IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 7

Today's Topics

Homework 4

I have posted Homework 4 here.

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

The is the first homework assignment that will require you to write a script.

Your script must meet certain requirements that I mentioned in the last class.

Let's review the requirements for homework scripts.

Quiz 1

I have posted the answers to Quiz 1 here.

Tips and Examples

Checking Class Exercises

If you run my testing scripts on your Class Exercise and you get no error messages ...
then your script is OK
If you cannot finish the exercise in class ...
you may do so at home
If I have not checked your exercise in class ...
I will do so later by running a script
If you did not create a script for the exercise ...
or there is a problem with your script ..
you will receive an email from me
If you receive no email ...
you received full credit for the exercise ...
minus any late penalty

An Easy Way to Create Homework Scripts

You may create your homework scripts anyway you like ...
as long as they meet the homework specification
But I would suggest the following approach
You can have more than one ssh window open ...
at any one time
Most ssh client packages allow you to copy and paste ...
between different windows
You can use this feature in creating homework scripts
First, open two ssh windows connected to .cs.umb.edu
In both windows go to the homework directory for the assignment
In one window run nano or a Unix text editor ...
to create the homework script file ...
with the name specified for the assignment
In this script file write the echo statement
```
echo Step 1
```
Now create two empty lines
Repeat this procedure for every step in the assignment
No go to the other window
Enter the Unix commands that will perform a step
When you get the commands working ...
copy and paste them into the script file ...
under the appropriate step number

Read the Specification

It's important to read the homework assignments carefully
If you leave out a step ...
or a part of step ...
you will lose points
I try to be careful to say exactly what I want ...
in a quiz, text or homework assignment
If you do not do everything I have asked ...
you will not get full credit
I do not do this to be obnoxious
I do this to prepare you for professional work
In technology, details are important
If you misspell a command in Linux/Unix you will get an error
If you skip a step in an installation procedure ...
the install will not work properly
In a well-run organization every project will have a specification
If you miss a step in this specification ...
you have not completed the project
Treat the homework assignments like a specification ...
and read them carefully
It is a good idea to read a specification more than once
When I was in school, I read each assignment immediately after class
That way I could look for anything that might cause me trouble
I could ask questions if I did not understand some point
Read over your homework assignment after you have done the work ...
to make sure that you have not skipped something
One of the purposes of a college education ...
is to teach you habits that will help you later in life
Reading the specification carefully ...
is a habit you should learn now

Always Check Your Work

Always run your scripts to see that they work ...
before the assignment is due
If you script does not run ...
or runs will errors ...
you will lose points
You should never assume that a script that you have written will work
You may read a script and think it looks fine ...
but spelling or other errors are easy to overlook
Whenever you make a change to a script ...
you should run it again
Always check your work

Common Mistakes with Pipes

There are some mistakes that student seem to make each semester
Two of these mistakes involve pipes
The first mistake is to use cat at the beginning of a pipeline ...
when it is not needed
Students will sometimes use cat to create input ...
that is then piped into grep, sort, head or tail
You don't need cat in a pipe with these commands ...
because these utilities can read a file directly ...
without using cat

So instead of using

cat names.txt | grep Glenn
cat names.txt | sort
cat names.txt | head
cat names.txt | tail

you should simply use

grep Glenn names.txt
sort names.txt
head names.txt
tail names.txt

This mistake is harmless ...
because it does not affect the result
But the second mistake will cause an error
The mistake is to use the filename ...
given to the first command in a pipeline ...
to the following commands
If I wanted to find all Red Sox games where the Sox won at home ...
I would write
```
grep Win red_sox.txt | grep vs
```

I would not write

grep Win red_sox.txt | grep vs red_sox.txt  # WRONG

If you do this, the output of the first grep ...
will be thrown away ...
and you will only get the output of the second grep

