as everything within those subdirectories. This can be a dangerous option so use it with caution.

-v, --verbose
This just tells you each file that "rm" is deleting. 

The following example is how to delete all the contents of your hard drive, DO NOT TRY THIS unless you actually want to remove all your files which is highly unlikely. I'm putting this simply to show you what you should not do.

rm -rf /

This will recursively delete everything in your Linux partition without prompts. Once again this is extremely dangerous so use with caution.


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5.10) touch
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This command simply updates a file to the current timestamp or makes a zero byte file. Here is how you use it:

touch <options> file

This will update "file" to the current timestamp. If "file" does not exist it will make a zero byte file. Use the help option to see the different times you can modify.


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5.11) mv
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This command stands for "move." It can not only move files but it can also be used to rename them as well. Here is the basic usage of this command:

mv <options> sources target

Here is an example of how to use this:

mv index.html /home/dingus/public_html

This will move the file index.html into /home/dingus/public_html. If you want to rename a file simply do this:

mv index.html blah.html

This will rename index.html to blah.html.


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5.11) echo
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The "echo" command prints out the text you put in. That may seem pointless but it can be used in very useful ways. Try executing the following:

echo $PATH

This will print out all your paths. You can also use it to append to other files like so:

echo "This will be added to the file" >> blahblah.txt

The above command uses redirection, which will be explained later.


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5.12) pwd 
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The "pwd" command stands for "print working directory" and that is exactly what it does. It tells you the current directory you are located in. For example if I was in my home directory which is /home/dingus and I typed "pwd" it would tell me just that.


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5.13) clear
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If your screen becomes cluttered with output from all the commands you have just learned, simply type "clear" and you will be presented with a nice clean prompt again.


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5.14) ps 
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This command means, "process status." It prints out all the processes that you are running with lots of information about each process. Here is the usage of “ps”:

ps <options>

Here are some common options, which are used with ps:

-a
This prints all processes

-r
Shows all running processes.


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5.15) date
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This command is self-explanatory. It prints the date and time that you configured your computer to.


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5.16) whoami
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This command has no real useful options. So just type "whoami" and it will display the user that you are. For example if I typed "whoami" (without the quotations of course) and my username was "dingus" it would dislpay "dingus" as the output of the command.


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5.17) uptime 
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This command shows how long the system has been up and how many users are logged on as well as the load averages.


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5.18) su
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If you ever need to switch users, there is no need to logout and then log back in with Linux. Simply use the "switch user" command. It is used in the following way:

su <options> <user> <shell-arguments>

If you do not specify a user to switch to it automatically switches to the root user. After you decide which user to switch to you are presented with a password prompt. Type the password in and press enter and you are now logged in as that user.


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5.19) passwd
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“passwd” lets you change your current password to a new one. If you are logged in as the user you want to change the password of simply type “passwd” and it will ask for the current and then the new password. If you are logged in as a privileged user and want to change the password of another user simply type “passwd user” replacing “user” with the user you want to change the password.


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6) Text Editors
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A variety of text editors come with Linux and it’s up to you to choose your favorite one. I will not explain how to use each one, as that would be a daunting task for some of them.


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6.1) vi
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“vi” has been around pretty much ever since UNIX was around. “vi” has its own set of commands which makes it harder to learn then other text editors and new user of Linux tend to shy away from it. It is a good idea to learn some of the commands as you might find that the only text editor available is “vi.” To start “vi” simple type “vi” at the command prompt. For help type “:help” and that should be enough to get you started. 


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6.2) pico
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“pico” is a popular text editor because of its simplicity. To start “pico” up, type “pico” at the command prompt. All commands are nicely laid out at the bottom of the screen for easy use. The “^” character means control. So for example to exit “pico” you would hold down the control key and press the letter “x.” 


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6.3) emacs
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“emacs” is a feature filled program. It is not only a text editor but also a web browser, mail reader and news reader. To start “emacs” type “emacs” at the command line. For help in “emacs” hold down control while pressing the letter “H.”


