IT 244: Introduction to Linux/Unix
Class 19 Exercise, Tuesday, April 27th

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 ex19 directory
  

      mkdir  ex19

• Go to your ex19 directory
  

      cd  ex19

Start Up a script Session

Execute the script command, using the --flush option

    script --flush

Running Processes

• Look at the processes currently running
  

      ps -f

  The -f option to ps gives you more information about running processes.
  
  

  UID	The username of the account that started the process
  PID	The process ID
  PPID	The process ID of the parent process
  STIME	When the process started
  CMD	The command line that started the process

  
  
• Run a job along with ps
  

      sleep 120 & ps -f

  Notice that the process ID of the parent process that called sleep and ps -f is the process ID of your shell.

The Process Hierarchy

• Run pstree while several sleep processes are running
  

      sleep 20 & sleep 20 & pstree -ph

  Look at the bold line of processes, which denotes your shell. You will notice sleep processes running under your bash shell.

Positional Parameters

• Copy print_positionals.sh to your current directory
  

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

• Look at the contents of this script
  

      cat print_positionals.sh

  The script simply prints the first four tokens on the command line.
  
  
• Run print_positionals.sh with three arguments
  

      ./print_positionals.sh foo bar bletch

  Notice that the first positional parameter printed, 0, is the name of the command.
  
  
• Copy print_arg_numbers.sh to your current directory
  

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

• Look at the contents of this script
  

      cat print_arg_numbers.sh

• Run this script with some arguments
  

      ./print_arg_numbers.sh foo bar bletch

  The script prints the number of arguments given to it on the command line.

Quotes with Variables

• Set a local variable
  

      city=Boston

• Display the value of this variable
  

      echo $city

• Set a new variable to the value of this variable without quotes
  

      new_city=$city

• Display the value of this new variable
  

      echo $new_city

  The value of the new variable has been set to that of the old variable.
  
  
• Repeat the above using single quotes
  

      new_city='$city'

• Display the new value of the variable
  

      echo $new_city

  Notice the dollar sign,  $  , indicating that the variable was not evaluated.
  
  
• Repeat the above using a backslash before the dollar sign
  

      new_city=\$city

• Display the new value of the variable
  

      echo $new_city

  Again the dollar sign appears.
  
  
• Set a variable using double quotes,  " " 
  

      new_city="$city $city $city"

• Display the new value of the variable
  

      echo $new_city

  The values of each of the variables was substituted properly in the new value, along with the spaces between them.
  
  
• Set a variable using a phrase
  

      cheer="Go Red Sox"

• Display the value of this new variable
  

      echo $cheer

Removing a Variable's Value

• Set two variables
  

      city=Cambridge ; state=MA

• Display the values of these new variables
  

      echo $city ; echo $state

• Unset city using unset
  

      unset city

• Display the value of city
  

      echo $city

  city now has no value.
  
  
• Remove the value of state by assigning it the empty string
  

      state=

• Display the value of state
  

      echo $state

  Now state has no value.
  
  

Making a Variable Read Only

• Create the variable team
  

      team=Patriots

• Display the value of team
  

      echo $team

• Change the value of team
  

      team="Red Sox"

• Display the value of team
  

      echo $team

• Make team read only
  

      declare -r team

• Try to change the value of team
  

      team=Celtics

  You can't change the value because team is now read only.

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 to your ex19 directory
  

      cd  ~/it244/ex/ex19

• Create the script ex19.sh
  

      nano  ex19.sh

• Write into this file commands from certain of the sections above
  
  Include Unix commands from the section Positional Parameters, to the section Making a Variable Read Only.
  
  
• Save this file
  
  Save and quit.
  
  
• Make this scipt executable
  

      chmod 755 ex19.sh

• Test the script
  

      ./ex19.sh

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