Terminal Environment & AFS

By now you should have a decent idea of how you can manage files and a basic understanding of some commonly-used commands. This article will build on that knowledge to acquaint you with some key elements of your terminal environment. We’ll also briefly talk about AFS, which is the environment you’ll be working on when you ssh into one of the Andrew Unix machines.

Printing Text (echo)

To print text, you can use the echo command, which just prints its arguments:

$ echo Hello, world!
Hello, world!

Important Directory Names

Some directories are more important than others, so they’re given some shorter names.

~ – the home directory

$ cd ~
$ pwd
/afs/andrew.cmu.edu/usr10/jezimmer

~andrewid – the home directory of user “andrewid”

Similar to the previous directory, but for any arbitrary user.

$ cd ~nmunson
$ pwd
/afs/andrew.cmu.edu/usr13/nmunson

. – the current directory

Shortcut for what would be printed out by pwd.

Note: pwd and . are not the same thing. pwd is a command which when run prints out the full path of the current directory. . (when used as a directory) is not a command. It’s merely a shortcut that can be used instead of typing out an entire directory name.

.. – the parent directory

Represents the directory above the current directory.

/ – the root directory

This is the folder that contains everything. That is to say it has no parent. Try running cd .. from this directory, and you’ll find that you end up back in /!

Environment Variables (export)

Bash keeps a collection of environment variables for every user who’s logged in. These variables hold key pieces of information about what’s going on. For example, there’s a variable called SHELL that says what shell you’re running, a variable named USER which contains the username of the current user, another named PWD that contains the present working directory, and more.

Setting Variables

You can control your environment by setting certain variables. Here are a few examples of how that’s done:

# set my_variable to the string "hello" (no spaces around the '='!)
$ my_variable="hello"

# assign to another_var and "export" it (see below)
$ export another_var="some string"

The difference between using export and not using it deals with what programs are able to read the contents of that variable. If a variable is not exported, then only you will be able to read its contents. That is, none of the programs that you run will be able to see the value of the variable that you just set.

In practice, you will almost always want to use export when setting variables.

Accessing Variables

It helps to be able to check and print the value of environment variables! To access to contents of a variable, you have to use the $ operator.

# get the value of my_variable and print it
$ echo $my_variable
hello

# print another_var surrounded by other text
$ echo lone${another_var}s
lonesome strings

Note that in the last example, had we not used ${...} around the variable name, like lone$another_vars, bash would have tried to look up the contents of another_vars, not another_var. Since this is a different variable entirely, something incorrect would have been printed.

AFS

AFS is a distributed file system that was invented at CMU. You have a quota of space and a home directory where you can put your files. You can access these files from any Andrew Unix server or cluster computer on campus.

When you’re using AFS, there’s a system of permissions (called access control lists, or ACLs) regulating who can access your files and what they can do to them. It’s important to know about how to use this system so that you can stop other people from getting access to your homework or other private files.

AFS is set up so that by default, you have a private directory where you can do your work: ~/private. If you don’t change its permissions, you can put all of your work for your classes in there and no one will be able to access it except for you.

By default, AFS also has a ~/public directory, where you can put things that you want other people to be able to see. Other users will be able to read files that you put there (and make copies of them), but not change them, delete them, or add their own files.

ACLs (fs)

You don’t need to memorize this information! It’s listed here solely for your reference.

It’s important to be able to control who can access your files on AFS, and there’s a command called fs that lets you do this.

For more information on any of the following commands, you can always run fs help <command> to get help on that fs subcommand.

Listing Permissions (fs la)

You can use fs la (or fs listacl) to see what the permissions on a directory are. The output will be an AndrewID or group followed by what they are allowed to do. For example:

jzimmerm@unix4:~$ fs la
Access list for . is
Normal rights:
  system:anyuser l
  jzimmerm rlidwka

There are several permissions that a user can have for a given directory:

Setting Permissions (fs sa)

You can use fs sa (or fs setacl) to change the permissions on a directory. The syntax is:

fs sa <directory> <user ><permissions>

where is some of the letters rlidwk or "none". For instance, to let the user bovik list, read, edit, create, lock, and delete files in the directory foo, you'd do:

fs sa foo bovik rlidwk

AFS Quota (fs lq)

You can use fs lq (or fs listquota) to see how much of your alloted AFS space you’re using. For example:

jzimmerm@unix14:~$ fs lq
Volume Name                    Quota       Used %Used   Partition
user.jzimmerm                2000000     385982   19%         46%