IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 9

Microphone

Questions

Are there any questions before I begin?

Homework 5

I have posted homework 5 here.

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

Tips and Examples

Mistakes That Make Your Scripts Hard to Score

I wrote the Rules for Homework Scripts to make the time I spend grading your scripts as efficient as possible
They may seem arbitrary but they make it easier for me to score your scripts
Whenever a student submits a script that violates the rules ...
it causes me extra work
That is why you lose points if you don't follow the rules
Most scripts I have seen follow these rules
But there are certain common mistakes that will cause you to lose points
The rule that causes me the most trouble when it is ignored is this

Testing Your Class Exercises

You should test each Class Exercise script using my test script
You will find these scripts in ~ghoffman/it244_test
The directory in which these script live is protected
That means you cannot use cd to go to this directory
Instead you must use the long pathname listed at the end of the Class Exercise page ...
like this for Class Exercise 8
```
~ghoffman/it244_test/ex08.sh
```
The script will then prompt you for you Unix username
```
Unix username:
```
Enter your username and hit Enter or Return

You will then see something like this

++++++++++++++++++++++++++++++++++++++++++++++++++
Error check
--------------------------------------------------
--------------------------------------------------

If there is no text between the last two lines of dashes
Then your script contains no Unix errors
Continue checking your script to see if you have used the right commands
The script is designed to give you only enough output to fit on your screen
When you see
```
--->  Next?
```
Hit Enter or Return to continue to the next series of tests

A Subtle Pathname Problem

Several semesters ago a student came to me with a problem that took me a few minutes to unravel
It all had to do with the absence of one character in a pathname
The student was trying to go to his it244 directory using the command
```
cd ~/it244
```
Once there he tried to create a file using the touch command ...
but he got a permission error
After staring at his prompt for a few minutes I realized what was happening
The student thought he had entered
```
cd  ~/it244
```
Which would have put him in his it244 directory
What he really typed was
```
cd ~it244
```
This command took him to the home directory of the it244 account, /home/it244
This account was created by Prof. Bo Sheng, who used to teach this course
It is a dummy account that he uses for teaching and testing ...
like my it244gh account
The only account that can create a file in this directory is it244 ...
Prof. Sheng's test account
Here again a single character was the source of the problem

A Trick When Creating a Long Pathname

I often make mistakes when typing
So when I have to create a long pathname ...
I often get it wrong
To deal with this I came up with a way to use ls ...
to slowly build the correct pathname
Let's say I'm in the directory holding test scripts for IT 244 ...
and I want to copy the file five_lines.txt ...
in my it244_files directory
I know that the file is somewhere under my home directory ...

So I'll start with

$ ls ~
bin                       drop_box                       it116       messages
bugs                      f24                            it116_test  public_html
code                      ghoffman                       it117       stdwrk
course_files              html                           it117_test  submitted
course_materials          hw1_piazza_filename_create.py  it244       tests_taken
course_scoring_documents  hw99.sh                        it244_test  tmp
demos                     issues                         mail        wrk

The directory I'm looking for is under the course_files directory
So I use the ↑ key to retrieve my previous command ...

then add course_files at the end

$ ls ~/course_files/
it116_files  it117_files  it244_files  it245_files

The file I want is in the it244_files directory
So I add this directory to the previous command ...
again using ↑
```
$ ls ~/course_files/it244_files/
```
The output of this command is very long but I can see the five_lines.txt file in the listitng

So I repeat this procedure to get the pathname to this file

$ ls ~/course_files/it244_files/five_lines.txt 
/home/ghoffman/course_files/it244_files/five_lines.txt

Now that I have the correct pathname ...
I can use command line editing to turn the previous command ...
into call to cp I need
I use ↑ to get the previous command ...
then use Control A to go to the start of the line ...
where I change ls to cp
Then I use Control E to go to the end of the line ...
where I add . (dot) after a space

This gives me

$ cp ~/course_files/it244_files/five_lines.txt .

Review

Hidden Filenames

A file whose filename begins with a period, . , is a "hidden" or "invisible" file
ls does not display these files unless you use the -a option
These files are used to configure your Unix environment

Startup Files

Files like .login and .bash_profile are located in your home directory
They are startup files
Startup files contain Unix commands that are run before you get the first shell prompt
These commands customize your Unix environment
You can set Unix variables which can help you with your work ...
and do other things

The . and .. Directory Entries

Every directory has at least two entries . and ..
When a new directory is created these are the first two entries
. stands for the current directory
. . stands for the parent directory of your current directory
. . is the directory immediately above your current location
. is most often used in two circumstances
- To run a program in your current directory
- To move or copy a file to your current directory

