Using Drop to Transfer Files

The drop command that I created is a general-purpose tool that transfers one file at a time to a destination. It transfers ownership of the file to the recipient, so that they can do with it whatever they want.

Think of this as like a physical drop box or like sliding an assignment under someone's door.

We can do this both from student teams to the CS 304 course account and from student to student.

In this explanation, I'm going to imagine that Hermione and Ron have worked together on an assignment using Hermione's account. Afterwards, they want to transfer the finished tarfile to Ron's account.

So, Hermione is the sender and Ron is the recipient.

Create a Drop Folder¶

To allow someone to drop a file to you, you have to create a drop folder in your home directory. This gives you a known place to look for files, and means that the sender can't drop them just anywhere in your account. So, do this first:

mkdir ~/drop

Ron needs to do this so that Hermione can drop a file to him.

Setting up a drop folder only needs to be done once, no matter how many files get dropped later.

Feel free to do an ls on the drop folder for the course account: ~cs304/drop

Drop the File¶

Then, Hermione can drop a file to Ron. Here's the command in action (here cs304guest is dropping a file to gdome):

[cs304guest@tempest ~]$ drop gdome unix.tar 
Copying unix.tar (from cs304guest) to /students/gdome/drop/ (uid 707)
/students/gdome/drop/cs304guest doesn't exist, making it.
Successful drop.

Structure and Permissions¶

If Hermione (cs304guest) is anxious and wants to be sure that the file is really there in Ron's (gdome's) drop folder, she can list the files that she has dropped:

[cs304guest@tempest ~]$ ls -l ~gdome/drop/cs304guest/
total 20
-r--r-----. 1 gdome cs304guest 20480 Sep  5 12:34 unix.tar

Ron (gdome) can also check that it's there:

[gdome@tempest ~]$ ls -l ~/drop/cs304guest/
total 20
-r--r-----. 1 gdome cs304guest 20480 Sep  5 12:34 unix.tar

You'll note in this example that there's a cs304guest (hermione) subfolder to Ron's (gdome's) drop folder. Think of that as "these are my files from Hermione". Each sender gets a subfolder with their account name. The drop command does this automatically.

This scheme allows all of the CS 304 students to drop a file called unix.tar to the course account and not step on each other. It also means that, next time, when Ron is working with Harry on a different assignment, Harry can drop the work to Ron and it'll go in the hpotter subfolder of Ron's drop folder.

Again, the subfolders are named for the sender.

You'll also notice, above, that the unix.tar file in Ron's drop folder is owned by gdome (Ron), not by Hermione. The drop command has transferred ownership.

One consequence of this is that Hermione can't delete the file after she has dropped it. If she needs to drop it again, either Ron has to delete the earlier one (since he's the owner) or Hermione needs to rename the file and drop that:

mv unix.tar unix-revised.tar
drop rweasely unix-revised.tar

That's it!

Summary¶

Ron (once) creates a ~/drop/ folder
Ron's partners can drop files to him.
The files go in subfolders named for the partner (sender)

Revising a Dropped Folder¶

Sometimes, you submit code that has an error in it, and I ask you to fix it and re-submit. Or maybe you realize the error before I ask, and you need to re-submit. Or maybe you're dropping code to your partner for a second time. If those situations arise, you can learn how to do that here.

First, and this is really confusing to people: there's a difference between the files and folders inside the tarfile, and the tarfile itself. Let's take a concrete example. Suppose you and your partner are working on the "Forms" assignment, and you create the following files:

forms/
    app.py
    templates/
        myform.html

Now, you go to the folder that contains the forms folder and you create a tarfile:

forms.tar
forms/
    app.py
    templates/
        myform.html

That tarfile contains the two folders and two files listed above, including their names.

Suppose you rename the tarfile, like this:

mv forms.tar forms-v1.tar

The folders and files inside the tarfile are not renamed! You named the original file forms.tar to remind you of what was in it, but it has no other functional role. You could just as easily have named it my_first_tarfile or anything else.

If you drop the forms-v1.tar file to the CS304 course account, it becomes my file. When I un-tar it, it will contain the files and folders listed above:

forms/
    app.py
    templates/
        myform.html

though they will now be in the CS 304 drop folder structure, in a subfolder named with your username.

Suppose there's an error. You can fix the original files, keeping their names the same, and create a new tarfile, but you should give it a different name, because you can't drop it again using the forms-v1.tar name, because there's already a file of that name in your drop folder. You can do that in two ways.

One way is to create the tarfile with a different name from the start:

tar cf forms-v2.tar forms/
drop cs304 forms-v2.tar

Notice that the tar command asks you for both the name of the folder to pack up, and the name of the file to put the folder in.

The second way is to create the tarfile the same way you always do, and just rename it before dropping it:

tar cf forms.tar forms/
mv forms.tar forms-v2.tar
drop cs304 forms-v2.tar

Either of these is fine.

Corollary: do not change the name of the forms folder, because that's what I look for when I'm grading. Don't do the following:

mv forms forms-v2
tar cf forms-v2.tar forms-v2
drop cs304 forms-v2.tar

Because when I grade, I'll get confused.

The flip side of this difference between the tarfile's name and its contents is that if your partner drops you a tarfile that contains folder foo and you untar it in someplace where there already is a folder named foo, the contents of the tarfile will replace the current folder. Often, that's what you want (as when you drop a folder for grading), but if you don't expect it, you'll be confused.