Description
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 on-line 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 Shrug, a subset of the version control system Git.
Git is a very complex program which 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.
Shrug is a contraction of Shell-running-git.
You must implement Shrug in Shell.
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 shrug-add should match the behaviour of 2041 shrug-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 straight forward, 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. Andrew may fix the bug, or indicate that you do not need to match the reference implementation’s behaviour in this case.
Shrug Commands
Subset 0
Subset 0 commands must be implemented in POSIX-compatible Shell. See the Permitted Languages section for more information. shrug-init
The shrug-init command creates an empty Shrug repository.
shrug-init should create a directory named .shrug, which it will use to store the repository. It should produce an error message if this directory already exists.
You should match this, and other error messages exactly. For example:
| $ ls -d .shrug ls: cannot access ‘.shrug’: No such file or directory
$ shrug-init Initialized empty shrug repository in .shrug $ ls -d .shrug .shrug $ shrug-init shrug-init: error: .shrug already exists |
shrug-init may create initial files or directories inside .shrug.
You do not have to use a particular representation to store the repository.
You do not have to create the same files or directories inside shrug-init as the reference implementation. shrug-add filenames…
The shrug-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 .shrug sub-directory. For example, you might choose store them in a subdirectory of .shrug.
Only ordinary files in the current directory can be added. You can assume filenames start with an alphanumeric character ( a-zA-Z09 ) and will only contain alpha-numeric characters, plus ‘.’, ‘-‘ and ‘_’ characters.
The shrug-add command, and other Shrug commands, will not be given pathnames with slashes. shrug-commit -m message
The shrug-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.
Shrug 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. shrug-log
The shrug-log command prints a line for every commit made to the repository: each line should contain the commit number, and the commit message.
shrug-show commit :filename
The shrug-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.
For example:
| $ ./shrug-init
Initialized empty shrug repository in .shrug $ echo line 1 > a $ echo hello world >b $ ./shrug-add a b $ ./shrug-commit -m ‘first commit’ Committed as commit 0 $ echo line 2 >>a $ ./shrug-add a $ ./shrug-commit -m ‘second commit’ Committed as commit 1 $ ./shrug-log 1 second commit 0 first commit $ echo line 3 >>a $ ./shrug-add a $ echo line 4 >>a $ ./shrug-show 0:a line 1 $ ./shrug-show 1:a line 1 |
Subset 1
Subset 1 is more difficult. You will need 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. shrug-commit -a -m message
shrug-commit can 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. shrug-rm –force –cached filenames…
shrug-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.
shrug-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. shrug-status
shrug-status shows the status of files in the current directory, the index, and the repository.
$ ./shrug-init
Initialized empty shrug repository in .shrug
$ touch a b c d e f g h
$ ./shrug-add a b c d e f
$ ./shrug-commit -m ‘first commit’
Committed as commit 0
$ echo hello >a
$ echo hello >b
$ echo hello >c
$ ./shrug-add a b
$ echo world >a
$ rm d
$ ./shrug-rm e
$ ./shrug-add g $ ./shrug-status a – file changed, different changes staged for commit b – file changed, changes staged for commit c – file changed, changes not staged for commit d – file deleted e – deleted f – same as repo
Subset 2
Subset 2 is extremely difficult. You will need 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. shrug-branch -d branch-name
shrug-branch either creates a branch, deletes a branch, or lists current branch names. shrug-checkout branch-name
shrug-checkout switches branches.
Note that, unlike Git, you can not specify a commit or a file: you can only specify a branch. shrug-merge branch-name|commit -m message
shrug-merge adds the changes that have been made to the specified branch or commit to the index, and commits them.
| $ ./shrug-init
Initialized empty shrug repository in .shrug $ seq 1 7 >7.txt $ ./shrug-add 7.txt $ ./shrug-commit -m commit-1 Committed as commit 0 $ ./shrug-branch b1 $ ./shrug-checkout b1 Switched to branch ‘b1’ $ perl -pi -e ‘s/2/42/’ 7.txt $ cat 7.txt 1 42 3 4 5 6 7 $ ./shrug-commit -a -m commit-2 Committed as commit 1 $ ./shrug-checkout master |
If a file contains conflicting changes, shrug-merge produces an error message.
| $ ./shrug-init
Initialized empty shrug repository in .shrug $ seq 1 7 >7.txt $ ./shrug-add 7.txt $ ./shrug-commit -m commit-1 Committed as commit 0 $ ./shrug-branch b1 $ ./shrug-checkout b1 Switched to branch ‘b1’ $ perl -pi -e ‘s/2/42/’ 7.txt $ cat 7.txt 1 42 3 4 5 6 7 $ ./shrug-commit -a -m commit-2 Committed as commit 1 $ ./shrug-checkout master |
