IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 10

Today's Topics

Reading Assignment

The reading assignment for this week is chapter 4 of Sobell, The Filesystem.

Homework 5

I have posted homework 5 here.

It is due this coming Sunday at 11:59 PM.

Mid-term

The mid-term exam for this course will be held on Tuesday, October 24th.

It will consist of 25 questions like those on the quizzes.

You will have the entire class period to work on the exam.

60% of the questions will come from the Ungraded Class Quizzes.

The last class before the exam, Thursday, October 19th, will be a review session.

You will only be responsible for the material in the Class Notes for that class on the exam.

The Mid-term is a closed book exam.

Tips and Examples

Using Pathnames

If you want to work with a file ...
you usually go to the directory that contains the file
When you do this you can use just the name of the file ...
when you need to run a command on it
I have some web pages that offer help with Linux
They are contained in the directory /home/ghoffman/public_html/linux
One of these files is common_unix_commands.html
If I wanted to know whether I had mentioned the finger command on this page ...
and I were in the directory that held the file ...
I would simply type
```
$ grep finger common_unix_commands.html
```
If I were in my home directory and didn't want to leave it ...

I could use a relative pathname

grep finger public_html/linux/common_unix_commands.html

The file I want to access is below my current directory ...
so I have to list all the directories between my current directory ...
and the directory that contains the file
What if I wanted you to make a copy of this file to practise with?
Each of you would be in a different directory when you copied the file ...
so I would give each of you an absoluted pathname ...

and you would type

cp  /home/ghoffman/public_html/linux/common_unix_commands.html   .

Practice with Access Permissions

You will need to know how to use chmod to change access permissions ...
on the mid-term and final exams
You will get some additional practice with chmod ...
in the Class Exercise for this class
I have also created a web page to give you extra practice with chmod
You will find this page here
The page also has a link to the answers
There is a link to the chmod practice page ...
on the class web page

Review

Access Permissions

All Unix files and directories have access permissions
The access permissions allow the owner of a file or directory ...
to decide who gets to do what with the file or directory
By default, the owner of a file or directory ...
is the user account that created it
Every file, directory or device on a Unix filesystem has three types of permissions
- Read
- Write
- Execute
Each access permission can either be on or off
If you have read permission on a file you can look at the data in the file
If you only have read permission, you cannot change a file
To change a file you need write permission
You cannot run a program or script file ...
unless you have execute permission on that file
Each of the three permissions is set either on or off ...
to three classes of users
- The owner
- The group
- Every other account
Every file or directory has an owner
The account that created the file is usually the owner
A group is a collection of Unix accounts
A group can only be set up by a system administrator
Every file or directory is assigned to a group
The last class of users is everyone else ...
any account that is not the owner or a member of the group

Viewing Access Permissions

To view the access permissions of a file or directory use ls -l

$ ls -l
total 5
-rw------- 1 it244gh libuuid 316 2011-09-20 21:32 dead.letter
lrwxrwxrwx 1 it244gh libuuid  34 2011-09-06 13:21 it244 -> /courses/it244/s12/ghoffman/it244gh
drwx------ 2 it244gh libuuid 512 2011-09-07 15:03 mail
drwxr-xr-x 2 it244gh libuuid 512 2011-09-25 15:48 test
-rw-r--r-- 1 it244gh libuuid  15 2011-09-20 16:18 test.txt

The character in the first column indicates the type of file
A dash, -, means an ordinary file
The letter d indicates a directory
The letter l (el) indicates a link
The next three characters indicate the permission of the owner
r means the owner has read permission
w means the owner has write (change) permission
x means the owner has execute (run) permission
-means the owner does not have the permission that would normally appear in this column
The next three characters give the permissions of the group
The last three characters are the permission of all other accounts
The next column is a number that indicates the number of links to the file or directory
The following column is the owner of the file or directory
Next, you will find the group assigned to the file or directory