Review

`uniq` - Eliminate Duplicate Lines

uniq prints the lines of a file to the screen ...
leaving out any adjacent lines that are identical
Notice that the lines must be adjacent
To make identical lines adjacent, uniq is often used with sort
The -i option makes uniq ignore case when looking for duplicate lines

`diff` - Differences between Files

diff compares two files and notes their differences
diff was created for use with the patch utility
Run diff with the -y option to get output that is easier to read

The Shell Prompt

By default, the shell prompt tells you three things
- Your Unix username
- The machine to which you are connected
- The location of your current directory
But before I changed it, my shell prompt looked like this
```
tsoro@vm74:~/it244/work$
```
The characters before the @ show my Unix username ...
the characters between @ and : are the hostname
of the machine I am using ...
and the characters from : to $ show my current directory
The ~ indicates your home directory
We'll talk more about this in a future class

`file` - See the File Type

The file utility can be used the determine the type of a file

$ file *
class_notes.css:           ASCII text
common_unix_commands.html: HTML document text
cs285L:                    directory
emacs_cheat_sheet.html:    HTML document text
index.html:                HTML document text
it244:                     directory
tips.html:                 HTML document text
unix_cheat_sheet.html:     HTML document text
work.txt:                  ASCII text

Pipes - Stringing Programs Together

A pipe takes the output of one command ...
and feeds it into the input of another command
Pipes allow you to chain together several Unix commands ..
into a single command
Commands joined in this way are sometimes called pipelines
Pipes are essential to the Unix philosophy of simple tools
Using pipes, you can string together many commands ...
to achieve exactly what you want
You form a pipe by placing the vertical line character, | ...

between two commands

$ head -5 red_sox.txt
2011-07-31  Red Sox @  White Sox    Win 5-3
2011-07-02  Red Sox @  Astros       Win 7-5
2011-07-03  Red Sox @  Astros       Win 2-1
2011-07-04  Red Sox vs Blue Jays    Loss 7-9
2011-07-05  Red Sox vs Blue Jays    Win 3-2

$ head -5 red_sox.txt | sort
2011-07-02  Red Sox @  Astros       Win 7-5
2011-07-03  Red Sox @  Astros       Win 2-1
2011-07-04  Red Sox vs Blue Jays    Loss 7-9
2011-07-05  Red Sox vs Blue Jays    Win 3-2
2011-07-31  Red Sox @  White Sox    Win 5-

Notice, in the command line above, that sort does not have an argument
Normally sort requires an argument that specifies the file to sort
But in a pipe, each command after the first ...
takes its input from the output of the preceding commands
You never need to specify the input when using a command in a pipeline ...
except the first command

`date` - Get the Date and Time

The Unix date command will give the time and the date
```
$ date
Tue Aug 21 10:20:05 EDT 2012
```
If we follow the command with a + and a string we can change the format
```
$ date +"%Y-%m-%d"
2012-08-21
```
Use info or man to see the various formatting options date provides

`which` - Finding a Program File

Unix commands are programs ...
that are located somewhere in the filesystem
The Unix utility which gives the location of an executable file
To find where the executable file for less is located ...
we can run which like this
```
$ which less
/usr/bin/less
```
which uses the PATH system variable to find the executable file
We'll discuss PATH in a future class

`whereis` - Finding Files Used by a Program

whereis is another program that can be used to locate program files
whereis takes a different approach than which
Every Unix or Linux system has certain standard places ...
where it stores programs and the files they use
whereis searches these locations
It returns a list of all files associated with a program
When we run whereis on the tar utility ...

we get more information than which returned

$ whereis tar
tar: /bin/tar /usr/include/tar.h /usr/share/man/man1/tar.1.gz

The first entry is the executable file
The second is a header file
The third is the file that man used to provide information about tar

