COMP(2041|9044) 23T2 — Assignment 1: Pigs
1/8
Assignment 1: Pigs
version: 1.3 last updated: 2022-07-04 930
Aims
This assignment aims to give you
practice in Shell programming generally
a clear concrete understanding of Git's core semantics
Note: the material in the lecture notes will not be sufficient by itself to allow you to complete this assignment. You may need to search online documentation for Shell, Git, etc. Being able to search documentation efficiently for the information you need is a very useful skill for any
kind of computing work.
Introduction
Your task in this assignment is to implement Pigs, a subset of the version control system Git.
Git is a very complex program that has many individual commands. You will implement only a few of the most important commands. You will
also be given a number of simplifying assumptions, which make your task easier.
Pigs stands for POSIX Implementation of Git in Shell.
As you must implement Pigs in POSIX Shell (dash).
Interestingly, early versions of Git made heavy use of Shell and Perl.
Reference implementation
Many aspects of this assignment are not fully specified in this document; instead, you must match the behaviour of a reference
implementation.
For example, your script pigs-add should match the behaviour of 2041 pigs-add exactly, including producing the same error messages.
Provision of a reference implementation is a common method to provide or define an operational specification, and it's something you will
likely need to do after you leave UNSW.
Discovering and matching the reference implementation's behaviour is deliberately part of the assignment.
While the code in the reference implementation is fairly straightforward, reverse-engineering its behaviour is obviously not so simple, and is
a nice example of how coming to grips with the precise semantics of an apparently obvious task can still be challenging.
If you discover what you believe to be a bug in the reference implementation, report it in the class forum. We may fix the bug, or indicate that
you do not need to match the reference implementation's behaviour in this case.
Pigs Commands
Subset 0
Subset 0 commands must be implemented in POSIX-compatible Shell.
See the Permitted Languages section for more information.
pigs-init
The pigs-init command creates an empty Pigs repository.
pigs-init should create a directory named .pig , which it will use to store the repository.
It should produce an error message if this directory already exists, or cannot be created.
You should match this, and other error messages exactly. For example:
$ ls -d .pig
ls: cannot access '.pig': No such file or directory
$ ./pigs-init
Initialized empty pigs repository in .pig
$ ls -d .pig
.pig
$ ./pigs-init
./pigs-init: error: .pig already exists
pigs-init may create initial files or directories inside .pig .
You do not have to use a particular representation to store the repository.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
2/8
You do not have to (and should not) create the same files and directories inside .pig as the reference implementation.
You can create whatever files or directories inside .pig you wish.
Do not store information outside .pig
pigs-add filenames...
The pigs-add command adds the contents of one or more files to the “index”.
Files are added to the repository in a two-step process. The first step is adding them to the index.
You will need to store files in the index somehow in the .pig sub-directory.
For example, you might choose store them in a sub-directory of .pig .
Only ordinary files in the current directory can be added.
You can assume filenames start with an alphanumeric character ( [a-zA-Z0-9] ) and will only contain alpha-numeric characters, plus . , -
and _ characters.
The pigs-add command, and other Pigs commands, will not be given pathnames with slashes.
pigs-commit -m message
The pigs-commit command saves a copy of all files in the index to the repository.
A message describing the commit must be included as part of the commit command.
Pigs commits are numbered sequentially: they are not hashes, like Git. You must match the numbering scheme.
You can assume the commit message is ASCII, does not contain new-line characters, and does not start with a - character.
pigs-log
The pigs-log command prints a line for every commit made to the repository.
Each line should contain the commit number and the commit message.
pigs-show [commit]:filename
The pigs-show should print the contents of the specified filename as of the specified commit.
If commit is omitted, the contents of the file in the index should be printed.
You can assume the commit, if specified, will be a non-negative integer.
Subset 0 examples
$ echo line 3 >>a
$ ./pigs-add a
$ echo line 4 >>a
$ ./pigs-show 0:a
line 1
$ ./pigs-show 1:a
line 1
line 2
$ ./pigs-show :a
line 1
line 2
line 3
$ cat a
line 1
line 2
line 3
line 4
$ ./pigs-show 0:b
hello world
$ ./pigs-show 1:b
hello world
Subset 1
Subset 1 is more difficult. You will need to spend some time understanding the semantics (meaning) of these operations, by running the
reference implementation, or researching the equivalent Git operations.
Note the assessment scheme recognises this difficulty.
Subset 1 commands must be implemented in POSIX-compatible Shell.
