IT 244: Introduction to Linux/Unix

IT 244: Introduction to Linux/Unix
Class 14

Today's Topics

Review

Correcting Mistakes on the Command Line
Retrieving Your Last Command Line Entry
Aborting a Running Program
Getting Help with Unix Commands
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
echo - Print Text to the Terminal
Pagers - View a File One Screen at a Time
Pathname Completion
grep - Finding Strings inside Files
head - View the Top of a File
tail - View the Bottom of a File
sort - Print a File in Sorted Order
diff - Differences between Files
Pipes - Stringing Programs Together
date - Get the Date and Time
which - Finding a Program File
locate - Search for Any File
finger - Get information on Users
The Hierarchical Filesystem
Case Sensitivity
Filename Extensions
Current Directory
Your Home Directory
Navigating the Hierarchical File Systems
Hidden Filenames
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
Access Permissions
Viewing Access Permissions
chmod
Using chmod with Numeric Arguments
The root Account
Directory Access Permissions
Links
The Two Types of Links
ln
Removing a Link
Command Options
The PATH System Variable
Running a Program in the Current Directory
Standard Input, Standard Output and Standard Error
The Keyboard and Screen as Standard Input and Standard Output
Redirection
Redirecting Standard Output
Redirecting Standard Input
Redirecting Standard Output Can Destroy a File
Adding Output to an Existing File
/dev/null

Homework 7

I have posted homework 7 here.

Review

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

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

Aborting 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

Special Characters

Some characters have special meaning in Unix

They are

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

Whitespace characters are special too
They are
- Space
- Tab
- Newline
Space and Tab separate commands, options, and arguments on the command line
The Enter key creates a newline character on a PC ...
while on a Mac it is the Return key
When the shell sees a newline ...
it executes the commands on the command line

Quoting and Escaping

You can turn off the special meaning of a character ...
by preceding it with a backslash, \
You can also turn off special meanings by enclosing a string in quotes
You can continue a command onto the next line ...
by using a backslash just before hitting Enter
The backslash turns off the special meaning of the newline character

`cd` - Change Directory

To change your directory use cd
If you run cd without an argument, it will take you to your home directory
To go to the directory above your current directory, use
```
cd ..
```

`pwd` - Show Your Current Directory

pwd will print you your current location in the filesystem
pwd takes no arguments

`ls` - List the Contents of a Directory

ls lists the contents of a directory
To see the contents of directory work, run
```
ls work
```
When you use ls without an argument ...
it lists the contents of the current directory
For more information run ls with the -l (long) option
ls -a displays the "invisible" files ...
whose name begins with a .

`cat` - Print the Contents of a File

To display the contents of a file, use cat

`rm` - Delete a File

To remove a file, use rm
To remove all files in a directory, use rm *
Be very careful when you use this construction
There is no file recovery mechanism in Unix
rm will not remove a directory unless you use a special option

`mkdir` - Create a Directory

You create a directory using mkdir

`rmdir` - Delete a Directory

rmdir is used to remove a directory
rmdir will not work unless the directory is empty

`cp` - Copy Files

cp copies files or directories
To copy a directory and all its files and sub-directories ...
use cp with the -r option

`mv` - Move a File or Directory

Use the mv command to move a file or directory ...
from one place to another
To rename a file or directory, you also use mv

`echo` - Print Text to the Terminal

echo prints text to the terminal
```
$ echo Hello
Hello
```
You can use echo to print the value of a system variable ...
if your precede the variable name with a $
```
$ echo $SHELL
/bin/bash
```

Pagers - View a File One Screen at a Time

Pagers are programs that display the contents of a file ...
one screenful at a time
The two pagers that Unix supplies are more and less
Hitting the Space bar advances to the next screen
Hitting the Enter or Return key takes you down one line
less, just to be confusing, has more features than more

Pathname Completion

When typing a long file name it is easy to make a mistake
To make life easier, Unix provides a feature called pathname completion
You type a few characters, then hit the Tab key
Unix will supply the rest ...
if there is only one file or directory that matches
If there is more than one match ...
Unix will supply as much as it can and then beep
If there is no match, it will also beep
If you don't get a complete match ...
hitting Tab twice ...
will display a list of all possible matches
This only works in the Bash shell