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7) Pipes and Redirection
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Pipes and redirection may seem scary at first but once you get the hang of it, it makes using Linux a lot more easier and productive. When piping and redirection is used together it can become a powerful tool.


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7.1) Pipes
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We briefly brushed over piping in the more and less commands but you may be wondering what exactly it was doing. The example given in "more" section was as follows:

ls -la | more

What that was doing was piping the output of "ls" into "more" making it easier to handle. Here is another useful example. As you learned earlier, "ps" shows the processes running and most of the times its output is extremely large. Say you only wanted to find out if “inetd” was running and you did not want to sift through a long list of processes. You could simply do this:

ps auxw | grep inetd

What that did was pipe the output of "ps" into "grep" and only print out “inetd.” When you type this you also see that “grep” is running and perhaps you do not want that show either. To take out “grep” from appearing in the output you can type the following:

ps auxw | grep inetd | grep -v grep

The last part "grep -v grep" is the part that excludes “grep” from showing up in the final output. So now what it really is doing is putting the output of “ps” into “grep” which will print out “inetd” if it is running and then pipe that through “grep” again so that “grep” does not appear in the output.


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7.2) Redirection
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Redirection is extremely useful. Next, I'll show you an example and explain it.

ps -auxxw > /home/dingus/psoutput

What this did was redirect the output of "ps" and send it to a file called "psoutput" in my home directory. Now you could open "psoutput" in your favorite text editor and browse through it in a leisurely manner. Say there already is a file named "psoutput" in my home directory and I want to add the output of another "ps" output to the end. To do this you would execute the following:

ps -auxxw >> /home/dingus/psoutput

That would append the output of "ps" to the end of the file. Now that we have "psoutput" in a file we want to see if crond is running so we can do this:

grep crond < /home/dingus/psoutput

What that will do is redirect "psoutput" into “grep” and look for a line that says "crond" in it. If it successfully finds it, then it will print out the line.


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7.3) Using Piping and Redirection Together
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Now you have hopefully learned piping and redirection you can begin putting them together. We could add both examples in the two previous sections together and do this:

ps auxw | grep inetd | grep -v grep > /home/dingus/psoutput

What this does it pick inetd out of the output from "ps" and puts in a file called "psoutput." If you truly understand this stuff you would probably be saying the previous command was absolutely pointless and you are correct. This is because the only thing in "psoutput" would be one line and that is rather pointless. 


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8) Shortcuts
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Linux makes it easy if you are a lazy person or just a person who has a lot to get done. Two wildcards are * which can substitue for any amount of characters and ? which substitutes for only one character.

Say you wanted to copy text1.txt, text2.txt, and text3.txt all into /tmp/texts from /home/dingus. You could simpley type:

cp /home/dingus/text?.txt /tmp/texts


That would copy all three text files into /tmp/texts. If those three text files were the only text files in the directory you could just type this:

cp * /tmp/texts


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9) Compiling and Installing Packages
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In this section I will attempt to teach you how to compile and then install packages as well as uninstalling them later if you have a need to. To install a standard .tar.gz file you first need to download it to a directory. It does not need to be any particular directory 
but I personally download packages to /usr/local/src. Just a note, there are files that are just .tar and just .gz as well as files like .bz, .tgz and many more. Those files are just other ways of compression and after decompressing them they will work the same if they contain the same files described next. In the case of this tutorial I am downloading the file we want to install into the /usr/local/src directory. Perform the following commands to install a package: 

cd /usr/local/src

As learned previously, "cd" changes directories, so we are going to the /usr/local/src directory to unpack the package.

tar -xvzf file.tar.gz

Here I am unpacking the the package. The "x" parameter means extract, the "v" parameter means verbose and shows all the packages that are being unpacked, the "z" option is for gzip and it makes gzip decompress the file before tar untars the file. The "f" option forcefully decompressed the archive. Now "file" will make a directory with the path of /usr/local/src/file. If the file name was "blah-1.2.3" it would make the directory /usr/local/src/blah-1.2.3. Now I will go to the /usr/local/src/file to compile and install "file." 

