Tips and Examples

• Mistakes That Make Your Scripts Hard to Score
• Testing Your Class Exercises
• A Subtle Pathname Problem

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

New Material

• Access Permissions
• Groups
• Viewing Access Permissions
• chmod
• Using chmod with Numeric Arguments
• chmod Practice

Graded Quiz

You can connect to Gradescope to take weekly graded quiz today during the last 15 minutes of the class.

Once you start the quiz you have 15 minutes to finish it.

You can only take this quiz today.

There is not makeup for the weekly quiz because Gradescope does not permit it.

Homework 5

I have posted homework 5 here.

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

Questions

Are there any questions before I begin?

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
• They are listed here
• Let's review them

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
• 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 by hitting Return or Enter
• When you see

  --->  Next?

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 also teaches 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

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 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

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 or script 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

• 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
• 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

• To learn the groups you belong to ...
• run the groups command

  $ groups
  faculty cs478-1 cs478-1G curcom oversight it-faculty it116-2 it116-2G it117-1 it117-1G it244-2 it244-2G

• If you run groups with a Unix username ...
• it will show the groups to which that account belongs

  $ groups it244gh
  it244gh : ugrad it-faculty

• To see which accounts are in a group use the group command

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

• Only a system administrator can add or delete users from a group

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

• 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 types of permissions
• And the third is the 3 categories 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

Class Exercise

Class Quiz