IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 4

Microphone

Homework 2

I have posted homework 2.

As usual, it will be due this coming Sunday at 11:59 PM.

First Graded Quiz

The first graded quiz will be given on Tuesday next week.

The questions from the graded quiz are taken from the ungraded quizzes of the previous week.

Unless I tell you otherwise, you should expect a graded quiz each week.

The quiz will be available on Gradescope on Tuesday.

The quiz will be available from the Gradescope web site but only for the last 15 minutes of the class.

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

If you do not take the quiz when it is offered you cannot take it at another time because Gradescope does not permit this.

If you fail to take the quiz but have a valid excuse, send me an email.

Gradescope

In order to take the quiz on Gradescope, you need to sign in to my Gradescope section.

Sometime during the next few days I will enter all your UMB email addresses into Gradescope.

You will receive an email from Gradescope.

What happens next depends on whether you have already registered with Gradescope.

If you are new to Gradescope

Click the "Set your password" link
Confirm your password
Click Save
You will then be taken to the page for your course

If you are already registered with Gradescope

Click the link to the course
At the login screen enter your UMB email address and your Gradescope password
If you have forgotten you password, click the "Forgot your password"link

You will find more information about Gradescope here.

Recordings of Class Meetings

You will find links to the recordings of class meetings here.

Questions

Are there any questions before I begin?

Tips and Examples

The `putty` Blank Screen Problem

Sometimes student who connect pe15.cs.umb.edu using putty get a blank screen
With no login prompt
This happens when you make 5 bad login attempts
The Unix system will block your login for 2 hours
This is a security feature to prevent password stealing

Use Flash Cards to Prepare for Graded Quizzes

To prepare for the graded quizzes you need to study the Class Quiz answers for the previous week
The best way to do this is to use flash cards
You take an index card and write the question on one side and the answer on the other
You should make a set of flash cards for each class
Label them with the Class Number and put a rubber band around them
I did this when taking courses for my Masters degree
I went through them on my way to work on the T
There are programs that you can use to create virtual flash cards
But the simple act of writing each card helps you remember the material

Always Check Your Work

It is easy to make a mistake when writing scripts
That's why it is very important to check every script you write
Doing this is easy for the Class Exercises
I have created scripts to test the scripts you write for each Class Exercise
You can use them too
You will find them in the directory ~ghoffman/it244_test
But you cannot go to that directory to run the script
To run the scripts use the pathname at the bottom of each Class Exercise
To run the test script for today's Class Exercise use
```
~ghoffman/it244_test/ex04.sh
```
Enter your Unix account name when prompted

Review

The Unix `script` Command

The script command creates a transcript of your Unix session
By default, script creates a text file named typescript in your current directory
Running script a second time will overwrite the previous typescript file
To prevent this from happening run
```
script -a
```
With the -a option, script will add to the previous typescript file
When you have finished with your script session you must type "exit"
Otherwise you will lose the entire session

Correcting Mistakes on the Command Line

You can correct mistakes on the Unix command line
Use the following Control key combinations
- Control A - Move the text insertion point to the beginning of the command line
- Control E - Move the text insertion point to the end of the command line
- Control K - Remove everything from the text insertion point to the end of the line
- Control U - Remove everything from the text insertion point to the beginning of the line
- Control Y - Pastes back on the command line what was removed with either Control K or Control U
The Left and Right arrow keys, ← and → move the text insertion point

Retrieving Your Last Command Line Entry

To retrieve the previous command, hit the Up arrow key, ↑
You can do this several times to go back to any previous command
The Down arrow key, ↓ takes you in the opposite direction

Stopping a Running Program

You can abort a running program using Control C

Getting Help with Unix Commands

Many Unix utilities have a help option
The help option provides some basic information about the command
Some commands use -h as the help option
But most use --help
For more information, use man or info
Follow man or info with the name of the command

Attendance

New Material

Special Characters

There are some characters that have special meaning in Unix
They should not be used in file or directory names

