School of Computer Science, McGill University
COMP-206 Introduction to Software Systems, Winter 2021
Mini Assignment 2: Intro to Shell Scripting
Due Date Feb 10th, 18:00 EST
This is an individual assignment. You need to solve these questions on your own. If you have ques-
tions, post them on Piazza, but do not post major parts of the assignment code. Though small parts
of code are acceptable, we do not want you sharing your solutions (or large parts of them) on Piazza.
If your question cannot be answered without sharing significant amounts of code, please make a pri-
vate question on Piazza or utilize TA/Instructors office hours. Late penalty is -15% per day. Even if
you are late only by a few minutes it will be rounded up to a day. Maximum of 2 late days are allowed.
You MUST use mimi.cs.mcgill.ca to create the solution to this assignment. You must not use your
Mac command-line, Windows command-line, nor a Linux distro installed locally on your laptop. You can access
mimi.cs.mcgill.ca from your personal computer using ssh or putty and also transfer files to your computer using
filezilla or scp as seen in class and in Lab A and mini assignment 1.
All of your solutions must be composed of commands from the list provided at the end of this
assignment description and your scripts must be executable as is in mimi.
For this assignment, you will have to turn in two shell scripts. Instructors/TAs upon their discretion may ask
you to demonstrate/explain your solution. No points are awarded for commands that do not execute at all. (Com-
mands that execute, but provide incorrect behavior/output will be given partial marks.) All questions are graded
proportionally. This means that if 40% of the question is correct, you will receive 40% of the grade.
Please read through the entire assignment before you start working on it. You can loose several
points for not following the instructions. There are also some helpful hints given at the end of this document
that can be useful to you.
Lab C provides some background help for this mini assignment.
Total Points: 20
Ex. 1 — Creating a backup script (10 Points)
1. Your first task is to create a shell script, backup.sh. This script will take a backup directory name and a
filename as its input and create a backup of the file in the said backup directory.
An example invocation is given below.
$ ls -l ~/backups206
total 9
-rwx------ 1 jdsilv2 root 702 Jan 22 10:05 tester.sh.20210122
$ ls -l ~/COMP206/finalproj/tester.sh
-rwx------ 1 jdsilv2 root 702 Jan 29 12:49 /home/2013/jdsilv2/COMP206/finalproj/tester.sh
$ ./backup.sh ~/backups206 ~/COMP206/finalproj/tester.sh
$ ls -l ~/backups206
total 17
-rwx------ 1 jdsilv2 root 702 Jan 22 10:05 tester.sh.20210122
-rwx------ 1 jdsilv2 root 702 Jan 29 12:50 tester.sh.20210129
$
Here the directory to which we want to create backup files is ~/backups206 . The file we are interested in
backing up is ~/COMP206/finalproj/tester.sh
1
The source file MUST NOT be removed once the file is backed up.
2. (1 Point)
If the script is not passed its two arguments, the script should throw a usage message and terminate its
execution immeidately with an error code of 1
$ ./backup.sh
Usage: ./backup.sh backupdirname filetobackup
$ echo $?
1
$
3. (1 Point) If the directory name passed to the script is not valid (does not exist or is not actually a directory),
the script should throw the following error and terminate with an error code of 1
$ ./backup.sh /nosuchdir ~/COMP206/finalproj/tester.sh
Error!! /nosuchdir is not a valid directory
4. (1 Point) If the file name passed to the script is not valid (does not exist or is not actually a file), the script
should throw the following error and terminate with an error code of 1
$ ./backup.sh ~/backups206 nosuchfile
Error!! nosuchfile is not a valid file
5. (1 Point) Your script should work correctly irrescpective of whether relative path or absolute path are used
for the backup directory name and file name arguments.
6. (4 Points) The correct execution of the script will result in a copy of the file being made to the backup
directory, but with the date of execution appended to its name. In the sample output given in 1, tester.sh is
stored in the backup directory with the date on which it was backed up (2021, Jan 29 formatted as 20210129).
You should follow this formatting convention. (see the Hints section). The script should then terminate with
code 0.
7. (2 Points) If the script detects that a backup of the said file was already taken for the day to that backup
directory, it must not make another backup or overwrite the backup file, but report this to the user and
terminate with error code 1.
So continuing from the example in 1, if we run the backup script again on the same file (to the same backup
directory), we will receive the following message.
$ ./backup.sh ~/backups206 ~/COMP206/finalproj/tester.sh
Backup file already exists for 20210129
8. (0 Points) Any error messages thrown from your script should be made by specific echo statements (which
results in the messages shown above). You must suppress any other error/output messages that might result
from the commands that you are using and not let it show up in the output. Violating this would result in
-1 point penalty.
9. (0 Points) At no point in your backup script should you create any other files (other than your backup file),
even for temporary purposes. Violating this would result in -1 point penalty.
10. (0 Points) Any instance of an incorrect termination code (from what is specified for each circumstance)
would result in -1 point penalty.