Pathnames

Every file has a pathname which is used to access the file
A pathname has two components
- A path to reach the file
- The name of the file
The path is a list of directories that you must go through ..
to reach the file you want
A path is like an address on a letter ...
or directions to get somewhere
The name of the file is always at the end of a pathname
When the slash, / appears between names in a pathname ...
it is a separator
It is used to separate a directory name from what comes after it
When a / is the first character in a pathname
it stands for the root directory
There are two types of pathnames
- Absolute
- Relative

Absolute Pathnames

The top of the filesystem is a directory called the root directory
The root directory is represented by a single slash character, /
It can stand alone or appear as the first character before a directory name
An absolute path is a list of directories starting with the root directory ...
and ending with the directory that contains the file
When you add the filename to the end of an absolute path ...
you have an absolute pathname

Tilde, ~, in Pathnames

Tilde, ~, is one of the characters that have special meaning to Unix
Bash replaces ~ with the absolute pathname of a home directory
~ by itself is replaced with the absolute pathname of
of your home directory
You can see this most clearly when you run echo on ~
```
$ echo ~
/home/ghoffman
```
~ followed by a Unix username ...
is replaced with the absolute pathname of that user's home directory
```
$ echo ~it244gh
/home/it244gh
```
If what follows the ~ is not a valid Unix username ...
no substituion will happen
```
$ echo ~xxxxx
~xxxxx
```
~ is a convenience feature provide by Bash ...
to make it easy to get to a home directory

Relative Pathnames

Absolute pathnames are useful because you can use them anywhere
But they are long and easy to mistype
For most purposes, it is easier to use relative pathnames
In a relative pathname, the path starts from your current directory
In an absolute pathname, the path starts from the root, /
While all absolute pathnames start with a slash, /, or a tilde, ~
Relative pathnames never do
As far as Unix is concerned ...
it makes no difference whether you use an absolute or relative pathname
There are four types of relative pathnames:
- When the file is in your current directory
- When the file is in a subdirectory of your current directory
- When the file is in a directory that is above your current directory
- When the file is in a directory that is neither above or below the current directory

Relative Pathnames in Your Current Directory

A relative pathname of a file or directory in your current directory is simple
It is the name of that file or directory

Relative Pathnames in a Subdirectory

Things get a little more complicated ...
when you are dealing with a file in a subdirectory
Here, you must list every directory ...
between your current directory and the file you want
You must use a slash, / after each directory name

Relative Pathnames above the Current Directory

When the file or directory is above the current directory ...
you can't list the directory names
Instead, you have to use the special . . entry in each directory
Use one . . for each directory up the chain in the path
Use a slash, /, between each ..

Relative Pathnames Neither above Nor below the Current Directory

What if the file is neither above nor below?
Here you have to go up to a common ancestor ...
and then down to the directory that holds what you want
The path starts with one or more . . ...
to get to the directory that is the common ancester of your current directory ...
and the directory holding the file you are trying to reach
Once you get to the common ancestor ...
you go down to the directory that holds the file

Attendance

New Material

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 account that created it
Every file, directory or device on a Unix filesystem ...
has three types of permissions
- Read
- Write
- Execute
If you have read permission you can look at the data in the file
You can run cat, more or less on these files
If you only have read permission on a file ...
you cannot change it
To change what is inside a file you need write permission
To run a program file you must have execute permission
If you make a Bash script executable ...
you can run it without having type bash before the name of the script
Each of the three types of permissions is either on or off
But the access permissions apply to three types of users
- The owner
- The group
- Every other Unix account
Every file or directory has an owner and a group assigned to it
The account that created the file is usually the owner
A group is a collection of Unix accounts
The last class of users ...
is any account that is not the owner or a member of the group
Unix calls this class of users "other"

Groups

A group is just a collection of Unix account names
For example there is a group called faculty ...
that contains Unix accounts of all instructors in the CS department
An individual account can be a member of more than one group
To see the groups that my account belongs to ...

I can use the group command

$ groups
faculty itclub curcom oversight it-faculty it116-1 it116-1G it117-1 it117-1G it244-2 it244-2G

To see the accounts belonging to a specific group ...

you run the group command

$ group it-faculty
msolah
tiago
teebanj
apotaszn
it244gh
ckelly
ghoffman
stran
tsoro
zyang
olivia93

On most Linux systems when a new account is created ...
a new group is also created for that account
That group has the same name as the account
At first this group only contains the username of the account
When the user creates a new file or directory ...
this special group is assigned to it
The owner can change the group assigned to a file
But they cannot change what accounts are in the group
Only the system administrator can add or remove account from a group
Our Unix machines do something different
All students accounts are assigned the default group ugrad
And all faculty are assigned the default group faculty

This group will be assigned to any file or directory I create

$ ls -l ~/tmp
total 4
drwxrwxr-x 2 ghoffman faculty 4096 May  3 15:59 dir1
-rw-rw-r-- 1 ghoffman faculty    0 May  3 15:59 foo.txt