cd /usr/local/src/file

After I go to /usr/local/src/file there will most likely be a file called "configure" and to see if there is a file called "configure" list the contents of the directory using the "ls" command. There might be .sh files instead which would be installed but that is not covered in this tutorial. Run "configure" by performing this command:

./configure

"configure" makes files called "makefiles" for your system which are responsible later for compilation. The next step is:

make

When you type "make" a bunch of output will fly by. This is because "make" is using the "makefiles" which were made by "configure" to compile all the hundreds or possibly thousands of source files. Now, install the newly compiled package by executing the following command:

make install

This will not take nearly as long as "make" and what it does is install the package. Make sure you are running "make install" as root or it will not work. To clean up do the following:

cd /usr/local/src

rm -rf file

What I just did was go to the "/usr/local/src" where "file.tar.gz" was downloaded and removed the directory in which it was extracted to since those files are no longer needed. 


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10) Network Tools
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The following commands are only the regularly used basic network tools, which I find useful. 


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10.1) ping
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The ping command is primarily used to check if a certain host is up by sending the specified host an ICMP ECHO_REQUEST packet. If the host is up you will receive an ICMP packet back and you will know that the host is up. The basic usage of "ping" is as follows:

ping <options> hostname/IP

Some useful options for ping are:

-c
This option allows you to specify how many packets you want to send to the host. For example you could do this:

ping -c 2 blah

That command would send 2 ICMP packets to host "blah."

-s
This option allows you to specify how large of a packet you want to send. If this option is not specified the default packet size used is 56. You need to be root to use this option.


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10.2) telnet
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"telnet" is an extremely useful program. It allows you to connect to a remote host as if you were actually on the machine. This is one of the many reasons why Linux and UNIX in general is preferred over other operating systems such as Microsoft Windows or Novell Netware. When first telneting to a remote machine you are displayed a login prompt, which asks for your username. After you type your username and press enter a password prompt is given. This is where many people get confused, as it does not put asterisks in place for each character of your password. It simply does not display anything that you have typed. So just type in your password and press enter. After doing this you will be given a shell prompt. Below is the basic usage of the telnet program:

telnet <options> <host <port>> 


Just a little warning about telnet, it sends everything in plain texts so you might consider using SSH (Secure Shell) if the machine you are connecting to has installed and enabled the SSH daemon.


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10.3) lynx
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"lynx" is a text based web browser for UNIX. If you do not run X then you most likely will be using “lynx” as it is a very popular and well-known browser. “lynx” is very simple to use. Just type "lynx" at the prompt and then lynx will take over the terminal. All the commands are laid out at the bottom of the screen and to navigate websites you simply use the up, down, right, left keys, and use the enter key to click on links.


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10.4) ftp
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FTP stands for File Transfer Protocol. FTP allows you to transfer files from one computer to another through this service. All Linux distributions come with an FTP client in the default installation. FTP clients allow you to connect to a remote FTP server. The client is you and the server is the computer you are connecting to. Next is an example of the basic usage of a standard FTP client:

ftp <options> <hostname/IP>

If you do not specify a hostname or IP an "ftp" prompt will come up instead. Once you connect to a remote server you will be asked your username and then password. Again, this is like "telnet" and nothing will appear when you type your password in, so just simply type it and press return. Below are some basic and useful "ftp" commands:

ls = list files
cd directory = change directory
put filename = upload a file
get filename = download a file 