`chmod`

When a file is created, is has certain default permissions

$ touch test.txt
$ ls -l test.txt 
-rw-r--r-- 1 it244gh libuuid 0 2012-09-17 14:40 test.txt

To change these permission, you need to use the chmod command
Only the owner of a file can do this
chmod requires two arguments
- The permissions you want to grant
- The name of the file(s) or directory(s) to which the change will be applied

The format for a call to chmod is

chmod  PERMISSIONS  FILES_OR_DIRECTORIES

The permission can be specified in two ways
- Symbolically
- Numerically
Symbolic permissions use letters and the plus and minus signs
The numeric form uses three digits running from 0 to 7
I will teach the numeric format for expressing permissions
You are free to read about the symbolic format in the textbook
I will not deduct points for using symbolic format ...
as long as you use it correctly

Using `chmod` with Numeric Arguments

The numeric permissions format uses three digits
Each digit is a number from 0 to 7
The first digit gives the permission of the owner
The second digit gives the permissions assigned to the group
The third digit gives the permissions for every other account
How do you get three pieces of information out of one number?
By adding powers of two
Each digit is the sum of three other numbers
When constructing the number you add
- 4 if you want to give read permission
- 2 if you want to give write permission
- 1 if you want to give execute permission
Notice that all the number are powers of two
If we write these values in binary notation
- 100 represents 4
- 010 represents 2
- 001 represents 1
A single decimal digit from 0 to 7 is represented by 3 binary digits
This is how we get three pieces of information out of one digit
For example, to give full permissions I would add
- 4 for read permission
- 2 for write permission
- 1 for execute permission
So the total, 7, represents all three permissions

Try to remember this chant

read    write   execute
4       2       1
owner group    everyone

Remember that you need three of these digits ...
to specify the full permissions for a file or directory

New Material

The root Account

On every Unix or Linux system there is a special account named root
This account is sometimes referred to as the superuser
root can access any file or run any program
root is an administrator account
It is used for system configuration and maintenance
Even a system administrator should not log in as root
Instead he or she should use a regular Unix account ...
and should switch to the root account only when they need its power
This can be done with the su (switch user) command
To become the root user you would enter the following on the command line
```
su -
```
This will only work if you know the root password
A better way of doing this is to use the sudo command
sudo stands for superuser do
You type sudo and then the command ...
that only the root user can run
You use it like this
```
sudo COMMAND_ONLY_ROOT_CAN_RUN
```
You will then be prompted for your password ...
not the root password
This will only work if the system administer ...
has added you to the sudoers list
sudo is safer than using su - ...
because the person using it does not have to know the root password
If all administrators knew the root password ...
you would have to change it every time one of them left
If they all use sudo ...
all you would have to do when they left ...
would be to delete their entry in the sudoers list

Setuid and Setgid Permissions

Sometimes a program needs to read or modify a file ...
to do the work it was designed to do
For example the passwd command ...
which is used to change the password for an account ...
has to make changes to the file /etc/shadow
When you need to change your password you run passwd
But /etc/shadow is owned by the root account ...
and no other account can change it
To deal with situations like this, two special permissions were created
- setuid
- setgid
If a program file has setuid permission ...
any account that runs the program ...
has all the permissions of the owner of that program file ...
but only while the program is running
This permission means a user can modify files ...
that the program needs to do its job ...
that ordinary users cannot change
If a prgram has setgid permission set ...
anyone who runs the program ...
has the permissions of the group assigned to that program...
while the program is running
setuid and setgid permissions only apply to executable files ...
that is, programs and scripts
A file with setuid permission ...
will have s in place of x ...
in the column for the owner's execute permission
A file with setgid permission ...
will have s in place of x ...
for the group execute permission
Since setuid and setgid permission ...
apply only to executable files ...
there is no ambiguity in replacing x with s
For example, consider the passwd command
The executable file for this utility is /usr/bin/passwd

Running ls -l on this file we get

ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 42824 2011-02-20 19:18 /usr/bin/passwd