`locate` - Search for Any File

which and whereis only work on programs
locate can be used to find any file
You don't need to know the full name of a file to use locate

locate will search on a partial file name

$ locate foot
/etc/update-motd.d/99-footer
/usr/share/doc/java-common/debian-java-faq/footnotes.html
/usr/share/emacs/23.3/lisp/mail/footnote.elc
/usr/share/emacs/23.3/lisp/org/org-footnote.elc
/usr/share/libparse-debianchangelog-perl/footer.tmpl
/usr/share/xml-core/catalog.footer
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/Kconfig
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/Makefile
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/Makefile.boot
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/include
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/include/mach
/usr/src/linux-headers-3.0.0-12/arch/arm/mach-footbridge/include/mach/debug-macro.S
...

locate does not actually search the file system itself
That would take too long
Instead it uses a database of all files on the system
This database is created by another program updatedb
updatedb is usually run automatically in the background to update the database

`who` - See Users Logged On

who prints a list of all users currently on the machine

$ who
rickm    pts/0        2017-12-07 03:05 (192.168.105.41)
apera    pts/1        2018-01-13 04:54 (73.16.254.131)
tsoro    pts/2        2018-01-14 15:23 (24.62.10.159)
apera    pts/5        2018-01-11 20:04 (192.168.107.83)
jkamby   pts/8        2018-01-11 15:21 (66.30.11.52)
onic     pts/9        2018-01-11 13:15 (158.121.18.145)

who also provides information about each user's login session

who am i will show the user who is logged into a terminal

$ who am i
tsoro    pts/2        2018-01-14 15:23 (24.62.10.159)

`finger` - Get information on Users

finger provides information about a user account

Login: tsoro                            Name: Torna O. Soro
Directory: /home/tsoro                  Shell: /bin/bash
On since Sun Jan 14 15:23 (EST) on pts/2 from 24.62.10.159
   7 seconds idle
Mail forwarded to tsoros@gmail.com
No mail.
No Plan.


			
$ finger ghoffman
Login: ghoffman       			Name: Glenn Hoffman
Directory: /home/ghoffman           	Shell: /bin/bash
On since Thu Sep 24 11:43 (EDT) on pts/5 from 158.121.249.7
   5 seconds idle
Mail forwarded to glennhoffman@mac.com
New mail received Fri Sep 18 14:00 2015 (EDT)
     Unread since Fri Sep 18 07:43 2015 (EDT)
Plan:
Office:        Science 3-92A (Pterosaur poster on door)           
Office Hours:  Tuesday & Thursday, 12:30 - 1:30 PM and 5:30 - 6:30 PM 
On Campus:     Wednesday 1 - 6PM Appointment needed on Wednesday
Classes:       IT 244-01   Introduction to Linux/Unix             TuTh 2:00 - 3:15 PM  Science S3-143 (IT Lab)
               IT 244-02   Introduction to Linux/Unix             TuTh 4:00 - 5:15 PM  Science S3-028 (Web Lab)  
               IT 341_02   Introduction to System Administration  TuTh 7:00 - 8:15 PM  Science S3-143 (IT Lab)
Home Address:  40 Central Street     617-821-3936 (home; 9am - 9pm only)
               Somerville, MA 02143

finger, like mv, has two functions

When used without an argument, finger shows ever user currently logged in

$ finger
Login     Name            Tty      Idle  Login Time   Office     Office Phone
eb        Ethan Bolker    pts/4    3:35  Sep 22 19:19 (pool-108-26-203-122.bstnma.fios.verizon.net)
ghoffman  Glenn Hoffman   pts/5          Sep 24 11:43 (158.121.249.7)
root      root           *tty1      52d  Mar 11  2015