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10.5) traceroute
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"traceroute" outputs all the hosts it travels to as the packet tries to get to it's destination. Each host the packet travels to is called a "hop." The usage of “traceroute” is displayed below:

traceroute <options> host <packetsize>


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10.6) wget
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"wget" is a useful tool for transfering entire websites such as in the case of mirroring when one server needs the exact same information as another server. "wget" can be used to download sites through either FTP or HTTP. To download a website's root directory simply execute the following command:

wget www.site.com 

or for an FTP site:

wget ftp.site.com

If you want to download an entire website and not just the root directory type the following into your terminal:

wget --recursive www.site.com 


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11) Ownership
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A certain user owns every file or directory. To see who owns a particular file or directory use the "ls" command previously learned with the long format option along with it. Here is an example:

ls -l /home

The output would look something like this except (the following is shortened to only one entry):

drwxr-xr-x 1 dingus blah 7368 Jul 14 2001 dingus/

In the case of ownership you can see that the user "dingus" owns the file which is in the third column and in the fourth column is the group which is "blah." To change the ownership of the directory to someone else which in this case is probably not a good idea because it is a home directory but if you wanted to you would type the following at your terminal:

chown phed /home/dingus

In the above example /home/dingus is now under the ownership of the user "phed." To change the group to "developers" from "blah" you would execute the following:

chgrp developers /home/dingus

If you are in need of changing both the user and group you can simply use chown alone as follows:

chown phed.developers /home/dingus

The above changed the user to "phed" and the group to "developers."


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12) Permissons
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Because Linux and other UNIX variants are multi-user operating systems a user needs to make sure that other user do not edit, modify, delete and other things that the owner maybe not want to be done. This is where permissions come into play. Permissions can be a bit confusing at first because there are two ways of reading permissions, the octal that is numeric and the letter format. I will first go over the octal version of permissions.

read = 4
write = 2
execute = 1 

The way the octal version works is by adding them. So if you wanted to give something read and execute permissions it would add up to 5. If you wanted it to have read, write and execute permissions it would add up to 7. The octal version is mainly used when chmoding a file or directory which is explained later.

The letter format is as follows:

read = r
write = w
execute = x

The letter format comes up when you list the contents of a directory in long format. The modes, which are the permissions, will show up in the first column.

drwx------ 2 dingus user 4096 Jun 18 21:04 mail
-rw-r--r-- 1 dingus user 0 Jul 13 22:50 file
drwxr-xr-x 2 dingus user 4096 Jul 13 04:44 public_html

In the above you can tell that "mail" is a directory because it has a "d" for the very first letter in the permission column. Since "file" is not a directory it simply has a "-" instead. After the first character, which signifies if it is a directory or file, it is split up into three parts, which are user, group and world. For the "mail" directory the user part is "rwx" which means the user which is me in this case has read, write and execute permissions to this folder while the group that I am in does not because it is marked with "---." World which is everyone else also does not have permission to my "mail" directory because it is also marked with "---" in its place. The file named file has read and write permissions for the user because it is marked with "rw-" but does not have execute permissions because where an "x" should be a "-" is there instead. People in the group I am only have read permission as it is signified with "r--." Everyone else which is world also only has read permissions as well. The directory "public_html" is a directory as you can see because it has a "d" in front of everything else. The user has read, write and execute permissions to this file so it is marked with "rwx." My group has read and execute permissions as it is marked with "r-x." Where write would normally be a "-" is put there to show my group does not have write permissions. The world also has read and execute permissions like my group.

Now, if we wanted to change the permissions of one of these directories or files we need to know the octal system, which was explained previously. If we wanted to give group users the power to write to my file named file you would do the following:

chmod 664 file

What “chmod” does is "change mode" which is the same as changing permissions. The first number is the user so we kept it the same so that I would have read and write access. Read is 4 and write is 2 so 4+2=6 and that is how we got the user permissions. Next is the group permissions. Originally it was only read so it would have been simply a 4 in the octal system but we want my group users to be able to write to it so we change it to 6, which is read and write. The world, which again is everyone else, cannot be trusted so we put that to 4 which is only read access.


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13) Administrative Commands
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The following commands are meant for the administrator of a computer and can only be run when that person has root access. If you are not logged in as root you can use the "su" command, which was explained earlier. 


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13.1) mount
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Linux treats your CD-ROM, floppy drives and other removable media as just another file system. In order to attach devices not listed in /etc/fstab to the filesystem you need use "mount." The following is the way to use "mount."

mount <options> <device> <directory>

"Device" can be a CD-ROM, DVD-ROM, floppy, tape drive etc. The "directory" can be any directory in the Linux file system. Next are some basic and useful options for mount.

exec
Allows you to execute binaries.

ro
Read-only access to the file system.

rw
Read-write access to the file system. 

