IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 4

Today's Topics

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

New Material

Special Characters
Quoting and Escaping
cd - Change Directory
pwd - Show Your Current 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

Reading Assignment

The reading assignment for this week is chapter 2 of Sobell, Getting Started.

You will find a list of all reading assignments here.

There is a link to this page on the class web page.

Homework 2

I have posted homework 2.

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

You will find a list of all homework assignments here.

The class web page has a link to this page.

First Graded Quiz

The first graded quiz will be given next week.

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

Review

The Unix `script` Command

The script command creates a transcript of you Unix session
By default, script creates a text file named typescript ...
in your current directory
If you have an existing typescript file ...
and run script in the same directory ...
script will obliterate the contents of the previous typescript file
To prevent this 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 ...
using 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 - Removes everything from the current text insertion point to the end of the line
- Control U - Removes everything from the current 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
In addition the left and right arrow keys, ← and → ...
can be used tho move the text insertion point left or right

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 ...
others use --help
For more information, use man or info
Follow man or info with the name of the command

New Material

Special Characters

There are some characters that have special meaning in Unix
They should never 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 entering text in a text editor ...
the newline moves you down to the next line
On the command line the newline tells the shell you are done typing a command ...
and the shell should run the command you just entered
When the shell sees a newline ...
it executes whatever is on the command line

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 ...
to turn off its special meaning
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/tsoro
```
You'll learn what ~ means in a future class
If I just wanted to print the ~ character ...
all I have to do is put a backslash, \ ...
immediately 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 meanings ...
but you don't need to worry about the difference for now

`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 the name of a directory ...
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
To go up one directory, use ..
as the argument for cd
```
cd ..
```
To go up more than one directory ...
use multiple instances of .. ...
each separated by a /
To go up two directories I would enter
```
cd ../..
```

`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

`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
The command line is not a GUI
It it easy to lose track of where you are
When this happens, certain things will not work ...
as you would expect
When ls is used without an argument ...
it lists the contents of your current directory
When you use ls followed by the name of a directory ...
it lists the contents of that directory
```
ls DIRECTORY
```
When you use ls followed by the name of a file ...
it simply displays the file name
When used with the -a (for all) option ...
ls will list all files ...
including those whose names begin with a . (dot)
Any file whose name begins with a . ...
will not show up when you use ls ...
unless you use the -a option
These "invisible" files are configuration files
Under normal circumstances ...
you pay no attention to them
We'll discuss these files later in the course
Another useful option to ls is -l ...

which displays 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

`cat` - Print the Contents of a File

cat (concatenate) displays the contents of a file
```
$ cat foo.txt 
foo
bar
blecth
```

When used with the -n option cat displays line numbers

$ cat -n lines.txt 
     1  line 1
     2  line 2
     3  line 3
     4  line 4
     5  line 5
     6  line 6
     7  line 7
     8  line 8
     9  line 9
    10  line 10

If you run cat on a long file ...
the contents of the file will pass by too quickly to read
To view long files, you should use a paging program
Paging programs, like more and less ...
show the content of a 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
```
rm does not ask you if you are sure ...
before deleting the file
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 foo.txt 
rm: remove regular file `foo.txt'? y [Enter]

It's good idea to use this option when running rm *
rm cannot be used on a directory ...
NEVER USE rm * IN YOUR HOME DIRECTORY
Every semester at least one student does this ...
and seems to lose their it244 directory
I can fix this ...
but it is 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 ...
unless you specify another location

`rmdir` - Delete a Directory

rmdir (remove directory) deletes a directory
```
rmdir IRECTORY_NAME
```
rmdir will not work on a directory ...
unless the directory is empty

`cp` - Copy Files

cp (copy) makes a copy of a file ...
or a directory
cp takes two arguments
The first argument is the source ...
the file or directory to be copied
The second argument is either the new filename ...
if you are making a copy in the same directory ...
or the directory into which the copy will go
```
cp FILENAME NEW_FILENAME_OR_DIRECTORY
```
cp can copy an entire directory ...
when used with the -r (for recursive) 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
When used to move something ...
the first argument is thing to be moved ...

and 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

Here I have moved the file foo.txt ...
from the current directory ...
to the directory work
When changing the name of a file or directory ...
the first argument is the old name ...

and the second is the new name

$ ls
foo.txt hold  it244  work

$ mv foo.txt bar.txt

$ ls
bar.txt  hold  it244  work