`grep` - Finding Strings inside Files

grep is used to find all lines in a file ...
that contain a certain string
grep takes two arguments
- The string you are searching for
- The file or files in which to search
grep has the following format
```
grep  STRING  FILE  [FILE ...]
```
To run grep on the files in a directory ...
use the -r (recursive) option
grep -r will search through all files in a directory ...
and in all subdirectories
grep, like Unix, is case sensitive
It thinks of "foo" and "FOO" as two different strings
To have grep ignore case ...
run it with the -i option
To have grep find all lines that do not match a string ...
run it with the -v option

`head` - View the Top of a File

head prints the first 10 lines of any file
If you want a different number of lines ...
follow head with a dash, - , and a number
```
head -20 foo.txt
```

`tail` - View the Bottom of a File

tail prints the last 10 lines of any file
If you want a different number of lines ...
follow tail with a dash, - , and a number
```
tail -20 foo.txt
```

`sort` - Print a File in Sorted Order

sort prints a sorted list of the lines in a file ...
to the terminal
sort does not change the file
sort sorts the lines of a file ...
by the first few characters in the line
To sort in reverse order use the -r option
sort, by default, sorts in alphabetical order
This is a problem when the first characters on a line are numbers
That's because 11 will sort before 2
To sort in numerical order use sort -n
To sort in reverse numerical order use sort -nr

`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

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

`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

`finger` - Get information on Users

finger provides information about a user account

$ finger ghoffman
Login: ghoffman                  Name: Glenn Hoffman
Directory: /home/ghoffman                Shell: /bin/bash
On since Wed Sep 17 16:09 (EDT) on pts/1 from dsl092-066-161.bos1.dsl.speakeasy.net
   1 second idle
Mail forwarded to glennhoffman@mac.com
Mail last read Thu Sep  4 15:12 2014 (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

finger, like mv, has two functions

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

$ finger
Login     Name                Tty      Idle  Login Time   Office     Office Phone
ghoffman   Glenn Hoffman       pts/0          Aug 18 11:13 (dsl092-066-161.bos1.dsl.speakeasy.net)
rouilj    John P. Rouillard   pts/1    4:34  Aug 18 06:44 (pool-74-104-161-40.bstnma.fios.verizon.net)
ubuntu    Ubuntu Dummy       *tty1      14d  Aug  4 04:53

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 Wed Sep 17 16:09 (EDT) on pts/1 from dsl092-066-161.bos1.dsl.speakeasy.net
   1 second idle
Mail forwarded to glennhoffman@mac.com
Mail last read Thu Sep  4 15:12 2014 (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: Dummy for Glenn Hoffman
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

The Hierarchical Filesystem

Unix uses a hierarchical filesystem
This means there is one directory at the top, called the root directory
The root directory is indicated by a simple slash character, /
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
A hierarchical filesystem also resembles a family tree
So we often use terms that describe family members when talking about directories
The directory up one level from your current directory is called the parent directory
All directories above the current directory are called ancestors
All directories inside the current directories are called child directories
All directories below the current directory can be called descendants
All directories and files within the same parent directory are called siblings

Case Sensitivity

Unix is case sensitive
This means that "Foo", "foo" and "FOO" ...
are three different things ...
as far as Unix is concerned
Unix utility and program names are always lowercase
Some operating systems do not distinguish ...
between UPPERCASE and lowercase characters
Windows is such a system
Make life easy for yourself
Use only lowercase characters in Unix filenames

Filename Extensions

Extensions are strings of characters ...
that appear at the end of the filename ...
after a period
Extensions are not recognized by the Unix filesystem
As far as Unix is concerned ...
they are just legal characters ...
that are part of the filename
Some Unix programs expect their files ...
to have certain extensions
For example, 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 ...
at the end of the filename
These extensions are required by the program ...
not by Unix

Current Directory

The way a Unix command works ...
depends, somewhat, on your environment
One of the most important parts of your environment ...
is your current directory
The pwd (print working directory) command ...
will always tell you your current directory
If a command expect a directory as an argument ...
you can usually omit it ...
and the program will assume you mean you current directory
For example, ls used with no arguments ...
will list the contents of your current directory