They are

& : | * ? ' " [ ] ( ) $ < > { } # / \ ! ~

Three other characters, called whitespace, are also special
The whitespace characters are
- Space
- Tab
- Newline
Spaces are used to separate entries on the command line
They separate the command from its options and its arguments
Tab has a special meaning we will discuss in another class
The newline character is what you get when you hit Enter on a PC ...
or Return on a Mac
When you are typing in a text editor, the newline moves you down to the next line
On the command line, hitting Enter or Return tells the shell you are done typing a command ...
and the shell should run the command you just entered

Quoting and Escaping

If you need to use a special character on the command line you must do two one of two things
- Use quotes
- Escape the special character
Escaping means putting a backslash, \ , in front of a special character
This turns off the special meaning of the character
The backslash must come immediately in front of the special character
The echo command prints whatever comes after it
It is used in scripts to print a message to the user
```
$ echo Hello World!
Hello World!
```
Here's what happens when I try to use echo with the special character ~
```
$ echo ~
/home/ghoffman
```
You'll learn what ~ means in a future class
I can use ~ with echo if put a backslash in front of it
```
$ echo \~
~
```
You can use quotes to do the same thing
```
$ echo '~'
~

$ echo "~"
~
```
There are two types of quotes
Single quotes - ' '
Double quotes - " "
They have slightly different purposes
But you don't need to worry about the difference for now

`pwd` - Show Your Current Directory

pwd (print working directory) displays your current directory
```
pwd
```
In the beginning, use pwd every time you use cd
This will keep you from getting lost

`cd` - Change Directory

cd (change directory) changes your current directory
You use cd to move from one directory to another
```
cd DIRECTORY_NAME
```
If you use cd with an argument it moves you to that directory
If you use cd without an argument it takes you to your home directory
The home directory is the directory you are in when you first log in to Unix
This can be useful if you get lost in the filesystem
To go up one directory, use ..
```
cd ..
```
To go up more than one directory use multiple instances of ..
Each .. must be separated by a /
To go up two directories enter
```
cd ../..
```

`ls` - List the Contents of a Directory

ls (list) is one of the most basic Unix commands
It shows you the files and directories inside a directory
You can use ls with or without an argument

If you give ls a directory name, it lists what is inside that directory

$ ls it244_test
1        ex06.sh  ex12.sh  ex20.sh      ex24.sh     five_lines.txt
1.sh     ex07.sh  ex13.sh  ex21.sh      ex25.sh     script_improvements.txt
ex02.sh  ex08.sh  ex16.sh  ex22.sh      ex26.sh     step_comment_check.sh
ex03.sh  ex09.sh  ex17.sh  ex22_sum.sh  ex27.sh
ex04.sh  ex10.sh  ex18.sh  ex23.sh      ex_list.sh
ex05.sh  ex11.sh  ex19.sh  ex23_sum.sh  exnn.sh

When you don't give ls an argument it lists the current directory

$ pwd
/home/ghoffman

$ ls
bin           course_materials  it116       it244        stdwrk       tmp
bugs          demos             it116_test  it244_test   submitted    wrk
code          haoyu             it117       mail         testing      xrchiv
course_files  html              it117_test  public_html  tests_taken

The command line does not give you much information
It it easy to lose track of where you are
When this happens, certain things will not work as you would expect
Running ls can help you know you are in the right directory
Like many Unix commands ls has many options
But two are used most often

The -l (el) option gives you a long listing

$ ls -l
total 20
-rw-rw-r--   1 ghoffman       103 Sep 11 14:34 basic.css
-rw-r--r--   1 ghoffman      3560 Aug 29 13:30 emacs_cheat_sheet.html
-rw-r--r--   1 ghoffman       701 Aug 29 13:30 index.html
drwxr-xr-x   6 ghoffman       512 Sep 15 14:11 it_244
-rw-r--r--   1 ghoffman      6831 Aug 29 13:30 tips.html
-rw-r--r--   1 ghoffman      6052 Aug 29 13:30 unix_cheat_sheet.html

We'll talk more about this when we discuss access permissions
The -a (for all) option also lists files whose names begin with . (dot)
These "invisible" files contain Unix configuration information
ls will not show them unless you use -a
Because you don't normally care about them
We'll discuss these files later in the course

