More details about LINUX

3. Understanding users and permissions


Linux is based on the idea that everyone using a system has their own username and password.



Every file belongs to a user and a group, and has a set of given attributes (read, write and executable) for users, groups and all (everybody).



A file or folder can have permissions that only allows the user it belongs to to read and write to it, allowing the group it belongs to to read it and at the same time all other users can't even read the file.



4. Who and what is root

Linux has one special user called root (this is the user name). Root is the "system administrator" and has access to all files and folders. This special user has the right to do anything.



You should never log on as this user unless you actually need to do something that requires it!



Use su - to temporary become root and do the things you need, again: never log into your sytem as root!



Root is only for system maintenance, this is not a regular user (LindowsOS don't have any user management at all and uses root for everything, this is a very bad idea!).



You can execute a command as root with:



su -c 'command done as root'



Gentoo Linux: Note that on Gentoo Linux only users that are member of the wheel group are allowed to su to root.



5. Opening a command shell / terminal

To learn Linux, you need to learn the shell command line in a terminal emulator.



In KDE: K -> System -> Konsoll to get a command shell)



Pressing CTRL-ALT-F1 to CTRL-ALT-F6 gives you the console command shell windows, while CTRL-ALT-F7 gives you XFree86 (the graphical interface).



xterm (manual page) is the standard XFree console installed on all boxes, run it with xterm (press ALT F2 in KDE and Gnome to run commands).



Terminals you probably have installed:



xterm http://dickey.his.com/xterm/

konsole (KDEs terminal)

gnome-terminal (Gnomes terminal)

Non-standard terminals should install:



rxvt http://www.rxvt.org/

aterm http://aterm.sourceforge.net

6. Your first Linux commands

Now you should have managed to open a terminal shell and are ready to try your first Linux commands. Simply ask the computer to do the tasks you want it to using it's language and press the enter key (the big one with an arrow). You can add a & after the command to make it run in the background (your terminal will be available while the job is done). It can be practical to do things like moving big divx movies as a background process: cp movie.avi /pub &. Jobs - the basics of job control



6.1. ls - short for list

ls lists the files in the current working folder. This is probably the first command to try out. It as a number of options described on the ls manpage.



Examples:



ls



ls -al --color=yes



6.2. pwd - print name of current/working directory

pwd prints the fully resolved name of the current (working) directory. pwd manpage.



6.3. cd - Change directory

cd stands for change (working) directory and that's what it does. The folder below you (unless you are in /, where there is no lower directory) is called "..".



To go one folder down:



cd ..



Change into the folder Documents in your current working directory:



cd Documents



Change into a folder somewhere else:



cd /pub/video



The / in front of pub means that the folder pub is located in the / (lowest folder).



7. The basic commands

7.1. chmod - Make a file executable

To make a file executable and runnable by any user:



chmod a+x myfile



Refer to the chmod manual page for more information.



7.2. df - view filesystem disk space usage

df -h



Filesystem Size Used Avail Use% Mounted on

/dev/hda3 73G 67G 2.2G 97% /

tmpfs 2.0M 24K 2.0M 2% /mnt/.init.d

tmpfs 252M 0 252M 0% /dev/shm





The flags: -h, --human-readable Appends a size letter such as M for megabytes to each size.



df manpage



7.3. du - View the space used by files and folders

Use du (Disk Usage) to view how much space files and folders occupy. Read the du manual page for flags and usage.



du is a part of fileutils.



Example du usage:



du -sh Documents/

409M Documents





7.4. mkdir - makes folders

Folders are created with the command mkdir:



mkdir folder



To make a long path, use mkdir -p :



mkdir -p /use/one/command/to/make/a/long/path/



Like most programs mkdir supports -v (verbose). Practical when used in scripts.



You can make multiple folders in bash and other shells with {folder1,folder2} :



mkdir /usr/local/src/bash/{old,new,dist,bugs}



mkdir manual page



The command rmdir removes folders.



7.5. passwd - changes your login password

To change your password in Linux, type:



passwd



The root user can change the password of any user by running passwd with the user name as argument:



passwd jonny



will change jonnys password. Running passwd without arguments as root changes the root password.



If you need to add several new users and give them password you can use a handy program like Another Password Generator to generate a large set of "random" passwords.



7.5.1. KDE

From KDE you can change your password by going:



K -> Settings -> Change Password

K -> Settings -> Control Center -> System Administration -> User Account

7.6. rm - delete files and folders, short for remove

Files are deleted with the command rm:



rm /home/you/youfile.txt





To delete folders, use rm together with -f (Do not prompt for confirmation) and -r (Recursively remove directory trees):



rm -rf /home/you/foo/





Like most programs rm supports -v (verbose).



rm manual page



7.7. ln - make symbolic links

A symbolic link is a "file" pointing to another file.



To make a symbolic link :



ln /original/file /new/link





This makes /original/file and /new/link the same file - edit one and the other will change. The file will not be gone until both /original/file and /new/link are deleted.



You can only do this with files. For folders, you must make a "soft" link.



To make a soft symbolic link :



ln -s /original/file /new/link





Example:



ln -s /usr/src/linux-2.4.20 /usr/src/linux





Note that -s makes an "empty" file pointing to the original file/folder. So if you delete the folder a symlink points to, you will be stuck with a dead symlink (just rm it).



ln manual page



7.8. tar archiving utility - tar.bz2 and tar.gz

tar (manual page) is a very handle little program to store files and folders in archives, originally made for tapestreamer backups. Tar is usually used together with gzip (manual page) or bzip2 (manual page), comprepssion programs that make your .tar archive a much smaller .tar.gz or .tar.bz2 archive.



kde



You can use the program ark (K -> Utilities -> Ark) to handle archives in KDE. Konqueror treats file archives like normal folders, simply click on the archive to open it. The archive becomes a virtual folder that can be used to open, add or remove files just as if you were working with a normal folder.



7.8.1. tar files (.tar.gz)

To untar files:



tar xvzf file.tar.gz





To tar files:



tar cvzf file.tar.gz filedir1 filedir2 filedir2...





Note: A .tgz file is the same as a .tar.gz file. Both are also often refered to as tarballs.



The flags: z is for gzip, v is for verbose, c is for create, x is for extract, f is for file (default is to use a tape device).



7.8.2. bzip2 files (.tar.bz2)

To unpack files:



tar xjvf file.tar.bz2





To pack files:



tar cvjf file.tar.bz2 filedir1 filedir2 filedir2...





The flags: Same as above, but with j for for bzip2



You can also use bunzip2 file.tar.bz2 , will turn it into a tar.



For older versions of tar, try tar -xjvf or -xYvf or -xkvf to unpack.There's a few other options it could be, they couldn't decide which switch to use for bzip2 for a while.



How to untar an entire directory full or archives?



.tar:



for i in `ls *.tar`; do tar xvf $i; done



.tar.gz: for i in `ls *.tar.gz`; do tar xvfz $i; done



.tar.bz2: for i in `ls *.tar.bz2`; do tar xvfj $i; done