Ex. 2 — Script to check for missing backups (10 Points),
In this task, you will write a script, chkbackups.sh that will help you check if there are any files in a directory that
you forgot to backup today.
1. (1 Point) The backup script expects the first argument to be the directory to which files are getting backed
up. The second argument is meant to be the directory which contains the files that you usually backup. If
the second argument is not provided to the script, it will assume that you want to check if all the files in the
current directory is backed up. In other words, if the script is not invoked in one of these forms, it must throw
an error and terminate with error code 1.
2
$ ./chkbackups.sh
Usage: ./chkbackups.sh backupdirname [sourcedir]
$ echo $?
1
2. (1 Point) Similar to the previous exercise, if any of the directory arguments passed to the script is not a
valid directory name, it should throw an error message and terminate with error code 1.
$ ./chkbackups.sh ~/backups206 /nosuchdir
Error!! /nosuchdir is not a valid directory
3. (2 Points) If the source directory is empty (has no files in it) then the script should display this message and
terminate with error code 1.
$ ./chkbackups.sh ~/backups206 ~/emptydir
Error!! /home/2013/jdsilv2/emptydir has no files
For simplicity, you can assume that source directories do not have any directories inside them.
4. (2 Points) If all the files in the source directory are already backed up for today in the backup directory, the
script should display this information and terminate with code 0.
$ ./chkbackups.sh ~/backups206 ~/COMP206/finalproj
All files in /home/2013/jdsilv2/COMP206/finalproj have backups for today in /home/2013/jdsi
lv2/backups206
5. (4 Points) If there are files in the source directory that is missing backup for today then the script should
list all such files and terminate with code 0.
$ ./chkbackups.sh ~/backups206 ~/COMP206/finalproj
tester.sh does not have a backup for today
You may list the file names (if multiple files are involved) in any order.
6. (0 Points) Any error messages thrown from your script should be made by specific echo statements (which
results in the messages shown above). You must suppress any other error/output messages that might result
from the commands that you are using and not let it show up in the output. Violating this would result in
-1 point penalty.
7. (0 Points) At no point in your backup script should you create any other files (other than your backup file),
even for temporary purposes. Violating this would result in -1 point penalty.
8. (0 Points) Any instance of an incorrect termination code (from what is specified for each circumstance)
would result in -1 point penalty.
You must write a reasonable amount of comments (to understand the logic) in both your scripts. You can lose
up to -2 points for not writing comments.
WHAT TO HAND IN
Upload both of your scripts, backup.sh and chkbackups.sh, to MyCourses. If MyCourses is not allowing files with
.sh extension, you may use the tar command as discussed in class to make a tar file or a tar.gz file that contains
your scripts and submit them. Please download your submitted tar files to double check if they are good. Errorneous
invocation of the tar command can sometimes result in a bad file. Such submissions cannot be unfortunately graded.
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ASSUMPTIONS
You may assume that any files and directories that your script needs to access will have the necessary permissions
for it to execute the tasks outlined in the assignment. You can also assume that the backup directory and source
directory do not have any subdirectories.
HINTS
• Explore the date command using its man page. Check what options you can pass to it so that the output
produced by the date command is formatted in the exact way you want it to be so that you can easily append
it to the name of the backup file.
• Explore the basename command to figure out how to extract only filename from the absolute path.
• How do you get rid of unwanted messages from the commands and utilities that you use in a shell script? Linux
provides a special file /dev/null to which you can discard any unwanted outputs. Explore how it is commonly
used in writing shell scripts.
• Learn the sytnax for performing logical OR, AND, NOT operations with your condition check/test operator.
COMMANDS ALLOWED
You may use any option provided by these commands, even ones that have not been discussed in class. Not all of
them may be required for you to build your solution.
[[ ]] ! basename dirname break
cd continue date diff echo
exit export for grep if
ls printf pwd $( ) cp
shift while mv
You may also use commands discussed in class but not listed here. You must not use the commands sed, awk, and
find.
MINITESTER
A tester script, mini2tester.sh, is provided with the assignment so that you can test how your scripts are behaving.
It is recommended that you first run your scripts yourself, test each of the options and arguments
using the examples above. Once you are fairly confident that your script is working, you can test it using the
tester script.
When you are ready, in order to run the tester, put the tester script in the same folder as your scripts for this
assignment and run
$ ./mini2tester.sh
The idea is that the tester’s output should be very similar in format or even identical to that of the example
output provided, minitester.out.txt except for directory names and timestamps.
QUESTIONS?
Please use piazza. However, before posting your question, use the search functionality to check it has been already
discussed. You should also look under “Mini 2 general clarifications” pinned post to check if a popular question has
been already included there. They will not get individual responses again.
FOOD FOR THOUGHT!
The following discussion is meant to encourage you to search independently for creative and optimal ways to perform
rudimentary tasks with less effort and does not impact the points that you can achieve in the above questions.
When you make the backup copy of the file, what option can you use to the utitlity used to copy the files, so that
it retains the modified timestamp (shown by ls -l) of the original file in the backupfile as well?
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