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Tutorial 12 - Shell Scripting - Part 2

6,842 bytes added, 14:10, 14 November 2021

→‎INVESTIGATION 3: exit AND break STATEMENTS

:In the next investigation, you will learn to use the '''exit''' statement to '''terminate the execution of a shell script''' if not run with the properly number of arguments and use the '''break''' statement that will '''terminate a loop''' but NOT terminate the running of the shell script.

= INVESTIGATION 3: ~~USING STARTUP FILES~~ exit AND break STATEMENTS = In this investigation, you will learn to use the '''exit''' and '''break''' statements in your shell scripts. '''THE EXIT STATEMENT''' The '''exit''' statement is used to terminate a shell script. This statement is very useful when combined with logic in a shell script to display an '''error message''' if the command was '''improperly executed''' and '''terminate''' the running of the shell script. The ''exit'' command can contain return a ''value'' to provide the '''exit status''' of your shell script (i.e. TRUE or FALSE value). '''Perform the Following Steps:''' # Make certain that you are logged into matrix account. # Confirm that you are currently located in the '''advanced''' directory. # Use a text editor like vi or nano to create the text file called '''exit.bash''' (eg. vi exit.bash) # Enter the following lines in the '''exit.bash''' shell script: #!/bin/bash if [ $# -ne 1 ] then   echo "USAGE: $0 [arg]" >&2   exit 1 fi echo "The argument is: $1" # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # '''Add execute permissions''' for this Bash shell script. # Issue the following command (without arguments): ./exit.bash What did you notice? Since there are no arguments, the test within the running shell script returns FALSE, then an '''error message''' with feedback of how to properly issue the shell script with an argument and then '''terminates''' the Bash shell script. Notice that the '''$0''' positional parameter displays the '''name''' of the currently running shell script in the USAGE message. This is useful in case you decide to '''change''' the ''name'' of the shell script at a later time. The symbol '''>&2''' redirects '''standard output''' from the USAGE message to '''standard error''' making like a real error message. This "''neat redirection trick''" will NOT be considered for evaluation for this coverage. # Issue the following Linux command: echo $? What does this '''exit status''' from the previously issued command indicate? # Issue the following command (with an argument): ./exit.bash osl640 What did you notice this time? # Issue the following Linux command: echo $? What does this '''exit status''' from the previously issued command indicate? # Issue the following command (with two arguments and redirecting stderr to a file): ./exit.bash osl640 Linux 2> error.txt What did you notice this time? # Issue the following Linux command: echo $? # Issue the following Linux command to confirm that stderr was redirected to a file: cat error.txt '''THE BREAK STATEMENT''' The '''break''' statement is used to '''terminate''' a '''loop''' without terminating the running shell script. '''Perform the Following Steps:''' # Make certain that you are logged into matrix account. # Confirm that you are currently located in the '''advanced''' directory. # Use a text editor like vi or nano to create the text file called '''break-1.bash''' (eg. vi break-1.bash) # Enter the following lines in the '''break-1.bash''' shell script: #!/bin/bash read -p "Enter an integer: " number while ! echo $number | egrep "^[0-9]{1,}$" > /dev/null 2> /dev/null || [ $number -ne 5 ] 2> /dev/null do   if [ $number -eq 5 ] 2> /dev/null   then      break   fi   read -p "Try again. Enter a valid integer: " number done echo "The number is: $number" # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # '''Add execute permissions''' for this Bash shell script. # Issue the following command (without arguments): ./break-1.bash When prompted, enter several '''invalid''' and '''valid''' integers. Then enter '''valid integers''' NOT containing the value of '''5'''. Finally, enter the integer with the value of '''5'''. What happens? Let's use the '''break''' statement with the '''for''' loop. # Use a text editor like vi or nano to create the text file called '''break-2.bash''' (eg. vi break-2.bash) # Enter the following lines in the '''break-2.bash''' shell script: #!/bin/bash for x do   if [ $x = "osl640" ]   then      break   fi   echo "Argument is: $x" done echo echo "Shell script has been completed" # Save your editing session and exit the text editor (eg. with vi: press '''ESC''', then type ''':x''' followed by '''ENTER'''). # '''Add execute permissions''' for this Bash shell script. # Issue the following command (with arguments): ./break-2.bash nwk680 prg600 osl640 osm620 What do you notice? How come '''osl640''' and '''osm620''' are NOT displayed but a message appeared at the end of the script that the script completed? # Issue the following to run a checking script: ~osl640/week12-check-3 # If you encounter errors, make corrections and '''re-run''' the checking script until you receive a congratulations message, then you can proceed. : In the next investigation, we will learn to create / modify '''startup files''' to customize your Linux shell environment.

= LINUX PRACTICE QUESTIONS =

Jason Michael Carman

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Tutorial 12 - Shell Scripting - Part 2

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