See the Permitted Languages section for more information.
pigs-commit [-a] -m message
pigs-commit can now have a -a option,
which causes all files already in the index to have their contents from the current directory added to the index before the commit.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
3/8
pigs-rm [--force] [--cached] filenames...
pigs-rm removes a file from the index, or, from the current directory and the index.
If the --cached option is specified, the file is removed only from the index, and not from the current directory.
pigs-rm , like git rm , should stop the user accidentally losing work, and should give an error message instead if the removal would cause
the user to lose work. You will need to experiment with the reference implementation to discover these error messages. Researching
git rm 's behaviour may also help.
The --force option overrides this, and will carry out the removal even if the user will lose work.
pigs-status
pigs-status shows the status of files in the current directory, the index, and the repository.
There are many different cases to consider for pigs-status .
You will need to experiment with the reference implementation to find them all.
Subset 1 examples
$ ./pigs-init
Initialized empty pigs repository in .pig
$ touch a b c d e f g h
$ ./pigs-add a b c d e f
$ ./pigs-commit -m 'first commit'
Committed as commit 0
$ echo hello >a
$ echo hello >b
$ ./pigs-commit -a -m 'second commit'
Committed as commit 1
$ echo world >>a
$ echo world >>b
$ echo hello world >c
$ ./pigs-add a
$ echo world >>b
$ rm d
$ ./pigs-rm e
$ ./pigs-add g
$ ./pigs-status
a - file changed, changes staged for commit
b - file changed, changes not staged for commit
Subset 2
Subset 2 is extremely difficult. You will need to spend considerable time understanding the semantics of these operations, by running the
reference implementation, and/or researching the equivalent Git operations.
Note the assessment scheme recognises this difficulty.
Subset 2 commands must be implemented in POSIX-compatible Shell.
See the Permitted Languages section for more information.
pigs-branch [-d] [branch-name]
pigs-branch either creates a branch, deletes a branch, or lists current branch names.
If branch-name is omitted, the names of all branches are listed.
If branch-name is specified, then a branch with that name is created or deleted,
depending on whether the -d option is specified.
pigs-checkout branch-name
pigs-checkout switches branches.
Note that, unlike Git, you can not specify a commit or a file: you can only specify a branch.
pigs-merge (branch-name|commit-number) -m message
pigs-merge adds the changes that have been made to the specified branch or commit to the index, and commits them.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
4/8
Subset 2 examples
Committed as commit 1
$ ./pigs-checkout master
Switched to branch 'master'
$ cat 7.txt
1
2
3
4
5
6
7
$ ./pigs-merge b1 -m merge-message
Fast-forward: no commit created
$ cat 7.txt
1
42
3
4
5
6
7
If a file has been changed in both branches pigs-merge produces an error message.
Note: if a file has been changed in both branches git examines which lines have been changed and combines the changes if possible. Pigs
doe not do this, for example:
$ ./pigs-init
Initialized empty pigs repository in .pig
$ seq 1 7 >7.txt
$ ./pigs-add 7.txt
$ ./pigs-commit -m commit-1
Committed as commit 0
$ ./pigs-branch b1
$ ./pigs-checkout b1
Switched to branch 'b1'
$ sed -Ei 's/2/42/' 7.txt
$ cat 7.txt
1
42
3
4
5
6
7
$ ./pigs-commit -a -m commit-2
Committed as commit 1
$ ./pigs-checkout master
Testing
Autotests
As usual, some autotests will be available:
$ 2041 autotest pigs pigs-*
...
You can also run only tests for a particular subset or an individual test:
$ 2041 autotest pigs subset1 pigs-*
...
$ 2041 autotest pigs subset1_13 pigs-*
...
If you are using extra Shell files, include them on the autotest command line.
Autotest and automarking will run your scripts with a current working directory different to the directory containing the script. The directory
containing your submission will be in $PATH .
You will need to do most of the testing yourself.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
5/8
Test Scripts
You should submit ten Shell scripts, named test00.sh to test09.sh , which run pigs commands that test an aspect of Pigs.
The test??.sh scripts do not have to be examples that your program implements successfully.
You may share your test examples with your friends, but the ones you submit must be your own creation.
The test scripts should show how you've thought about testing carefully.
You are only expected to write test scripts testing parts of Pigs you have attempted to implement. For example, if you have not attempted
subset 2 you are not expected to write test scripts testing pigs-merge .
Permitted Languages
Your programs must be written entirely in POSIX-compatible shell.
Your programs will be run with dash, in /bin/dash . You can assume anything that works with the version of /bin/dash on CSE systems is
POSIX compatible.
