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Linux permissions

  • 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:

1 2 3 4 5 6 7 8 9 10
type owner group others
read write execute read write execute read write execute
$ 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.

More info


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
~/public_html$ mv sample.html index.html

Now we can check out our hompage at:<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.


HTML is a type of XML markup language, it consists of tags:

...  content

It usually isn't case sensitive, but there are many types. The basic structure of HTML:

<!DOCTYPE ...>
  ... content descriptions, meta-data
  ... the homepage itself

There are tags that don't need to be closed, e.g.:

 <img ...>

Instead of:

 <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.

  <!-- Comment -->


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">
        <meta charset="utf-8">
            Title of the page

See the source of this page for example to start with.

  1. Provide a title to your homepage e.g.: Homepage of XY (title tag).
  2. Write a welcome messege for your homepage (h1, h2, etc. tags).
  3. 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.
  4. 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)
  5. Put a picture on your homepage, it should be a picture of you, but for the sake of practice it can be anything.
  6. 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.
  7. Validate your page with the validator, green means okay, if there are some red errors, try to correct those.
  8. Beautify your site, write about yourself.

Useful links

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