- Permission groups
- superuser (su), sys admins
- owner, the user who made the file/directory, ex. someone
- group, a subset of users with a distinct name, ex. student
- others, everyone else, public, guest, untrusted unsers
- Types of permissions
- w: Write, can modify
- r: Read, can read
- x: eXecute, run a program, or list a directory
- -: none
It is specified who can do what, ex:
- owner can write, read and execute
- its group can read and execute, but not modify
- others cannot do anything
The superusers (su) can do anything at any time, only a superuser can make an other user super.
These can be expressed with a set of three alpha-numeric charactes:
- u: user, owner
- g: group
- o: others
- a: all of the above
- read: r or 4
- write: w or 2
- execute: x or 1
- none: - or 0
The a is not the same as o, because it is possible that a group does not have a permission, but someone outside of the groups has.
The permissions can be represented with a 10 character string:
$ ls -l ~ drwxr-xr-x 8 borbely student 4096 Aug 30 23:24 Desktop drwxr-xr-x 2 borbely student 4096 Mar 27 2012 Downloads drwxr-xr-x 2 borbely student 4096 Oct 20 2009 Drives drwx------ 2 borbely student 4096 Apr 20 10:42 mail drwxr-xr-x 7 borbely student 4096 Sep 6 13:01 public_html $ _
The public_html folder is owned by borbely, its group is student, permissions: drwxr-xr-x
- type: it is a directory
- my permissions rwx means that owner can do anything
- other students' permissions: r-x, they can read but not write
- others: r-x, they can still read but not write
Numerically, you can encode this in 3 digits. The three numbers are (from left to right): owner, group and others. The munbers 4: read, 2: write, 1: execute. They can be added. Example: /home/student/borbely/public_html has permission: 755 meaning
- owner (borbely) 7=4+2+1: read, write, execute
- group (student) 5=4+1: read and execute
- other 5=4+1: read and execute
You can change the permissions with chmod Examples:
- chmod 700 ~/info_hazi: nobody can see anything, except me (of course superusers still can see it).
- chmod 750 ~/important_work: In this folder one can share data with group members, but not others. Also group can only see it, not modify.
- chmod 754 -R ~/public_html: The -R applies the permissions recursively in the subfolders of the folder, and every file in it.
- man chmod
We will make our own homepage today!
Creating your homepage
Using the Linux konsole create a directory named public_html in your home directory:
$ cd ~ $ mkdir public_html $ cd public_html
~/public_html$ wget math.bme.hu/~kkovacs/sample.html ~/public_html$ mv sample.html index.html
Now we can check out our hompage at: math.bme.hu/~<username>. Check out the contents of index.html:
~/public_html$ gedit index.html
Every browser has a View source function. This is the raw HTML data of the site, when you open a webpage this is the first thing that gets downloaded to show you the site.
<tag> ... content </tag>
It usually isn't case sensitive, but there are many types. The basic structure of HTML:
<!DOCTYPE ...> <html> <head> ... content descriptions, meta-data </head> <body> ... the homepage itself </body> </html>
There are tags that don't need to be closed, e.g.:
<img ...> <br/>
<img ...>XYZ</img> <br> ... </br>
Some tags have optional attributes:
<div align="center"> ... </div> <img width="100" ... />
We can provide comments in the source code. These don't appear on the webpage itself, but can be read through the source of the webpage.
<body> Content <!-- Comment --> </body>
Before starting these tasks rename your index.html to sample.html and start with a new file named index.html that only includes this HTML code:
<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <title> Title of the page </title> </head> <body> Website </body> </html>
See the source of this page for example to start with.
- Provide a title to your homepage e.g.: Homepage of XY (title tag).
- Write a welcome messege for your homepage (h1, h2, etc. tags).
- Write a short CV for your homepage, it doesn't have to be perfect, but try to use nice formatting with div, p, table, list tags.
- Write some links, maybe a list of links (ol, ul, dl these are list tags), you can write links that point to useful sites, or someone else's homepage. (a tag for links)
- Put a picture on your homepage, it should be a picture of you, but for the sake of practice it can be anything.
- Create another page (a separate .html file), on this page create a table with your current timetable. Provide a link on your main page (index.html) that points to this page. And another from this timetable page to the main page.
- Validate your page with the validator, green means okay, if there are some red errors, try to correct those.
- Beautify your site, write about yourself.