Review

• 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
• Pipes - Stringing Programs Together
• which - Finding a Program 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
• Running a Program in Unix
• 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.

It is NOT due this coming Sunday.

Instead it is due Sunday, March 24th.

This is to give you time to study for the midterm.

And to give me time to score it.

Midterm

The Midterm exam for this course will be held on Tuesday, March 19th.

That is the first Tuesday after Spring Break

It will be given in this room.

It will consist of 25 questions like those on the quizzes.

60% of the questions will come from the Ungraded Class Quizzes.

The other 40% of points will be questions that I create specially for this exam.

For these questions you will have to know

• Absolute and relative pathnames
• The PATH system variable
• Access permissions
• Redirection & and pipes
• grep
• Utilities

Today's class will be a review session.

You will only be responsible for the material in today's Class Notes on the exam.

You will find the Midterm review Class Notes here.

If for some reason you cannot take the exam on the date mentioned above you must contact me to make alternate arrangements.

The Midterm is given on paper.

I scan each exam paper and upload the scans to Gradescope.

I score the exam on Gradescope.

You will get an email from Gradescope with your score when I am done.

The Midterm is a closed book exam.

You are not allowed to any resource, other than what is in your head, while taking the exam.

Cheating on the exam will result in a score of 0 and will be reported to the Administration.

Remember your Oath of Honesty.

To prevent cheating, certain rules will be enforced during the exam.

Quiz 5

I have posted the answers to Quiz 5 here.

No Class Exercise or Class Quiz

Today is a review session.

There will no Class Exercise or Class Quiz today.

Questions

Are there any questions before I begin?

Review

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
• The help option for GNU programs is --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 ...
• or the the Return key on a Mac
• 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 typing 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
• But you must put a $ before the variable name

  $ 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, hit Tab a second time
• bash will display a list of all possible matches

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

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 get 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 command
• You never need to specify the input ...
• when using a command in a pipeline ...
• except the first command

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

finger - Get information on Users

• finger, like mv, has two functions
• When run with a name or username as an argument
• finger provides information about that user

  $ finger ghoffman
  Login: ghoffman       			Name: Glenn Hoffman
  Directory: /home/ghoffman           	Shell: /bin/bash
  On since Thu Oct 20 07:00 (EDT) on pts/12 from 130.44.154.89
     3 seconds idle
  Mail forwarded to glenn.hoffman@umb.edu
  
  New mail received Fri Jul  8 13:31 2022 (EDT)
       Unread since Fri Jul  8 13:30 2022 (EDT)
  Plan:
  Office:		Science 3-92A (Pterosaur poster on door)
  Office Hours:   Tuesday & Thursday, 12:30 - 1:30 PM and 5:30 - 6:30
  Classes:	IT 244-11    TuTh 11:00 - 12:15		McCormack M03-0430
  		IT 116-11    TuTh  2:00 -  3:15		McCormack M03-0430
  		IT 117-11    TuTh  4:00 -  5:15		Wheatley W01-0061
  Home Address:  	40 Central Street     
                 	Somerville, MA 02143
                 	617-821-3936 (9am - 9pm only)

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

  $ finger
  Login    Name                    Tty      Idle  Login Time   Office     Office Phone
  emperor  Jean Gerard             pts/13   7:14  Oct 19 19:34 (73.61.22.4)
  emperor  Jean Gerard             pts/20   8:41  Oct 19 19:46 (73.61.22.4)
  ericode  Eriksson  Xavier Osori  pts/0      4d  Oct 15 13:39 (10.200.5.52)
  ericode  Eriksson  Xavier Osori  pts/23     4d  Oct 15 18:09 (10.200.5.52)
  estevao  Estevao Lordeiro        pts/10   1:54  Oct 20 05:15 (66.31.196.187)
  gghinita Gabriel Ghinita         pts/4      18  Oct 20 04:25 (176.202.52.62)
  ghoffman Glenn Hoffman           pts/12         Oct 20 07:00 (130.44.154.89)
  kkfletch Kenneth Fletcher       *pts/19     1d  Oct 13 17:00 (172.18.23.172)
  ldavid   Leonard David          *pts/9       9  Oct 20 06:58 (172.18.23.176)
  ldavid   Leonard David           pts/11     11  Oct 20 06:41 (172.18.23.176)
  morgan   Sean Geary              pts/25  13:14  Oct 19 17:50 (10.200.5.63)
  rickm    Richard Martin         *pts/8      1d  Oct 18 22:23 (24.147.140.64)
  rouilj   John P. Rouillard      *pts/6    7:32  Oct 13 13:44 (71.184.142.113)
  teebanj  Tayo Banjo              pts/14  16:30  Oct 12 15:27 (158.121.33.16)
  tmullaly Thomas Mullaly         *pts/1   16:55  Oct 17 16:13 (158.121.33.14)
  tmullaly Thomas Mullaly         *pts/15     1d  Oct 17 19:06 (216.180.91.178)

