Lab 4: Conditionals and Variables

In this lab, you will be writing more complex scripts. You must still ensure your syntax (the spelling of commands and placement of characters) is correct, but as labs progress you will need to pay more and more attention to the logic of your scripts (how the commands and control blocks interact). To help with this, try writing out the logic of your script in comments before adding shell commands or control blocks. For example, let's write a simple script that prints the third line of a file, if that file exists. Otherwise print an error message. The script will be executed as ./print_third.sh file.txt

#!/bin/bash

# get file name from command line

# check if file exists

# if file doesn't exist, print error message and exit

# if file exists, print 3rd line

Ok, now let's add code

#!/bin/bash

# get file name from command line

# check if file exists
# if file doesn't exist, print error message
if [[ ! -f $1 ]]
then
    1>&2 echo ERROR
    exit 1
else
    # if file exists, print 3rd line
    head -n 3 $1 | tail -n 1
fi

Some of these comments are a bit redundant, so we can tidy them up. In finished code, your comments should be brief explanations of why and how the code works. You shouldn't comment what every line is doing, but adding a comment to explain complex operations is totally fine.

#!/bin/bash

# check if file exists, if not print error message
if [[ ! -f $1 ]]
then
    1>&2 echo ERROR
    exit 1
else
    # if file exists, print 3rd line
    head -n 3 $1 | tail -n 1
fi

Planning your scripts this way not only helps avoid errors, it also helps other people to understand your script. The most important person who will have to read and understand your script is your future self. You might be very confident that you understand the script now, but in a few weeks time when you come back and read what you wrote you will be very thankful for clear and commented code!

Questions

1. Modify print_third.sh so that it prints a different error if there is no input file (i.e. the first command line argument is an empty string). You can print to STDERR like this:

   1>&2 echo "THIS IS AN ERROR MESSAGE TO STDERR"
   

   The 1>&2 is redirecting the stdout from echo (1>) into stderr (&2)
   
   
2. What is the difference between an integer comparison and a string comparison? Give an example of when you would use each.
3. Write a script find_longer.sh which accepts two files as command line arguments and prints the name of the longer file. If both files are equal, print "equal length". If an argument is missing or refers to a non-existent file, print an appropriate error message and set a nonzero exit status.
   Hint: you will want to use wc for this task, but wc filename.txt prints the file name as well as the counts. To prevent wc from printing the file name, you can pipe in the output of cat. For example, cat filename.txt | wc will not print the file name.
4. Write a script same_head_and_tail.sh which checks whether the first and last lines of a file are the same.
5. Modify your lab3 script sandwichsay so that users can pass an optional second command line argument to define how the bread should look. You may assume that the bread will only be a single line.

   ./sandwichsay "ham and cheese"
   
   %============================%
   ham and cheese
   %============================%
   

   ./sandwichsay "ham and cheese" "[++BREADBREADBREADBREAD++]"
   
   [++BREADBREADBREADBREAD++]
   ham and cheese
   [++BREADBREADBREADBREAD++]
   

6. You have decided that you want to run sandwichsay from anywhere on the system without having to specify its location (you want to just call sandwichsay not ~/wherever/sandwichsay). How could you do this?

Department of Computer Science & Software Engineering The University of Western Australia Last modified: 8 February 2022 Modified By: Daniel Smith