Lab 13 mnjk

Introduction

In this lab you will create and modify some basic BASH scripts.

Lab Procedure

Prerequisites

1. Have completed the reading on BASH scripting. If you haven't done this you will almost certainly have difficulty completing the lab!
2. Open an SSH console to your Linux system using the PuTTY software, login with your standard user account

Creating Simple BASH Scripts

At their core BASH scripts are really just lists of commands which could have been entered directly into the command line but have been pre-entered into a file to be run later instead. We can see how this works by creating a couple of very simple BASH scripts which each just run a single command. Your ability to write BASH scripts that do something useful is really only limited by how well you know Linux commands and utilities which could be included in the script. The echo command can be used on your system to print information out to the standard output.

1. Lets try running a command echo on your computer:
echo Hello World!
You should see the text "Hello World!" printed out on the screen below your command.
2. Create a new text file named hello.sh with the following text:
• In order to turn that command into a script we just need to put the command into a text file and add one more line, called a sha-bang because it is made up of a hash (pound) sign and a bang (exclamation) sign. This line tells the system what program to use to interpret the commands in the script. In this case we're going to be writing BASH scripts so we'll set it for the BASH program.

#!/bin/bash
echo Hello World!

• Set the hello.sh file to be executable by all users on the system so that we will be able to run it.
• NOTE: As a safety precaution against mistakenly executing something you don't really want to the shell will not allow you to run a file by just typing the filename, even if you are in the correct directory. The location of the file must either be included in your system's PATH, you must specify the full path to the file (such as /home/jsmith/hello.sh), or if you are in the correct directory already you can put in a relative path like ./hello.sh. Remember, a single period is a shortcut which means the current directory so ./ means a file in the current working directory. Try running your hello.sh script now. If your system outputs Hello World! just like when you ran the echo command from the command line directly you have successfully written your first BASH script!
3. Now try using what you have learned to create and test another script named simple-backup.sh which executes the command tar -czf /var/jsmith-backup.tar.gz /home/jsmith/ when the script is run.

Using Variables and Conditionals in BASH Scripts

Variables are a key part of turning a simple list of instructions you could type into a command line into something much more powerful by allowing you to reuse code and do something very similar (remember programmers are lazy!) but with a little tweak each time without re-writing the whole thing. Variables can be defined in the script itself but there are also special variables which are part of the BASH environment we can use. Variables can be identified in bash scripts as starting with a $. One type of these variables allows us to access arguments which have been supplied as part of the command line calling the script. Sometimes we only want to run certain sections of the script or we want to modify the script depending on the outcome of some test, called a conditional. This is often used in combination with variables as we'll see in the example below. In BASH the script filename is assigned to the $0 variable, the first argument to $1 and so on. Let's see how we can modify our hello.sh script to make it a bit more personable by including a name.

1. Try adjusting your hello.sh script to look like this:

#!/bin/bash
echo Hello $1!

• What do you think will happen if you run the script like this ./hello yourfirstname? Try it and find out if you were correct in your hypothesis!
• Wait, what if someone doesn't know they are supposed to include their name as an argument though? Try just running ./hello.sh again and see what happens.
It might be the case that we want to go back to the simple Hello World! and maybe add some instructions if they don't specify an argument.
2. Try updating your hello.sh script again and adding a conditional like this:
See if you can predict what these changes will do when the script is run and then try it out to see if you are correct.

#!/bin/bash
if [ -n "$1" ]; then
    echo Hello $1!
else
    echo Hello World!
    echo Try running $0 yourfirstnamehere to get a personal greeting!
fi

• It's also possible to capture the standard output of a command executed by the script and put that into a variable for later use. For example, try running the date +%Y%m%d command. We can use the output of this command in a variable to make our backup script a little bit nicer.
3. Save this script as better-backup.sh with the appropriate permissions to run it as a script:

#!/bin/bash
USER="jsmith"
TODAY=$(date +%Y%m%d)
BACKUPFILE="/var/"$USER"-backup-"$TODAY".tar.gz"
echo Beginning backup for $USER on $TODAY
tar -czf $BACKUPFILE /home/$USER/
echo Backup completed for $USER saved to $BACKUPFILE

• See if you can predict what this script will do and then try it out to see if you are correct. Once you have figured out how this script works see if you can modify it so you can specify a username as an argument instead of as a fixed variable. Can you do this with a change to only one line?
• After successfully doing that try modifying the script so that it gives instructions about how to use the script if you forget to give a username as an argument.
It is important to note that while this script works, it's just a simple example and not well written because it is easily broken by giving a username which doesn't actually exist on the system, or trying to backup files you don't have permission to access, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input such as an argument.