-t <type>
Enables you to specify the filesystem type. There are too many file systems to list here but some basic ones are ext2, iso9660 and msdos.

-v
Prints out everything that is happening. Like other commands it means "verbose."


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13.2) umount
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The command "umount" means "unmount" if could not guess and it does exactly that. "umount" is used as follows:

umount <options> <device/directory>

The following are the only options for “umount”:

-a
Unmount all filesystems in /etc/mtab.

-n
Unmount specified file system but do not log it in /etc/mtab.

-t type
Unmount the file systems of the specific type specified.

You can specify either the device or the directory which to unmount. Either one will work.


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13.3) shutdown
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The "shutdown" command obviously shuts down the computer. The great thing about Linux and other UNIX variants is that you can specify when you want to shutdown. 

shutdown <options> when <message>

The message part is so that when the time to shutdown comes closer it is displayed more and more frequently. The most common options are listed below:

-r
Restart the computer.

-h
Halt the computer after the shutdown is complete. After that just press the power button to turn your computer off.

-c
Cancel a shutdown that is in progress.

The when part of the shutdown command is specified in minutes in the following form: +x, x equaling the number of minutes before shutdown. So for example if you wanted to have your computer shutdown in 10 minutes you would enter "+10." If you want your computer to shutdown that very instant you would type "now" instead of an amount of minutes. Here is an example if you wanted to shutdown your computer right then and there.

shutdown -h now

Here is another example:

shutdown -r +15 "Shutdown at 3PM"

This would restart the computer in 15 minutes and display the message "Shutdown at 3PM" to users.


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14) Users and Groups
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In Linux and other UNIX variants you can add users and groups, edit and delete them with just a text editor. There are three main important files that go along with users and groups which are the /etc/passwd, /etc/shadow and /etc/group files. Here are some terms which will help you in the following sections.

UID: A numerical number for a user and stands for User ID. Every user has his or her own unique UID.

GID: A numerical number for a group and stands for Group ID. 

In this section “Users and Groups” only privileged users may edit the files or execute the commands. 


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14.1) /etc/passwd
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The /etc/passwd is a text file which can be edited with your favorite text editor but I will later explain the other ways of changing information by using commands. A line in the /etc/passwd file would look something along the lines of this:

dingus:boTrvszaCXaMe:120:156:Jonathan Blah:/home/dingus:/bin/bash

Colons for easy reading delimit each line. The first part is the username which is dingus, next is the encrypted password, the UID, GID, real name, home directory and default shell. 

Each user has its own line in the /etc/passwd file and it may not be more then one line. Every user also must have his or her own unique UID except the root user who has a UID of zero and is able to share its UID with anyone else. The root should be the only account to have it because if you do decide to allow more then one user to have super user privileges it will increase the chances of your system being compromised.

You may want to temporarily or permanently disable an account for a reason. To do so you simply change the encrypted password. The most efficient and guaranteed way to work is to simply add an asterisk to the end of the encrypted password. So if you wanted to disable my account the new entry would look like this:

dingus:boTrvszaCXaMe*:120:156:Jonathan Blah:/home/dingus:/bin/bash

It is a common practice that if you do disable an account in this manner you put a short and concise note after the asterisk as a reminder why you disabled the account. For example the encrypted password might now look like this “boTrvszaCXaMe*trash talk.”

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14.2) /etc/shadow
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The need for the /etc/shadow file came about as dictionary cracking became easier as computers became faster. The /etc/shadow file now keeps the encrypted password rather then the /etc/passwd file. The /etc/passwd file looks like this when the passwords are shadowed:

dingus:x:120:156:Jonathan Blah:/home/dingus:/bin/bash

The file needed to be kept the same so that other programs could still refer to it. Everyone still has read access to the /etc/passwd file as it has always been that way. 