You can also use a last or first name with finger

$ finger hoffman
Login: ghoffman       			Name: Glenn Hoffman
Directory: /home/ghoffman           	Shell: /bin/bash
On since Thu Sep 24 11:43 (EDT) on pts/5 from 158.121.249.7
   2 seconds idle
Mail forwarded to glennhoffman@mac.com
New mail received Fri Sep 18 14:00 2015 (EDT)
     Unread since Fri Sep 18 07:43 2015 (EDT)
Plan:
Office:        Science 3-92A (Pterosaur poster on door)           
Office Hours:  Tuesday & Thursday, 12:30 - 1:30 PM and 5:30 - 6:30 PM 
On Campus:     Wednesday 1 - 6PM Appointment needed on Wednesday
Classes:       IT 244-01   Introduction to Linux/Unix             TuTh 2:00 - 3:15 PM  Science S3-143 (IT Lab)
               IT 244-02   Introduction to Linux/Unix             TuTh 4:00 - 5:15 PM  Science S3-028 (Web Lab)  
               IT 341_02   Introduction to System Administration  TuTh 7:00 - 8:15 PM  Science S3-143 (IT Lab)
Home Address:  40 Central Street     617-821-3936 (home; 9am - 9pm only)
               Somerville, MA 02143
               

Login: ghoffmn        			Name: Glenn Legacy Hoffman
Directory: /home/ghoffmn            	Shell: /bin/bash
Last login Mon Jul 13 13:14 (EDT) on pts/5 from dsl092-066-161.bos1.dsl.speakeasy.net
Mail forwarded to glennhoffman@mac.com
Mail last read Thu May 28 18:30 2015 (EDT)
Plan:
Office:         McCormack M-3-607                 Fall 2014
Office Hours:   Tuesday & Thursday, 10:00 - 12:00 PM and by appointment
Classes:        
	IT 341-2  Introduction to System Administration  TuTh  12:30-1:45  S3-148  (IT Lab)
	IT 244-1  Introduction to Linux/Unix             TuTh   2:00-3:15  S3-028  (Web Lab)
	IT 244-3  Introduction to Linux/Unix             TuTh   4:00-5:15  S3-148  (IT Lab)
	IT 244-2  Introduction to Linux/Unix             TuTh   5:30-6:45  S3-028  (Web Lab)
Home Address:   40 Central Street     617-821-3936 (home; 9am - 9pm only)
                Somerville, MA 02143


Login: it244gh        			Name: Glenn Hoffman Dummy
Directory: /home/it244gh            	Shell: /users/nologin
Never logged in.
Mail forwarded to glennhoffman@mac.com
No mail.
Plan:
This account is a test account for Glenn Hoffman teaching it244

New Material

The Hierarchical Filesystem

A filesystem is an arrangement of data on a storage device ...
and the software needed to store and retrieve that data
All files are contained in directories
On Windows and the Mac these directories are called folders
A directory can contain other directories
A hierarchical filesystem starts with a special directory at the top
This directory is called the root directory

In Unix, we represent this directory with a single slash character, /

$ ls /
bin     initrd.img  proc tmp
boot    lib         root        tools
courses lib32       run         users
data    lib64       sbin        usr
dev     lost+found  selinux     var
etc     media       sources     vmlinuz
groups  mnt         spool
home    nobackup    srv
home.ORIG  opt      sys

All other directories are contained within the root directory ...
or one of its many subdirectories
This structure is called a tree because it looks like a tree turned upside down
Unlike in nature, this tree grows down from the root
A hierarchical filesystem is endlessly extensible
We can have as many files and directories as we want ...
as long as there is enough disk space to hold them
A hierarchical filesystem resembles a family tree
We often use family terms when talking about directories
The directory one level up is called the parent directory
Any directory above another directory can be called an ancestor
Directories inside the a directory are its child directories
Any file or directory below the current directory is a descendant
Directories and files within the same directory are called siblings

Unix Files and Directories

Files are linear arrangements of data ...
on some storage device
Some files are programs
Program files contain binary instructions that the machine understands
Some files contain data in various formats ...
such as text
Some Unix text files contain configuration information
These configuration files modify your Unix environment
They perform the same function as the Windows Registry
Since these files are text ...
you can read and edit them easily
You only need a text editor to read or change them ...
not some special utility ...
like the Windows Registry Editor
Unix treats directories as files ...
although they are a special kind of file
Unix also treats devices as special kind of file
But this is invisible to the ordinary user
You can't run cat or less on a directory or a printer
Treating directories and devices as files ...
allows Unix to be simpler than it might be ...
and in engineering simpler is always better ...
than complicated
Directories and devices are just special kinds of files