Start your programs with:
#!/bin/dash
If you want to run these scripts on your own machine — for example, one running macOS — which has dash installed somewhere other than
/bin , use:
#!/usr/bin/env dash
You are permitted to use any feature /bin/dash provides.
On CSE systems, /bin/sh is the Bash (Bourne-again shell) shell: /bin/sh is a symlink to /bin/bash . Bash implements many non-POSIX
extensions, including regular expressions and arrays. These will not work with /bin/dash , and you are not permitted to use these for the
assignment.
You are not permitted to use Perl, Python or any language other than POSIX-compatible shell.
You are permitted to use only these external programs:
b
Only a few of the programs in the above list are likely to be useful for the assignment.
Note you are permitted to use built-in shell features including: cd , exit , for , if , read , shift and while .
If you wish to use an external program which is not in the above list, please ask in the class forum for it to be added.
You may submit extra shell files.
Assumptions/Clarifications
Like all good programmers, you should make as few assumptions as possible.
You can assume pigs commands are always run in the same directory as the repository, and only files from that directory are added to the
repository.
You can assume the directory in which pigs commands are run will not contain sub-directories apart from .pig .
You can assume where a branch name is expected a string will be supplied starting with an alphanumeric character ([a-zA-Z0-9]), and only
containing alphanumeric characters plus '-' and '_'. In addition a branch name will not be supplied which is entirely numeric. This allows
brnach names to be distinguished from commits when merging.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
6/8
You can assume where a filename is expected a string will be supplied starting with an alphanumeric character ([a-zA-Z0-9]) and only
containing alphanumeric characters, plus '.', '-' and '_' characters.
You can assume where a commit number is expected a string will be supplied which is a non-negative integer with no leading zeros. It will
not contain white space or any other charcters except digits.
You can assume that pigs-add , pigs-show , and pigs-rm will be given just a filename, not pathnames with slashes.
You do not have to consider file permissions or other file metadata. For example, you do not have to ensure files created by a checkout
command have the same permissions as when they were added.
You do not have to handle concurrency. You can assume only one instance of any pigs command is running at any time.
You can assume that only the arguments described above are supplied to pigs commands. You do not have to handle other arguments.
You should match the output streams used by the reference implementations. It writes error messages to stderr: so should you.
You should match the exit status used by the reference implementation. It exits with status 1 after an error: so should you.
You can assume the directory containing your scripts is in $PATH .
You can not assume the directory containing your scripts is the same as the repo.
Your scripts are always run in the directory containing the repository.
Autotests and automarking will put your scripts (pigs-init, pigs-add, ...) in a different directory to the repository. This may break scripts which
run or source other scripts or file and assume they are in the current directory. Autotests and automarking will add the directory containing
your scripts to $PATH . This allows you to access other scripts or files by just specifying their name. For example: . library_functions.sh
will source the shell commands in . library_functions.sh even though it is another directory, because the directory has been added to
$PATH .
Note running . ./library_functions.sh will break during autotests and automarking. There can be subtle problem related to directories,
ask for help in the forum.
You can assume arguments will be in the position and order shown in the usage message from the reference implementation. Other orders
and positions will not be tested. For example, here is the usage message for pigs-rm :
$ 2041 pigs-rm
usage: pigs-rm [--force] [--cached] <filenames>
So, you assume that if the --force or --cached options are present, they come before all filenames, and if they are both present the
--force option will come first.
Pigs error messages include the program name. It is recommended you use $0 however it is also acceptable to hard-code the program
name. The automarking and style marking will accept both.
Do not use the modification time of a file to determine whether it has changed. You must use the file contents.
Change Log
Assessment
Testing
When you think your program is working, you can use autotest to run some simple automated tests:
$ 2041 autotest pigs
2041 autotest will not test everything.
Always do your own testing.
Version 1.0
(2022-06-21 1200)
Initial release
Version 1.1
(2022-06-23 2300)
[Reference] Update pig-status to include a usage message
Version 1.2
(2022-06-29 130)
[Reference] Update pigs-branch to give an error message when the current branch is deleted
[Reference/Autotest] Update pigs-branch to give a better error message when the default
branch is deleted
Version 1.3
(2022-07-04 930)
[Reference] Fixed pigs-commit -a failing when trying to remove the last file in the index
[Reference] Update pigs-show to display a usage message when the first argument is not in the
correct format
[Reference/Autotest] Update pig-status to show better status when a file is removed from the
index
COMP(2041|9044) 23T2 — Assignment 1: Pigs
7/8
Automarking will be run by the lecturer after the submission deadline, using a superset of tests to those autotest runs for you.