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
• At least as far as Unix is concerned

Filename Extensions

• Extensions are 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

• When you connect to a Unix machine you always have a position in the filesystem
• This location is your current directory
• Relative pathnames always start with 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 .plan
• 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

Navigating the Hierarchical File Systems

• 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

Attendance

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
• Slash, / is used to mark the end of a directory name
• 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

• bash replaces ~ with the absolute pathname of your home directory

  $ echo ~
  /home/ghoffman

• ~ followed by a username ..
• is replaced it with the absolute pathname of that user's home directory

  $ echo ~it244gh
  /home/it244gh

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, /
• 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 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 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 put 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  ..
• You keep going up until you get to a directory ...
• that is the ancestor of your current directory ...
• and an ancestor of 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 can only be changed by the owner of a file or directory
• The permissions determine 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 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

  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/s24/ghoffman/it244gh/
  lrwxrwxrwx 1 it244gh ugrad   36 Sep 23  2017 it117 -> /courses/it117/s24/ghoffman/it244gh/
  lrwxrwxrwx 1 it244gh ugrad   36 Sep 23  2017 it244 -> /courses/it244/s24/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
• 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 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 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
• 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

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

• Access permissions work 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 or delete the directory itself
• To do that you must write permission on its parent directory
• Execute permission on a directory allows you to enter it using cd

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/s24/ghoffman/it244gh

• This is a link to/courses/it244/s24/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 see that if you use pwd ...
• with the -P (note the capitalization) option

  $ pwd
  /home/it244gh/it244
  
  $ pwd -P
  /courses/it244/s24/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 on the same disk partition
• 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

Running a Program in Unix

• When you run a program in Unix you create a process
• A process is a running program
• Processes have resources
  • Memory in RAM in which they run
  • The data streams standard input, standard output, and standard error
  • Time on the CPU to do their work
• bash cannot create a process
• Only the kernel can
• A process starts when you type a command and hit Enter
• bash asks the kernel to create a process for the program
• It then goes to sleep until the process ends
• When the process ends it sends a status code back to the kernel
• The kernel then wakes up bash and gives it the status code
• A status code of 0 means the process ran without error
• A number greater than 0 indicates some kind of error

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 pathname for the executable file
• Most of the time when you run a program you don't use a pathname ...
• you simply use the name of the program
• To get the 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 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

• What if the file is not found in PATH?
• In that case you will get an error message
• If you do not have execute privileges you will also get error message
• You can modify the PATH variable in your own Unix environment

Running a Program in the Current Directory

• For security reasons, you shold never 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 data streams
  • Standard Input
  • Standard Output
  • Standard Error

• Standard input is where the program normally gets input
• By default, standard input is the keyboard
• Standard output is where the program normally sends its output
• By default, standard output is the terminal
• Standard error is where the program normally 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 program does not specify the source of input ...
• it comes from standard input
• By default, standard input is keyboard
• When a program does not specify the destination of output ...
• it goes to standard output
• By default, standard output is the screen
• When a program 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 it to, a different place
• 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
• A pipe sends the output from one program into the input of another
• The output of the first command is redirected from the screen to the input of the second command

Redirecting Standard Output

• To redirect standard output use the greater than symbol, > ...
• followed by a filename
• The format for output redirection is

  COMMAND  [ARGUMENTS]  >  FILENAME

• If the file does not already exist it will be created

Redirecting Standard Input

• Redirecting standard output sends output to something other than the screen
• Redirecting standard input 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

• Redirecting standard output to an existing file overwrites its contents
• You lose the original contents of that file

Adding Output to an Existing File

• Redirecting standard output to an existing file overwrites its contents
• 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
• Redirecting input to /dev/null gives a program the empty strin
• /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