Filenames

When you ask Unix for a file ...
you must give it two pieces of information
- The name of the file
- The location of the file in the hierarchical file system
Every file has a name - the filename
Different Unix flavors allow filenames to be of different length
Most Unix systems allow filenames to be no more than 255 characters
It's best to keep filenames short
This makes them easier to type
You're less likely run into trouble ...
if you move the file to another system ...
where the maximum filename length may be shorter

To learn the maximum length of a filename on any machine use stat

$ stat -f /home | grep Namelen
ID: 0        Namelen: 255     Type: autofs

Notice that I used a pipe to get just the information I wanted
You can use a space in filenames if you enclose them in quotes
This is a VERY BAD IDEA
When you run ls on a directory that has a filename with a space in it ..
it looks like you have more than one file
```
$ touch 'foo bar'
$ ls
foo bar
```
It looks like you have two files, foo and bar
In reality, you only have one file, foo bar
If you encounter such a file ...

you have to escape the space character to work with the file

$ ls -l foo\ bar 
-rw-r--r-- 1 it244gh it244-2G 0 2011-09-24 14:34 foo bar

Instead of a space in a filename ...
use an underscore, _
```
$ touch foo_bar
$ ls
foo_bar
```
To avoid problems, only use the following characters in file names
- Uppercase letters (A-Z)
- Lowercase letters (a-z)
- Digits (0-9)
- Underscore _
- Dash -
- Period .
You cannot have two files with the same name ...
in the same directory

Case Sensitivity

Unix is case sensitive
This means that Foo, foo and FOO ...
are three different files ...

as far as Unix is concerned

$ ls

$ touch Foo foo FOO

$ ls
foo  Foo  FOO

Unix commands are always lower case
This is just a convention
But it is a helpful convention
Otherwise you would have to remember capitalization ...
as well as the name of the command
Some operating systems are not case sensitive ...
like DOS and Windows
Stick to the convention of always using lowercase filenames ...
this will make it easier to move between different operating systems

Filename Extensions

Extensions are strings of characters ...
that appear at the end of the filename ...
after a period
Unix ignores extensions
As far as Unix is concerned, extensions are just part of the filename
Some Unix programs, do require certain extensions
The C compiler, gcc, expects the filenames of source code files ...
to end in .c
The Java compiler, javac, expects Java source files ...
to have .java extension
These programs enforce this rule
Unix couldn't care less
As far a Unix is concerned an extension ...
is just part of the filename ...
with no special significance

Current Directory

The way a command works depends ...
on your Unix environment
One of the most important parts of your environment ...
is your current directory
Your current directory as your current location ...
in the filesystem
The pwd (print working directory) command
will always tell you your current directory
If run ls without an argument ...
it will list your current directory

Your Home Directory

Whenever you log in to a Unix machine ...
you will always find yourself inside your home directory
This directory belongs to your Unix account only
You can do anything you want in your home directory ...
including blowing it all away
You can create files or directories in your home directory
You can let other users have access ...
to the contents of your home directory
You can always get back to your home directory ...
by running cd (change directory) with no argument
```
$ cd

$ pwd
/home/tsoro
```
Your home directory contains a number of "hidden" files ...
which customize your environment
These files begin with a period, . ...
and do not ordinarily appear in an ls listing ...
unless you run ls with the -a option
On most Unix systems, your home directory ...
will be in the /home directory
On a Mac it will appear in the /Users directory

Any file or directory in the hierarchical filesystem ...
will be one of four positions ...
relative to your current directory
- Inside your current directory
- Below your current directory
- Above your current directory
- Off to the side of your current directory
In this last case, you must go up before you can go down ...
to reach the file or directory