Submission
When you are finished working on the assignment, you must submit your work by running give :
$ give cs2041 ass1_pigs pigs-* test??.sh [any-other-files]
You must run give before Week 7 Monday 115959 2023 (midday) to obtain the marks for this assignment. Note that this is an individual
exercise, the work you submit with give must be entirely your own.
You can run give multiple times.
Only your last submission will be marked.
If you are working at home, you may find it more convenient to upload your work via give's web interface.
You cannot obtain marks by emailing your code to tutors or lecturers.
You can check your latest submission on CSE servers with:
$ 2041 classrun check ass1_pigs
You can check the files you have submitted here.
Manual marking will be done by your tutor, who will mark for style and readability, as described in the Assessment section below. After your
tutor has assessed your work, you can view your results here; The resulting mark will also be available via give's web interface.
Due Date
This assignment is due Week 7 Monday 115959 2023 (midday) (2023-07-10 115900).
The UNSW standard late penalty for assessment is 5% per day for 5 days - this is implemented hourly for this assignment.
Your assignment mark will be reduced by 0.2% for each hour (or part thereof) late past the submission deadline.
For example, if an assignment worth 60% was submitted half an hour late, it would be awarded 59.8%, whereas if it was submitted past 10
hours late, it would be awarded 57.8%.
Beware - submissions 5 or more days late will receive zero marks. This again is the UNSW standard assessment policy.
Assessment Scheme
This assignment will contribute 15 marks to your final COMP(2041|9044) mark
15% of the marks for assignment 1 will come from hand-marking. These marks will be awarded on the basis of clarity, commenting, elegance
and style: in other words, you will be assessed on how easy it is for a human to read and understand your program.
5% of the marks for assignment 1 will be based on the test suite you submit.
80% of the marks for assignment 1 will come from the performance of your code on a large series of tests.
An indicative assessment scheme follows. The lecturer may vary the assessment scheme after inspecting the assignment submissions, but it
is likely to be broadly similar to the following:
HD (85+) All subsets working; code is beautiful; great test suite
DN (75+) Subset 1 working; good clear code; good test suite
CR (65+) Subset 0 working; good clear code; good test suite
PS (55+) Subset 0 passing some tests; code is reasonably readable; reasonable test suite
PS (50+) Good progress on assignment, but not passing autotests
0% knowingly providing your work to anyone
and it is subsequently submitted (by anyone).
0 FL for
COMP(2041|9044)
submitting any other person's work; this includes joint work.
academic
misconduct
submitting another person's work without their consent;
paying another person to do work for you.
Intermediate Versions of Work
You are required to submit intermediate versions of your assignment.
COMP(2041|9044) 23T2 — Assignment 1: Pigs
8/8
COMP(2041|9044) 23T2: Software Construction is brought to you by
the School of Computer Science and Engineering
at the University of New South Wales, Sydney.
For all enquiries, please email the class account at cs2041@cse.unsw.edu.au
CRICOS Provider 00098G
Every time you work on the assignment and make some progress you should copy your work to your CSE account and submit it using the
give command below. It is fine if intermediate versions do not compile or otherwise fail submission tests. Only the final submitted version
of your assignment will be marked.
Attribution of Work
This is an individual assignment.
The work you submit must be entirely your own work, apart from any exceptions explicitly included in the assignment specification above.
Submission of work partially or completely derived from any other person or jointly written with any other person is not permitted.
You are only permitted to request help with the assignment in the course forum, help sessions, or from the teaching staff (the lecturer(s) and
tutors) of COMP(2041|9044).
Do not provide or show your assignment work to any other person (including by posting it on the forum), apart from the teaching staff of
COMP(2041|9044). If you knowingly provide or show your assignment work to another person for any reason, and work derived from it is
submitted, you may be penalized, even if that work was submitted without your knowledge or consent; this may apply even if your work is
submitted by a third party unknown to you. You will not be penalized if your work is taken without your consent or knowledge.
Do not place your assignment work in online repositories such as github or anywhere else that is publicly accessible. You may use a private
repository.
Submissions that violate these conditions will be penalised. Penalties may include negative marks, automatic failure of the course, and
possibly other academic discipline. We are also required to report acts of plagiarism or other student misconduct: if students involved hold
scholarships, this may result in a loss of the scholarship. This may also result in the loss of a student visa.
Assignment submissions will be examined, both automatically and manually, for such submissions.