Notice the s in the place ...
where the owner's execute permission should be
The passwd command needs to modify /etc/shadow
which is a file that stores the encypted passwords

Only root can change this file

ls -l /etc/shadow
-rw-r--r-- 1 root shadow 926 Jul 16  2013 /etc/shadow

But you need to change this file ...
for your entry only ...
when you change your password
The setuit permission allows you to run passwd and modify /etc/shadow ...
but passwd knows who you really are ...
and it will only let you modify your own entry ...
not the entry for any other account

Setting Setuid and Setgid with `chmod`

How do you assign setuid or setgid permissions to a file?
You do it with chmod ...
and an extra digit
If I wanted to assign 755 permission to a script ...
along with setuid permission ...
I would use the 4755
When 4 is the first digit in a series of four digits used with chmod ...

setuid permission is assigned to the file

$ chmod 4755 work.sh 

$ ls -l work.sh 
-rwsr-xr-x 1 ghoffman grad 0 Mar  3 12:30 work.sh

To assign the setgid permission ...

I would use 2 instead

$ chmod 2755 work.sh 

$ ls -l work.sh 
-rwxr-sr-x 1 ghoffman grad 0 Mar  3 12:30 work.sh

Setuid and setgid will not be on any test or quiz

Directory Access Permissions

Unix permissions work a little differently for directories
Read and write permissions for a directory ...
are similar to those for a file
Read permission on a directory ...
allows you to list the contents using ls
So read permission on a directory ...
only allows you to run ls on it
To read the files in the directory ...
you need read permission for each file
So read permission on a directory ...
does not allow you to read the files in that directory
You need read permission on the file to do that
Write permission on a directory ...
allows you to create or delete anything in that directory ...
and to change the name of anything inside that directory
But you cannot change the files themselves ...
unless you have write permission on those files
Note that write permission on a directory ...
only apllies to the contents of a directory ...
not to the dirctory itself
You cannot change the name of a directory ...
or delete it
unless you have write permission on its parent directory
Execute permission on a directory is very different ...
from execute permission on a file
You can't run a directory from the command line ...
like a program or script
Execute permission on a directory allows you to do two things
- Enter the directory using cd
- Run ls on a file in the directory
  if you know its name
So if you know the name of a file in some directory ...
and you do not have read access to the directory ...
but you have execute access ...
and you know the name of a file ...
you can run ls or ls -l on that file

Links

Sometimes, it is convenient to have more than one way ...
of getting to a file or directory
Windows, for example has shortcuts
You can create a shortcut anywhere ...
and clicking on it will take you to the real file ...
somewhere else on disk
This allows the user another way of getting to the file
Instead of going to the directory that holds the file
you can click a shortcut in some other directory
In Unix these pointer files are called links
Links can be very useful when moving to a directory ...
that is far away from your current directory
If your current directory has a link to another file or directory ...
you can use the link to access that file or directory
This saves you the bother of using a long absolute ...
or relative pathname

For example, each of you has an entry in your home directory called it244

$ ls -l it244
lrwxrwxrwx 1 ghoffman faculty 36 Oct  5 11:42 it244 -> /courses/it244/sum17/ghoffman/ghoffman

Notice the l (el), the first character in the permissions string
This tells you that you have a link named it244, not a directory
The real directory is ghoffman in /courses/it244/sum14/ghoffman
But you can use the link just as if it were a directory
In the home directory of my test account it244gh is link ...
to a directory for this account in the course directory /courses/it244/f14/ghoffman

If you cd to the link, you will go to the real directory

If you cd into this location and use pwd

$ pwd
/home/it244gh

$ cd it244

$ pwd
/home/it244gh/it244

The path that pwd prints ...
is the route you took to get to the current directory
But it is not the real path to the directory
You can only get the true location ...

if you use pwd with the -P option

$ pwd
/home/it244gh/it244

$ pwd -P
    /courses/it244/sum14/ghoffman/it244gh

You must use a capital P, not a lowercase p
Unix tries to hide your real location when you use a link ...
so as not to confuse you
In this case, we used a link named it244 ...
inside the home directory of my it244gh account ...
to get to /courses/it244/sum14/ghoffman/it244gh

I can get back to where I came from using ..

