For Linux newbies a quick lesson on every day Linux commands.
Welcome! We meet again. It has been some time now since I've heard you swear at your computer so I assume your Linux installation went well and you have been experimenting with your new system However If you are one of those wanderers who ever so often chance upon random webpages on the net, here you will find a detailed guide to installing Linux.
Now during installation you chose a 'root' password which you used to login when you were prompted for a username and a password for the first time. 'Root' on Linux is the centre of all power. He is the System Administrator who has access to all files and folder and can add or delete users or change their passwords.
Now absolute power can corrupt absolutely. For eg. If you put in a virus infected floppy as Root ,(Not that there are many viruses for Linux, as of now I have come across just two ) the virus might be able to infect all files , since you have access to all of them. Users other than root however have write access to a limited part of the file-system which often excludes crucial library and configuration files thus reducing the risk of damage due to such incidents. So you must create a new user for yourself and when not performing administrative tasks should log in using that username rather than root.
Lets create a new user.
First open the Linux Console which is found on the Panel at the bottom of your screen generally shown by an icon that looks like TV switched off with a black screen. The following commands are to be type in the console, pressing enter after you have entered the full command.
The 'useradd' Command
The general syntax for the useradd command is :
useradd -d home_directory -e expire_date -g initial_group -p password login_name
home_directory : Is the main directory within which the user is expected to story all his files and subdirectories.
For a user named 'foo' the home directory generally is /home/foo
expire_date : This is the date on which the user's account expires and he/she cannot access the computer
anymore unless the account is renewed. This is somewhat similar to your ISP account
expiring in 6 months or one year. The expire date is in yyyy-mm-dd format (2002-08-30)
initial_group : Every user in Linux belongs to a group which affects his file permissions. The initial group
must be a group which already exists.
Password : This will be the user's password to access his account
login_name : This will be the user name with which the user will access his account.
Eg :
useradd -d /home/einstein -e 2002-08-30 -g root -p relativitysucks einstein
creates a user named einstein on my computer.
His home directory is /home/einstein
His expirty date is 30th august 2002
He belongs to the 'root' group
His password is relativitysucks
His username is einstein.
Incase you do not enter one of the parameters group, home, expire or shell they are replaced by their default values. These default values can be viewed using the " useradd -D" command and can also be changed which however we will cover in a later document.
The ls command
The ls command is equivalent of the DOS dir command. It lists the files and subdirectories contained within the present directory.
Some possible flags which can be used with the ls command are :
ls -a
List all files (Some configuration files starting with a dot '.' are otherwise not listed). Often the number of files in a directory is too large to be fitted within one screenfull of data. In such a case we use dir/p for DOS. For linux a similar command is
ls | more
Lists files and directories page after page on keystroke. The above command actually is a combination of two commands. It introduces a new concept called 'Piping'. It is done using the logical OR or | character found just above the Enter key on your keyboard. In Linux it is possible to give the output of one command to another command as an input.The ls command lists files & subdirectories and the more commands divides its input into page length views. Thus piping the ls output to more results in page length views of files and subdirectories.
ls -R
It lists the files and subdirectories of a directory and further lists the contents of each subdirectory recursively. The output of this command is usually large and is best seen when piped through more.
The pwd command
The pwd or the present working directory command gives you the path to the directory in which you presently are. It is used without flags simply as 'pwd'
The su command
Many a times you might have logged in as a normal used and might need to be root to install a software or for some other small task. You could logout then login as root complete the work logout and login back as a normal user. Instead, you can just use the su command. The format is :
su username
eg : su root
when you 'su' to become root from a normal user, you are asked for the root password. But if you are root, you can use 'su' to become any user without using a password. Once your work is finished, use 'exit' to become yourself.
The whoami command
Sorry folks! This command won't solve your teenage identity crisis but it will tell you which user you are logged in as. Useful when you have used 'su' many times and now don't know who you are.
The cp command
This one copies files / directories from one place to another it's syntax is
cp source_file_with_path destination_path
eg : cp /home/aarjav/secret.txt /ftp/pub
This would make all my secrets public :). But my secrets wouldn't fit on my 8.4 Gb hard-disk ;) The cp command can be used with some useful flags also :
cp -i
Interactive copying, prompts before overwriting files or directories
cp -l source_file_with_path destination_path
Makes a link (shortcut) to the source_file at the destination path instead of actually copying it there.
cp -p
Preserve file attributes while copying if possible
cp -R
Copy Recursively . Used when copying directories. This command also copies the contents of the subdirectories.
cp -u
Update i.e. Copy only if the source file is newer than the destination file or the destination file does not exist.
The rm command
The rm command is used to remove or delete files or directories. Its general format is:
rm -flag file_or_directory_with_path
eg : rm /home/aarjav/waste.txt
Some flags which can be used with the rm command are
rm -v file.txt
Remove verbosely, explain what is being done.
rm -r my_directory
Remove the directory and its contents recursively.
The mkdir command
This command is used to create new a new directory. Its syntax is
mkdir -optional_flag directory_name
The possible flags are
mkdir -v directory_name
Tell what is going on.
mkdir -p directory_with_path
This is a cool command. Suppose you need a directory named SEIT within another directory called PVPP in /usr/local and the parent directory PVPP itself does not exist, then you can use :
mkdir -p /usr/local/PVPP/SEIT
This command creates the PVPP directory and the SEIT subdirectory in one go.
The man command
For someone new to linux, the man command is one of the most important commands. The syntax is:
man command_name
Suppose you have not understood fully one of the above commands or want to find out about a new command you have learnt , the man command provides a manual for that command
Thus
man cp
will show you a manual on the cp command and so on.
I think that is enough material to keep you busy for a few hours and get you through some of the elementary tasks in Linux. So farewell friends, until we meet again.