`cat` - Print the Contents of a File

cat (concatenate) prints a file
```
$ cat foo.txt 
foo
bar
blecth
```

When used with the -n option cat displays line numbers

$ cat -n numbered_lines.txt 
     1  Line 1
     2  Line 2
     3  Line 3
     4  Line 4
     5  Line 5

If you run cat on a long file, the output can scroll off the screen
To view long files, you should use a paging program
Paging programs show the file one screenful at a time
I'll discuss them more in a future class

`rm` - Delete a File

rm (remove) deletes a file
```
rm FILENAME
```
There is no undelete feature in Unix
Deleted files cannot be recovered if not backed up
Don't delete a file unless you are sure you won't need it
To remove all the files in a directory use
```
rm *
```
Be careful when using * with rm
It will delete everything and there is no way to get back what was deleted
If you run rm with the -i option it will ask you before deleting each file
```
rm -i * 
rm: remove regular file `foo.txt'? y [Enter]
```
It's good idea to use this option when running rm *
rm cannot remove a directory unless you use a special option
NEVER USE rm * IN YOUR HOME DIRECTORY
Every semester at least one student does this ...
and lose their it244 directory
I can fix this
But it's a pain in the butt

`mkdir` - Create a Directory

mkdir (make directory) creates a directory
```
mkdir DIRECTORY_NAME
```
The directory will be created in the current directory

`rmdir` - Delete a Directory

rmdir (remove directory) deletes a directory
```
rmdir DIRECTORY_NAME
```
rmdir will not work unless the directory is empty

`cp` - Copy Files

cp (copy) makes a copy of a file or a directory
```
cp  FILENAME  DIRECTORY_OR_NEW_FILENAME
```
cp takes two arguments
The first argument is the source ...
the file or directory to be copied
The second argument is the destination
Where the copy should go
Often the destination is a directory

Like this

$ ls
dir  foo.txt

$ ls dir

$ cp foo.txt dir

$ ls dir/
foo.txt

But to make a copy in the same directory the 2nd argument is a new name
```
$ cp foo.txt bar.txt

$ ls 
bar.txt  dir  foo.txt
```
To copy a directory you must use the -r option

`mv` - Move a File or Directory

mv (move) is a command that does two different things
It can change the location of a file or directory
```
mv  FILENAME_OR_DIRECTORY_NAME  NEW_DIRECTORY
```

It can also change the name of a file or directory

mv  FILENAME_OR_DIRECTORY_NAME  NEW_FILENAME_OR_NEW_DIRECTORY_NAME

In either case, mv takes two arguments
The first argument is always the the name of the file or directory

When used to move, the the second argument is the new location

$ ls
foo.txt  it244  work

$ ls work

$ mv foo.txt work

$ ls
it244  work

$ ls work
foo.txt

When changing the name of a file or directory, the second is the new name

$ ls
foo.txt hold  it244  work

$ mv foo.txt bar.txt

$ ls
bar.txt  hold  it244  work

Studying

Some Students Are Having a Hard Time

Tips and Examples

Review

New Material

Studying

Homework 2

First Graded Quiz

Gradescope

Recordings of Class Meetings

Questions

Tips and Examples

The putty Blank Screen Problem

Use Flash Cards to Prepare for Graded Quizzes

Always Check Your Work

Review

The Unix script Command

Correcting Mistakes on the Command Line

Retrieving Your Last Command Line Entry

Stopping a Running Program

Getting Help with Unix Commands

Attendance

New Material

Special Characters

Quoting and Escaping

pwd - Show Your Current Directory

cd - Change Directory

ls - List the Contents of a Directory

cat - Print the Contents of a File

rm - Delete a File

mkdir - Create a Directory

rmdir - Delete a Directory

cp - Copy Files

mv - Move a File or Directory

Studying

Class Exercise

Class Quiz

The `putty` Blank Screen Problem

The Unix `script` Command

`pwd` - Show Your Current Directory

`cd` - Change Directory

`ls` - List the Contents of a Directory

`cat` - Print the Contents of a File

`rm` - Delete a File

`mkdir` - Create a Directory

`rmdir` - Delete a Directory

`cp` - Copy Files

`mv` - Move a File or Directory