$ pwd
/home/it244gh/it244

$ cd ..

$ pwd
/home/it244gh

Why does your home directory have a link to your it244 class directory?
To make things easier for instructors like me
If the directory in which you do your course work were in your home directory ...
I would have to go to many places to collect your assignments
Instead I only have to go one place ...

and if I run ls is see each of your course directories

$ ls
ahmadzaidan  enwolny   jnouider  mohammad  stephani
almonte2     etsai001  jswesson  natalief  subrina
ariley2      ghoffman   jwiriadj  newton    sullivan
bedewy99     GROUP     keller01  nholobo   test
bghanem      hhxxxooo  khartman  patrick8  thall
bozheng      htran18   ktgav     piperb1   thill
bradena      hung721   ktung     pjadik    tjo02124
bstoker      ifiallo   litav     qleegain  vlimage
cribeiro     it244gh   maher     rehmank   wwong89
deng5        jason123  MAIL      rhalhass  ymao
dhorard      jflores   mfdal     rtamw     zfritz
diemtr       jnewman   mlebrun7  sayaz     ziliang

The it244 link in your home directory ...
points to your course directory in /courses/it244/f14/ghoffman
The name of your course directory is your Unix username

The Two Types of Links

There are two types of links
- Hard links
- Symbolic, or soft, links
Hard links are older
But they are seldom used these days
A hard link is like a duplicate file name
If you have a hard link to a file ...
and the original filename is deleted ...
the file will still be there
The file will remain until the last hard link is removed
Hard links have some disadvantages
Hard links can only point to files, not directories
Our Unix filesystem appears to be a single hierarchy
In reality it is a collection of different file systems ...
on different hard disc volumes
The different file systems are stitched together ...
so they look like a single system
Unix hides this fact from users
But this causes problems for hard links
You can only have a hard link to a file in the same volume ...
as the link you are creating
That means you can't link to a file on a different disk or partition
Symbolic links are much more flexible
Symbolic links are sometimes called soft links
You can use either an absolute or relative pathname
when creating a symbolic link
A symbolic link can point to a file or directory ...
on any disk or partition
Deleting a soft link does not delete the file or directory it points to
You can delete a file or directory that has a soft link pointing to it ...
without deleting the link
The symbolic link remains ...
but it points to nothing

`ln`

Use ln to create a link
To create a symbolic or soft link, use ln with the -s option
Otherwise you will create a hard link
That is not what you want
ln takes two arguments
```
ln  -s  PATHNAME  LINK_NAME
```

For example

$ pwd
/home/it244gh

$ ln  -s  ~ghoffman/course_files/it244_files  examples

$ ls -l examples
lrwxrwxrwx 1 it244gh libuuid 28 2012-09-17 17:53 examples -> /home/ghoffman/examples_it244

Removing a Link

To delete a link, use rm
This will work whether the link points to a file or a directory
If you delete a symbolic link ...
it will not affect the file or directory it points to
If you delete a hard link ...
you will not delete the file ...
unless the link is the last connection to the file

Today's Topics

Tips and Examples

Review

New Material

Reading Assignment

Homework 5

Mid-term

Tips and Examples

Using Pathnames

Practice with Access Permissions

Review

Access Permissions

Viewing Access Permissions

chmod

Using chmod with Numeric Arguments

New Material

The root Account

Setuid and Setgid Permissions

Setting Setuid and Setgid with chmod

Directory Access Permissions

Links

The Two Types of Links

ln

Removing a Link

Attendance

Class Quiz

`chmod`

Using `chmod` with Numeric Arguments

Setting Setuid and Setgid with `chmod`

`ln`