Using Loops and Reading User Input

Sometimes you want a program to do something over and over again several times. This is typically done using a loop. Let's say you wanted to create a bunch of sample files to do some further practice with in the current directory named sample-fileX.extension where X was a number that would keep incrementing by 1 each time and you could specify any extension you wanted. We could certainly use arguments to capture that user input but it is also possible to accept input from the user directly while the script is running. This would create what we call an "interactive" script because the user is interacting with it while it runs.

1. Create a file-generator.sh script on your system like the one below and see if you can figure out how it works before testing it:

   #!/bin/bash
   BASENAME=sample-file
   echo "Welcome to the sample file generation utility!"
   echo "Files will be created using the filename format "$BASENAME"X.extension"
   echo "Enter the number of files you wish to create, then press Enter:"
   read NUM
   echo "Enter the extension (without the leading period) to put on the files"
   read EXTENSION
   for CURNUM in `seq 1 $NUM`; do
       touch $BASENAME$CURNUM.$EXTENSION
   done
   echo "Created "$NUM" file(s)."

   There are a few things to note about this script. First, we have used one type of loop, a FOR loop, but there are others and other ways to make this work. Second you can see several more examples of calling another program inside your script. In this case we have used the read, seq, and touch programs to accomplish our goal. The seq program is especially useful while writing scripts. It can be used to generate a series of numbers from a starting number (1 in this case) to a stopping number ($NUM in this case). There are many other helpful programs you can use while writing scripts including some you know about already such as grep and find, and others you don't such as sed and awk. You can even use programs like wget to get information from the Internet and process it for use in a script instead of using it for just downloading a file. For example, if you know of a web site that lists the sunrise and sunset times you can use a combination of wget and grep/awk/sed to pull that time out of the webpage and use it as part of a script. The possibilities are almost endless, time spent learning about the ins and outs of these little utilities is time well spent if you will be writing shell scripts!
• Try running the script and see if you were correct about what it does and how it works.
NOTE: Also, it is important to note that while this script works, it's just a simple example and not well written because it is easily broken by the user entering something other than a number for the number of files to be created, etc. A better script could be written to check for all of these things as well as other errors. If the script is to be used by more people than the original author (and in the best cases, even then) it is important for usability and security purposes to do these sorts of checks, particularly if the script accepts user input! This is how security bugs are born.
• See if you can modify the script so you can specify a starting and an ending number for the files rather than just a number of files to be created.
• See if you can add a check to the script which will display an example of a filename that will be created and asking the user if that looks correct before actually creating all the files. If the user enters N then do not create the files.

Functions and Experimenting

One final programming structure we should discuss is the function. In shell scripting you can think of a function like something you could have put into a separate script so that you could call it to do something multiple times from inside of your script but instead of making it a completely separate file you decided to include it in a single script.

1. Here is a sample script using a function:

#!/bin/bash
function helloname {
    echo "Type your name, followed by enter:"
    read NAME
    echo "Hello "$NAME"!"
}
echo "This program will welcome you twice to be extra nice!"
helloname
echo "At this point we could be doing lots of other stuff in our script or be in a loop, etc."
helloname
echo "Thanks for playing!"

• See if you can figure out what will happen when you run it, then try it out and see if you were correct. Note that functions need to be declared (listed) before you use them in your script so it would be common to see them at the beginning of the file as in this example. Also, it's possible to pass information to a function using arguments just like you would with a separate script.
2. Now that you have a grasp on the fundamentals of BASH scripting see if you can write a script which can be used to modify the name of all files in the current directory which share the same extension such as those created by file-generator.sh. The goals for your script should be:
   • The user has a choice to enter the original and the new extension for the files either using command line arguments or interactively by answering questions your script asks.
   • The script should display each old file name and then the new file name next to it so you can see what is going on
   • You will want to use conditionals to figure out if the user entered arguments to use as the old and new extensions or should be prompted for them interactively.
   • You should use functions and loops in order to make the most efficient script possible. Hints: The actual renaming of the files and displaying old and new names would be a good thing to put in a function because you will be calling it over and over again. Also, loops can be used to operate on a list of things such as a list of filenames.
   Don't be afraid to try using search engines for help with this. The goal of this lab is not to make you into a professional programmer but to introduce you enough to shell scripting so that you can create useful scripts with resources such as search engines available to you.