Your Home Directory

Whenever you log in to a Unix host ...
you will always find yourself in your home directory
This a directory that belongs to your Unix account only
You have full control of permissions within this directory
If you use cd with no arguments ...
it will take you to your home directory
```
$ cd

$ pwd
/home/ghoffman
```
Your home directory contains a number of hidden files ...
which customize your environment ...
like .forward
On most Unix systems, home directories are found inside the /home directory
The name of your home directory ...
is the same as your Unix username

Any file or directory in the filesystem ...
will be one of four positions ...
relative to your current directory
It can be inside your current directory
It can be below your current directory
It can be above your current directory
It can be off to the side of your current directory
In this last case, you must go up ...
before you can go down to reach this file

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

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
- The name of the file
- A path to reach the file
The path is a list of directories that you must go through ...
to reach the file you want
A pathname is like an address on a letter ...
a name and directions to get there
The name of the file is always at the end of a pathname
When the slash, / appears between names in a pathname ...
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

There is one form of absolute path that is very short
This is the tilde character, ~
Tilde stands for your home directory
This means you can use tilde, ~ ...
anywhere you would normally use a path to your home directory
When you put a tilde in front of a Unix username ...
it stands for the home directory of that account
~ always means an absolute path

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 and 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 an ancestor of your current directory
- When the file is in a directory that is neither an ancestor or descendant of the current directory

Relative Pathnames in Your Current Directory

A relative pathname of a file or directory ...
inside your current directory ...
is simply 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, / ...
to separate the name of each directory ...
from what comes after it

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 ..
You keep going up ...
until you get to a directory ...
that is both an ancestor of your current directory ...
and the file you are trying to reach
Once you get to the common ancestor ...
you go down to the directory that holds the file

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 user account that created it
Every file, directory or device on a Unix filesystem has three types of permissions
- Read
- Write
- Execute
Each access permission can either be on or off
If you have read permission on a file you can look at the data in the file
If you only have read permission, you cannot change a file
To change a file you need write permission
You cannot run a program or script file ...
unless you have execute permission on that file
Each of the three permissions is set either on or off ...
to three classes of users:
- The owner
- The group
- Every other account
Every file or directory has an owner
The account that created the file is usually the owner
A group is a collection of Unix accounts
A group can only be set up by a system administrator
Every file or directory is assigned to a group
The last class of users is everyone else ...
any account that is not the owner or a member of the group

Viewing Access Permissions

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

$ ls -l
total 5
-rw------- 1 it244gh libuuid 316 2011-09-20 21:32 dead.letter
lrwxrwxrwx 1 it244gh libuuid  34 2011-09-06 13:21 it244 -> /courses/it244/s12/ghoffman/it244gh
drwx------ 2 it244gh libuuid 512 2011-09-07 15:03 mail
drwxr-xr-x 2 it244gh libuuid 512 2011-09-25 15:48 test
-rw-r--r-- 1 it244gh libuuid  15 2011-09-20 16:18 test.txt

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
The next three characters indicate the permission 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 that would normally appear in this column
The next three characters give the permissions of the group
The last three characters are the permission of all other accounts
The next column is a number that indicates 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

`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 permission, you need to use the chmod 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) to which the change will be applied

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 for expressing permissions
You are free to read about the symbolic format in the textbook
I will not deduct points for using symbolic format ...
as long as you use it correctly

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 permission of the owner
The second digit gives the permissions assigned to the group
The third digit gives the permissions for every other account
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 constructing the number you 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, represents all three permissions

Try to remember this chant

read    write   execute
4       2       1
owner group    everyone

Remember that you need three of these digits ...
to specify the full permissions for a file or directory

The root Account

On every Unix or Linux system there is a special account named root
root can access any file ...
or run any program
root is an administrator account
It is used for system configuration and maintenance

