IT 244: Introduction to Linux/Unix
Class 18 Exercise, Tuesday, April 20th

Log in to pe15

• Run the ssh client
  

  Use the values in the table below to connect to pe15.

  ssh configuration parameter	value
  protocol	ssh
  ssh version	2
  ssh port	22
  host	pe15.cs.umb.edu
  user name	your Unix username
  Authentication method	password

  
  
• Enter your password

  A dialog box will appear on the screen into which you must type the password for your Unix account.

Directory for This Exercise

(When you are ready for me to check this exercise, please scroll back up to where you entered these commands!)

• Go to your ex directory

      cd  ~/it244/ex

• Create an ex18 directory
  

      mkdir ex18

• Go to your ex18 directory
  

      cd  ex18

Start Up a script Session

Execute the script command, using the --flush option

    script --flush

Continuing a Command on the Next Line

NOTE: Where you see the following notation:

    \[Enter]

...that means to input the backslash character,  \  , and then press the Enter key.

• Backslash with echo
  

      echo  Go  \[Enter]
      Red  \[Enter]
      Sox  [Enter]

  The greater than symbol,  >  , that you see after all but the last time you hit Enter is a secondary prompt.
  
  It tells you that the shell is awaiting more input.
  
  This will not work unless you hit the Enter key immediately after typing the backslash.
  
  
• More practice with backslash
  

      echo  The University  \[Enter]
      of Massachusetts  \[Enter]
      at Boston  [Enter]

• Try typing a space before hitting Enter after backslash
  

      echo  Hello \[Space][Enter] 

  The echo command does not wait for further input, it prints its argument and you don't get a secondary prompt.
  
  The backslash is a quoting character that turns off the special meaning of whatever character comes next.
  
  When you hit Enter immediately after the backslash, you are turning off the special meaning of the new line character, which tells the shell to execute a command.
  
  If you put a space, or any other character immediately after the backslash, that is the character that the backslash applies to, not the new line character which the shell receives after you hit Enter.

Changing Prompts

• Change your primary prompt
  

      PS1='Prompt-> '

  Note the single quotes.
  
  You must use them
  
  Since you have not added this to your .bash_profile, this will only be your prompt for this terminal session.
  
  
• Change your secondary prompt
  

      PS2='>> '

• Show your new secondary prompt
  

      echo  Go  \[Enter]
      Red  \[Enter]
      Sox  [Enter]

  You should see your new secondary prompt with each new line.

Semi-colon,  ; , to Separate Commands

• Use three echo commands on the same command line
  

      echo  foo;  echo  bar;  echo  bletch

  Each echo command generates its own line of output.
  
  
• Use three more echo commands on the same line
  

      echo  Unix;  echo is not;  echo  for the faint of heart

• Use different commands on the same line
  

      echo;  echo  This is what is in my home directory:;  ls  -al ~

& to Separate Commands

• Copy hi_bye.sh from ~ckelly/course_files/it244_files
  

      cp  ~ckelly/course_files/it244_files/hi_bye.sh  .

• Display the contents of hi_bye.sh
  

      cat hi_bye.sh

• Run mulitple instances of hi_bye.sh with & as a separator, then run the jobs command
  

      ./hi_bye.sh  & ./hi_bye.sh  & jobs

  The ampersand causes each of the two invocations of hi_bye.sh to run in the background.
  
  This means the ampersand has two functions, to cause a command to run in the background and to separate one command from the next.
  
  Notice that jobs is able to report on the two invocations of hi_bye.sh running in the background, because it is running in the foreground.
  
  
• Wait for completion of jobs
  
  After the second Goodbye appears, hit Enter.
  
  You should see a report that the two jobs have terminated.

Important Keyword Shell Variables

• Display PATH
  

      echo  $PATH

• Display HOME
  

      echo  $HOME

• Display SHELL
  

      echo  $SHELL

• Display PS2
  

      echo  $PS2

• Display some of your global variables
  

      env

Local Variables

• Create a local variable
  

      greeting=Hello

• Display the local variable you just created
  

      echo  $greeting

• Assign a string with a space in it to this local variable
  

      greeting='Hello there'

• Display the new value of this variable
  

      echo  $greeting

• Copy print_foo.sh from ~ckelly/course_files/it244_files
  

      cp  ~ckelly/course_files/it244_files/print_foo.sh  .

• Print this script to the screen
  

      cat  print_foo.sh

  This script simply prints the value of the variable foo to the screen.
  
  
• Create the local variable foo
  

      foo=FOO

• Print the value of foo
  

      echo $foo

• Run print_foo.sh
  

      ./print_foo.sh

  The script prints no value for foo, since foo was a local variable defined in your original shell.
  
  The script runs in a subshell and cannot see the local variables of its parent shell.

Global Variables

• Set foo to a new value and export it
  

      export foo=BAR

• Print foo in your current shell
  

      echo $foo

• Run print_foo.sh
  

      ./print_foo.sh

  Since the export command makes foo a global variable, print_foo.sh can now see this variable in the subshell in which it is run.

The Directory Stack

• Go to ~ckelly/course_files/it244_files/dir_tree
  

      cd  ~ckelly/course_files/it244_files/dir_tree

• Look at your current location
  

      pwd

• Look at the contents of the directory
  

      ls

• Use dirs to look at the directory stack
  

      dirs

  It only lists the current directory.
  
  
• Go down one level using pushd
  

      pushd  proj

  Two directories are now listed.
  
  
• Display your location
  

      pwd

• Look at the contents of the directory
  

      ls

• Go down another level
  

      pushd  proj1

  Three directories are now listed.
  
  
• Display your location
  

      pwd

• Go up one level using popd
  

      popd

  Now, only two directories are listed, because you poped the previous directory from the stack.
  
  
• Display your location
  

      pwd

• Go up one level, suppress output and get location, all on one line
  

      popd  >  /dev/null;  pwd

  By redirecting output to /dev/null, you have stopped popd from printing out the directory stack.
  
  By using a semi-colon followed by pwd, you have displayed your current location without having to wait for a prompt.

Exit Your script Session

Use the exit command

    exit

You should see a confirmation message confirming that the script session is finished, resulting in a file called typescript. I will look at the typescript file, along with the other files in your directory.

Bash Script for This Exercise

• Go back to your ex18 directory
  

      cd  ~/it244/ex/ex18

• Create the script ex18.sh
  

      nano  ex18.sh

• Write into this file commands from certain of the sections above
  
  Include Unix commands from the section Important Keyword Shell variables, to the section The Directory Stack.
  
  
• Save this file
  
  Save and quit.
  
  
• Make this scipt executable
  

      chmod 755 ex18.sh

• Test the script
  

      ./ex18.sh

  Fix any errors you find.
  
  If you have trouble, see me.