The format for the /etc/shadow file is as follows:

dingus:boTrvszaCXaMe:10568:0:99999:7:-1:-1:12232355

In order this is how the format is laid out: username matching the entry in /etc/passwd, the encrypted password, if it is left blank it means that there is no password for the account which is a bad idea. After the password is the days since January 1, 1970 that the password has last been changed. Next is the number of days before the password may be changed, if the entry is “0” then it means it may be changed whenever. The next entry is the amount of years before the password must be changed. The next entry is the number of days to warn a user before his or her password expires. Next is the number of days to disable a user after the password expires. Next is the number of days since January 1, 1970 that the account has been disabled. The last column is a reserved field. The fields that have –1 signify infinity. 

If you need to disable account you can do it the same way as explained in the /etc/passwd section. Simply put an asterisk at the end of the encrypted password and it will prevent them from logging in. 


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14.3) /etc/group
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Each user is in at least one group. The user’s default GID is specified in the /etc/passwd file and is linked to the /etc/group file. Here is an example of the /etc/group file: 

blahers:baHrElKPNjrPE:102:dingus,pete,phed

Like the /etc/passwd and /etc/shadow files, the /etc/group file is also delimited by colons. The first column is the group name, next is the encrypted password, then the GID, and the users within the group, which are separated by commas. 


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14.4) Users
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In order to add a user you will need to use one of these commands: “useradd” or “adduser” depending on the distribution you are using. If you are using a distribution that supports “adduser” then you are lucky because it is easier and more straight forward then “useradd.” To run “adduser” simply type “adduser” and it will ask you everything it needs in order to make a user, very simple. If your distribution only has “useradd” you would use the following example:

useradd <options> username

This will not set the password, groups etc so you will most likely want to use a few more parameters with the “useradd” command. Below are some useful options for “useradd.”

-c comment
This allows you to set the username. If it has spaces you need to use quotation marks like all other Linux commands. Here is an example:

useradd –c “dingus blah” 

-d directory
This lets you specify the home directory of a user. By default the home directory is /home/user.

-g group
This lets you specify the group the user will be added to.

-G group <,group>
If you want a user to be in multiple groups then you can add the user to them using this command. For example, if you wanted to add “dingus” to groups “users” and “admins” you could do this:

useradd dingus –G users,admins

-s shell
Some users like to use other shells then the default one so in place for “shell” you put the full path to the shell.

-u uid
This lets you specify the UID rather then having one assigned by default.

After creating a member you will want to add a password or you will greaten security problems. If you are root user you can type:

passwd username

and then change or add a password that way. If you are logged in as the user simply type “passwd.” Passwords should include, lower case and upper case letters, numbers and random characters. The number should not spell a word either. The password should not make sense and you should not use birthdays, social security numbers etc. For example, a good password would be something along the lines of this: 4pX%*L!82h#. 

After you have made users, you may want to modify them. To do this, use the “usermod” command. It is used practically the same as “useradd.” To change the group the user is in you would type:

usermod dingus –g groupname

Someday you might want to get rid of user “dingus” and to do that you would type:

userdel <options> dingus

When you execute that it will delete all entries in the /etc/passwd and /etc/shadow files. If you use its one option, which is “-r”, it will delete all files owned by user “dingus.”


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14.5) Groups
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Groups are important for organizing users. All users belong to at least one group. The basic usage of the command is as follows:

groupadd <options> groupname

Below are some useful options:

-g gid
This lets you specify the GID instead of having the computer assign one that is available.

If you happen to want to modify a user you can use the “groupmod” command. Its usage is displayed below:

groupmod <options> groupname

Below are the only options for “groupmod.”

-g gid
This will change the GID of the group.

-n newgroupname
This option will change the group name.

You may find that you need to delete a group and the following is the command to do so. 

groupdel groupname

There are no options for “groupdel.”


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15) Conclusion
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Now that you have read through the tutorial you should have a fairly good idea how to navigate around a Linux system as well as get some work done on it. This text file will be updated and modified in the future so for the latest version of this text file please visit my website at http://www.rootslash.com.