Directory Access Permissions

The Unix access permissions work a little differently for directories ...
than they do for files
Read and write permissions for a directory are similar to those for a file
Read permission allows you to list the contents of that directory using ls
Write permission allows you to create, delete ...
or change the name of entries in that directory
Write permission on a directory ...
does not allow you to change the contents of a file ...
in that directory
Write permission on a directory does not apply to the directory itself
If you have write permission on a directory you can change what's inside it ...
but you cannot rename the directory or delete it ...
unless you have write permission on its parent directory
Execute permission on a directory allows you to do two things
It allows you to enter that directory using cd
It also allows you to read a file in that directory
if you already have read permission on that file ...
and know the name of that file

Links

Links are like shortcuts on a Windows machine
or aliases on a Mac
Links allow you to move around the filesystem using short names
Each of you has an entry in your home directory called it244

In the home directory of my test account, it244gh, I have such a link

$ ls -l it244
lrwxrwxrwx 1 it244gh faculty 34 Sep  2 13:10 it244 -> /courses/it244/f14/ghoffman/it244gh

This is a link to/courses/it244/f14/ghoffman/it244gh

If you cd into this location and use pwd

$ pwd
/home/it244gh

$ cd it244

$ pwd
/home/it244gh/it244

This path reflects the route you took to get here
But it is not the real pathname of the directory
You can only get that information if you use pwd with the -P (note the capitalization) option
```
$ pwd
/home/it244gh/it244

$ pwd -P
/courses/it244/f14/ghoffman/it244gh
```

The Two Types of Links

There are two types of links
- Hard links
- Symbolic, or soft, links
Hard links are older
A hard link is like a duplicate file name
Hard links can only point to files ...
not directories
You can only have a hard link to a file ...
if that file is on the same hard disk volume ...
as the link
Symbolic links are much more flexible
You can use either an absolute or relative pathname ...
when creating a symbolic link
A symbolic link can point to a file or directory anywhere in the filesystem
Deleting a symbolic link does not delete the file or directory it points to

`ln`

To create a symbolic or soft link, use ln with the option

ln takes two arguments, a pathname and the name for the link

$ pwd
/home/it244gh

$ ln -s ~ghoffman/examples_it244 examples

$ ls -l examples
lrwxrwxrwx 1 it244gh libuuid 28 2012-09-17 17:53 examples -> /home/ghoffman/examples_it244

Removing a Link

To delete a link, use rm
If you delete a symbolic link ...
it will not affect the file or directory it points to

Command Options

Options modify the behavior of the command
Options are usually preceded by one or two dashes, -
GNU programs frequently have options that are preceded by two dashes, --
The options in GNU programs are usually words
The options in other Unix programs are usually a single letter
Utilities that report the size of files usually do so in bytes
Such utilities often have a -h, or --human-readable, option
With this option, the file size will be displayed in kilobytes, megabytes or gigabytes, as appropriate
Many commands display a help message when run with the --help option
Almost all GNU utilities accept this option

The PATH System Variable

To run a program, a process must be created
The shell cannot create a process ...
only the kernel can
The shell asks the kernel to start the process ...
but in order to do that, it has to give the kernel a pathname11
for the executable file for that program
Most of the time when you run a program you don't use a pathname ...
you simply use the name of the program
To turn the name of a program into a pathname for the executable file ...
the shell uses the PATH variable
PATH contains a list of directories to search to find the program file
The shell searches each of these directories in turn ...
until it finds an executable file with the name of the command
PATH always has a default value ...
which is set when Unix or Linux is installed

The absolute pathname of each directory is separated from the next by a colon, :

echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games

If the shell reaches the end of the directory listings in PATH ...
without finding the command ...
it will display an error message
If the shell finds executable file ...
but you do not have execute privileges
it will tell you this in an error message
You can modify the PATH variable in your own Unix environment

Running a Program in the Current Directory

For security reasons, it is never advisable to put the current directory, . ...
in the PATH list
Then how do you run a program inside your current directory?
You can do this using the following construction
```
./PROGRAM_NAME
```
This will always work ...
regardless of the contents of PATH

Standard Input, Standard Output and Standard Error

Every Unix process always has access to three different "files"
- Standard Input
- Standard Output
- Standard Error
Unix thinks anything it can write to ...
or read from ...
is a file
Standard input is where the program gets input ...
when a specific source of input ...
like a file or a device ...
is not specified
By default, standard input is the keyboard
Standard output is where the program sends its output
if a specific file or device is not mentioned
By default, standard output is the terminal
Standard error is where the program sends error messages
By default, standard error is the same as standard output: the terminal