Viewing Access Permissions

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

it244gh@itserver6:~$ ls -l
total 4
-rw-r--r-- 1 it244gh ugrad    0 May  7 09:47 foo.txt
lrwxrwxrwx 1 it244gh ugrad   36 Sep 23  2017 it116 -> /courses/it116/s19/ghoffman/it244gh/
lrwxrwxrwx 1 it244gh ugrad   36 Sep 23  2017 it117 -> /courses/it117/s19/ghoffman/it244gh/
lrwxrwxrwx 1 it244gh ugrad   36 Sep 23  2017 it244 -> /courses/it244/s19/ghoffman/it244gh/
drwxrwxrwx 5 it244gh ugrad 4096 Apr  7 09:34 tmp

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
We will discuss links in the next class
The next three characters indicate the access permissions 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 for that column
The next three characters indicate the permissions of the group
The last three characters are the permission of all other accounts
After the permissions is a number ...
that counts 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
Following this is the size of the file in bytes
Next is the date and time the file or directory was created or last modified
The last column is the name of 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 permissions you use the chmod (change mode ) 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) which will be changed

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
The numeric format is initially confusing ...
but in my experience, it is the better way to go
You must use the numeric method in all homework assignments, quizzes and tests
Scoring answers that use the symbolic method takes time I do not have

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 permissions of the owner
The second digit gives the permissions assigned to the group
The third digit gives the permissions for every other account
Each of these classes of users ..
must be assigned values for read, write and execute permissions
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 calculating a permission number you start with 0
Then 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, grants all three permissions
7 is 111 in binary
Let's look at some other digits
6 in binary is 110
The leftmost digit is 1 indicating read permission
The center digit is 1 indicating write permission
The last digit is 0 indicating that execute permission is not granted
5 in binary is 101
The first digit is 1 so read permission is granted
The second digit is 0 so write permission is not granted
The last digit is 1 so execute permission is granted
The following table shows the permissions granted

by each of the digits from 0 to 7

#	`ls -l`	Permissions
0	`---`	None
1	`--x`	Execute only
2	`-w-`	Write only
3	`-wx`	Write and execute
4	`r--`	Read only
5	`r-x`	Read and execute
6	`rw-`	Read and write
7	`rwx`	Read, write, and execute

Only a few of these combination are commonly used ...
like rwx and r--
This scheme is confusing when you first encounter it ...
but it becomes easier as you use it

Try to remember this chant

4      2      1
read   write  execute
owner  group  everyone

The first line contains the numbers added to 0 ...
to form the permissions digit
The second is the type of permission
And the third is the category of users
Repeat this to yourself several times and it should sink in
You must call chmod with three digits
Let's look at some examples
When you create a new file ...

it will have certain default permissions

$ touch foo.txt

$ ls
foo.txt

$ ls -l
total 0
-rw-r--r-- 1 it244gh libuuid 0 2012-02-09 15:51 foo.txt

The owner can read and write the file, but not execute it
The group and everyone else can only read the file

To make the file unreadable to everyone except the owner, run

$ chmod 600 foo.txt 

$ ls -l
total 0
-rw------- 1 it244gh libuuid 0 2012-02-09 15:51 foo.txt

To change the file back to its default permissions, run

$ chmod 644 foo.txt 

$ ls -l
total 0
-rw-r--r-- 1 it244gh libuuid 0 2012-02-09 15:51 foo.txt

Here are some common arguments to chmod ...

along with what you will see if you run ls -l

Digits	`ls -l`
600	`rw-------`
644	`rw-r--r--`
664	`rw-rw-r--`
666	`rw-rw-rw-`
755	`rwxr-xr-x`
777	`wxrwxrwx`

`chmod` Practice

To help you learn to use chmod I have created two pages
Permission Practice and Permission Practice Ansers
The first asks you to calculate numbers representing a certain set of permissions
The second shows you the answers

Tips and Examples

Review

New Material

Questions

Homework 5

Tips and Examples

Mistakes That Make Your Scripts Hard to Score

Testing Your Class Exercises

A Subtle Pathname Problem

A Trick When Creating a Long Pathname

Review

Hidden Filenames

Startup Files

The . and .. Directory Entries

Pathnames

Absolute Pathnames

Tilde, ~, in Pathnames

Relative Pathnames

Relative Pathnames in Your Current Directory

Relative Pathnames in a Subdirectory

Relative Pathnames above the Current Directory

Relative Pathnames Neither above Nor below the Current Directory

Attendance

New Material

Access Permissions

Groups

Viewing Access Permissions

chmod

Using chmod with Numeric Arguments

chmod Practice

Class Exercise

Class Quiz

`chmod`

Using `chmod` with Numeric Arguments

`chmod` Practice