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Directories and Files


Basics on Directories and files

Linux stores data and programs in files. These are organized in directories. In a simple way, a directory is just a file that contains other files (or directories).

The part of the hard disk where you are authorised to save data is calle your home directory. Normally all the data you want will be saved in files and directories in your home directory. To find your home directory (if you need), type:

   echo $HOME
The symbol ~ can also be used for your home directory.

There is a general directory called /tmp where every user can write files. But files in /tmp usually get removed (erased) when the system boots or periodically, so you should not store in /tmp data that you want to keep permanently.



What is in a name?

A file can be fully and uniquely identified by its full name, including all directories to which it belongs. The system starts at the root directory, with name / The it "splits" into (sub)directories, and these split further, and so on, until you get to a file. For example, a home directory could be /usr15/pablo, on which there is a directory called programming, with a directory inside called include, on which there is a file called time.h The the full path of this last file will be

   /usr15/pablo/programming/include/time.h


Creating and Removing directories

To make a new directory do:

   mkdir directory-name
To remove a directory that does not files inside do:
   rmdir directory-name
If the directory has files, you can do the following:
   rm -rv directory-name
and you will be asked for each file (or subdirectory) if you want to remove it; or
   rm -frv directory-name
that will remove all files (and subdirectories) without asking any questions.

Changing the working directory

To change ("enter") into a directory do:

   cd directory-name
This assumes that the new directory is a subdirectory of the one you are currently working on. If that is not the case, you will have to type the name, for example:
   cd /usr/local/share/bin
To go to your home directory do simply:
   cd


Renaming directories

To change the name of a directory do:

   mv directory-name new-name
It will move all the files inside as well.

Creating and Removing files

Creating files can be done in many different ways. Here are a few examples:



Renaming files

Similar to renaming a directoty:

   mv file-name new-name


Looking at the content of a file

Again, there are many ways to look at it:

Not all files can be read in the screen. Some times after looking at the contents of a file your screen gets into some "funny" characters; to go back to the regular characters use
   reset
Type this command even if you can't read what is shown in the screen. Perhaps you need to do it twice before your screen comes back to normal.

Here is a list of some utilities to open/read the contents of files.

Type of file Common Extensions Programs to open file
Text .txt, .text less, more, cat
Images .jpg, .gif, .pnm, .xpm, .bpm xv, display
PostScript .ps gv, ghostview
PDF .pdf acroread, gv
DOS-generated .rtf, .doc, .xls openoffice (soffice), abiword
ZIP compression .zip unzip
GZIP compression .gz gunzip
BZIP compression .bz, .bz2 bunzip, bunzip2
Tar archive .tar tar
Tar and gzip compression .tar.gz, .tgz tar -z
Tar and bzip compression .tar.bz, .tar.bz2 tar -j
DOS excutable .exe strings
Movie .mpeg, .mpg mplayer, totem, xmovie
cpio archive .cpio cpio
ar archive .ar ar
Debian archive .deb less
Redhat archive .rpm rpm
sound .wav bplay
MP3 sound .mp3 mpg321
HTML .html, .htm firefox, mozilla, netscape, lynx


Types of files

The system has files of many different type (though for the operating system they are all equivalent). To find the type of a file do

   file file-name

As discussed above, a directory is just a file whose contents are file (names). Another interesting \lq\lq type\rq\rq\ of files are symbolic links; a symbolic link is just another name for a file. Why should you do that? Suppose you have some interesting information on how to use ppp, and you want to have it in your directory called ppp-info as well as in the directory called doc. You could save the file in the ppp-info directory and then copy it to the doc directory. Or you can just have the data in the file howto-ppp in the ppp-info directory and then do (from the doc directory, assuming doc and ppp-info are in the same \lq\lq level\rq\rq ):

   ln -s ../ppp-info/howto-ppp howto-ppp
Then you can see the howto-ppp from either the ppp-info or the doc directory. Having two files with the same name in this case is okay since the full names are different (the full names will include the directories names, which are different). Both files are equal, editing one will make the changes in the other, since, after all, one file contains the data and the other is just a name. However, there is a difference at the time of removing the files: if you remove the file ppp-info/howto-ppp then the file doc/howto-ppp will be a name for a non-existing file, so it will have nothing in it. But if you remove doc/howto-ppp the data will remain in ppp-info/howto-ppp, since after all what you have done is just to remove a name.

Permissions

In Linux files come with permissions, a way to decide who can read, write (or execute) a file. These permissions are divided into three parts: those for the owner (user) of the file, those for the group to which the owner belongs and then permissions for all the other users (each account, besides a name, has a group or groups to which it belongs; the groups are generally used for administrative stuff). From the point of view of what is allowed to do in a file or directory, permissions are for reading, writing and executing.

To look at the permissions of a file you can use the command

   ls -l file-name
The first ten characters will give you the type of file (- for a file, l for a link and d for a directory), the permissions of the owner, the group and all other users. So, if we forget the first character we have read, write and execute permissions. If the permission is granted then the letters r, w or x will appear in the output; otherwise a symbol - will appear. For example, a file with read and write permissions for the owner, read for the group and execute for others (a strange combination!) will show as
-rw-r----x
To change the permissions of a file or directory you can use the chmod command. Just put first the accounts to which you want to apply the changes (u for user or owner, g for group and o for others), then whether to grant (+) or remove (-) permissions and then r, w or x for the corresponding type of permission. For example
   chmod g+rw file-name
   chmod o-rw file-name
will grant read and write permissions to the group for file-name and remove those same permissions for all other users (of course, the owner falls in the group, so he/she will have permissions).

Some other commands on directories and files (mc)

The Midnight Commander is a browser for easy management of your files and directories. You can use the \lq\lq F\rq\rq\ keys, usually located on top of the keyboard, to do different operations on files. The help for these keys is at the bottom of the screen, but they are just labeled with numbers, no F in it. Here is a screen shot of mc:


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