The Keyboard and Screen as Standard Input and Standard Output

When a command or script does not specify ...
where input is to come from ...
it comes from standard input
By default, standard input is keyboard
When a command or script does not specify ...
where output should go ...
it goes to standard output
By default, standard output is the screen
When a command or script does not specify ...
where error messages should go ...
they goes to standard error
By default, standard error also is the screen

Redirection

Redirection is telling Unix to take data from ...
or send data to ...
a different place than usual
Redirection is one of the features that makes Unix so flexible
You can take input from something ...
other than the keyboard ...
like a file
You can send output to something ...
other than the screen ...
like another file
Redirection is what makes pipes possible
When you set up a pipe ...
you are sending the output from one program ...
into the input of another
You are redirecting the output of the first command ...
from the terminal ...
to the input of the second command
This allows next command to take its input ...
from something other than a file

Redirecting Standard Output

To redirect standard output use the greater than symbol, > ...
followed by a filename
This tells Unix to send the output from a command ...
to the file or device that appears after the symbol
The format for output redirection is
```
COMMAND  [ARGUMENTS]  >  FILENAME
```
If the file does not already exist ...
it will be created

Redirecting Standard Input

When we redirect standard output ...
we send output to something other than the screen
When we redirect standard input ...
we take input from something other than the keyboard
To do this, we use the less than symbol, <
Here is the format
```
COMMAND  [ARGUMENTS] <  FILENAME
```

Redirecting Standard Output Can Destroy a File

If you redirect standard output to a file that already exists ...
you will overwrite the contents of that file
You will replace the original contents of the file ...
with new data

Adding Output to an Existing File

If you redirect standard output to a file that already exists ...
you will lose the original contents of that file
But Unix allows you to add to the bottom of a file
This is called appending
The append symbol is two greater than symbols ...
with no space in between, >>
The format is
```
COMMAND  [ARGUMENTS] >>  FILENAME
```

/dev/null

Sometimes a program will do something useful ...
but produce output you don't want
For situations like this, Unix provides /dev/null
Any output you send to /dev/null will disappear
It will never appear on the screen
If you redirect input to come from /dev/null ...
the command will receive an empty string
/dev/null is most useful when dealing with error messages
Since error message normally go to the terminal ...
they will be mixed up with the regular output
Sending error messages to /dev/null ...
prevents this from happening

Today's Topics

Review

Homework 7

Review

Correcting Mistakes on the Command Line

Retrieving Your Last Command Line Entry

Aborting a Running Program

Getting Help with Unix Commands

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

echo - Print Text to the Terminal

Pagers - View a File One Screen at a Time

Pathname Completion

grep - Finding Strings inside Files

head - View the Top of a File

tail - View the Bottom of a File

sort - Print a File in Sorted Order

diff - Differences between Files

Pipes - Stringing Programs Together

date - Get the Date and Time

which - Finding a Program File

locate - Search for Any File

finger - Get information on Users

The Hierarchical Filesystem

Case Sensitivity

Filename Extensions

Current Directory

Your Home Directory

Navigating the Hierarchical File Systems

Hidden Filenames

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

Access Permissions

Viewing Access Permissions

chmod

Using chmod with Numeric Arguments

The root Account

Directory Access Permissions

Links

The Two Types of Links

ln

Removing a Link

Command Options

The PATH System Variable

Running a Program in the Current Directory

Standard Input, Standard Output and Standard Error

The Keyboard and Screen as Standard Input and Standard Output

Redirection

Redirecting Standard Output

Redirecting Standard Input

Redirecting Standard Output Can Destroy a File

Adding Output to an Existing File

/dev/null

Attendance

`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

`echo` - Print Text to the Terminal

`grep` - Finding Strings inside Files

`head` - View the Top of a File

`tail` - View the Bottom of a File

`sort` - Print a File in Sorted Order

`diff` - Differences between Files

`date` - Get the Date and Time

`which` - Finding a Program File

`locate` - Search for Any File

`finger` - Get information on Users

`chmod`

Using `chmod` with Numeric Arguments

`ln`