FAQ

FAQ

Legacy FAQ

Please read these frequently asked questions before emailing a guru.

If you’re unsure of a command, you can type in man CMD at the prompt, where CMD is the name of the command. Try typing man cd or man ls or man man at the prompt.

While this page is updated occasionally to correct or remove answers no longer accurate or relevant to the computer science department systems, it is worth noting that this document has been around for a long time, and may make references to things with which you are not familiar. If this makes you feel frightened or lonely, try drinking a warm cup of tea, or use Google until the feeling passes.

If none of the answers here help, don’t hesitate to contact us!

  • How can I get a new program installed?
  • How can I get to files on my UNET webspace from a department Linux machine?
  • How can I get to my files in my CS account from home?
  • How do I change my password?
  • How do I filter my email?
  • How do I get a CS Account?
  • How do I password protect my webpages to allow limited access?
  • How do I print out to the printers in Volen?
  • How do I protect my files?
  • How do I remotely login to a computer?
  • How do I set up my CS account to forward my email to another account?
  • How do I use Emacs?
  • How do I use Vim?
  • How do I use the RHEL operating system (the operating system on all of the public workstations)?
  • How much space do I have on my CS account?
  • How often are my web pages backed up?
  • How often are the files in my home directory backed up?
  • I forgot my password, or it expired. What can I do?
  • I have a friend who doesn't have a CS account. Can I let him/her use mine?
  • I need to leave the Vertica Lounge. Do I have to logout?
  • I think someone has my password. What do I do?
  • I tried to log in and received the message "Your account has expired; please contact your system administrator." What does this mean? What should I do?
  • It's 9 pm on a Sunday night, and I really really really need help from a guru. What do I do?
  • There's something wrong with a program/computer/printer. What do I do?
  • What is SCP, and how do I use it?
  • What is SFTP, and how do I use it?
  • What is SSH, and how do I use it?
  • What is a CS Account?
  • What is the Vertica Lounge?
  • What's the best editor, Vim or Emacs?
  • Where can I plug my laptop into the network?
  • Where do I put webpages? How can people see them?
  • Which dynamic languages am I allowed to use on my personal webpage?
  • Why has my IP address been blocked by an ssh server?
  • Why is the machine I'm on so slow?
  • Why shouldn't I shut down the iMacs in Vertica?
  • How can I get a new program installed?

    Email us and tell us what program you need and why (or what for) it is useful. As a rule we have no problem installing software that someone needs for a legitimate reason.

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    How can I get to files on my UNET webspace from a department Linux machine?

    You can use SCP or SFTP. LTS changes their shell servers from time to time. As of this writing, a list of their available shell servers can be found at http://lts.brandeis.edu/techresources/accounts/unet/serversfilestorage.html, which states that you should use limpet.unet.brandeis.edu or starfish.unet.brandeis.edu for the hostname/server. For the folder use /usr/users/<USERNAME>, for the username use your UNET ID.

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    How can I get to my files in my CS account from home?

    You need to log in to one of the public workstations and copy files from there over SSH. You can use SCP or SFTP to copy files to your local machine. Use the hostname of the workstation you are connecting to, your CS user name, and the full path to your home directory.

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    How do I change my password?

    Connect to a public workstation, run passwd, and follow the prompts.

    Longer version:

    When you run passwd, you will be prompted to type in first your old (current) password (to authenticate), then your desired new password, then your desired new password once more to confirm. No characters will be echoed to your terminal as you type in either your old or new password. Hit enter after you have typed in each password. If you enter your existing password incorrectly or if your desired new password fails to pass the automatic strength checks, an alert will be written to standard error.

    Note: Password changes may take up to one hour to synchronize to all CS machines. If you change your password and the new password doesn’t work right away, either use your old password or wait an hour and try again.

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    How do I filter my email?

    See the Mail Settings page.

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    How do I get a CS Account?

    To have a CS account, you must either be enrolled in a CS class, be a declared CS major or minor, or be an officially affiliated postbac or grad student, postdoc, or staff member/department employee. If you meet one of these requirements, visit accounts.cs.brandeis.edu to get a CS account.

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    How do I password protect my webpages to allow limited access?

    See our htaccess guide.

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    How do I print out to the printers in Volen?

    As of fall 2020, mcnally, the designated printer for undergraduates in the computer science department, has been removed from the Vertica Lounge. If you are an undergraduate who needs to print documents for a COSI course, please use the printers in Goldfarb. If you are a graduate or PhD student who needs access to department printers, we provide a list of available printers as well as detailed instructions on how to access them from any device via the CUPS print server.

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    How do I protect my files?

    All files and directories have permissions on them which describe who is allowed to do what with them. You can see the permissions on a file or folder by running ls -l. Permissions have have a form of 10 character string that is grouped as follows:

    Except for the item description character, each set of three characters is organized in the same way:

    For files, these permissions are fairly self explanatory: read allows you to read the file, write allows you to edit the file, and execute allows you to run a file (like a shell script). For directories, read allows you to see the contents of the directory, write allows you to create and delete files in the directory, and execute allows running programs (including a web browser) to access the files in the directory.

    Keep in mind that even if you disallow someone from editing a file by denying them write access, if the directory allows them write access, they can read the file, copy its contents, and then delete and recreate the file with whatever permissions they desire. If you deny write access to a directory, but allow write access to the parent directory, they can delete and recreate the whole directory in the same way. Be careful with who you give write access to!

    To change permissions on a file or folder, use the command chmod <mode> <file>, where <mode> is how you want to change the permissions, and <file> is the file or folder whose permissions you want to change. To describe to chmod how you wish to change the permissions of the file, you can either describe only the change you wish to make, or describe the permissions for the file as a whole.

    To describe just the permission change, the mode will be a character representing the group you want to change (u for user/owner, g for group, o for other/world, or a for all three) followed by a character representing how you want to change the permission (+ to grant the permission, - to revoke it), followed by a character representing the permission to change (r for read, w for write, x for execute). For example, to remove write permissions for other on a file, the mode is o-w, while to add execute permissions for everyone, the mode is a+x. You can also combine multiple groups and and permission settings in one mode: to add read access for user and group, but not other, you the mode is ug+r, while to remove write and execute permissions for other, the mode is o-wx.

    To describe the permissions for the file as a whole, we use a number, instead of a string of characters. Each permission type is assigned a number: read is 4, write is 2, execute is 1. The mode we will use is a set of three sums, one for each permissions group. For example, let’s say we want user to have read, write, and execute access, group to have read and execute access, and other to have read access only. The sum for user is 4+2+1=7, the sum for group is 4+0+1=5, and the sum for other is 0+0+1=1, so the mode is 751.

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    How do I remotely login to a computer?

    You can remotely access other computers over the network using SSH. Please see the welcome guide for more information on SSH. Previously, people used telnet for remote access: however, telnet is an insecure protocol and is no longer supported.

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    How do I set up my CS account to forward my email to another account?

    Create (if necessary) the file /mnt/mail.cs.brandeis.edu/`whoami`/.forward (whoami evaluates to your username) by using your favorite text editor (emacs, pico, vim, etc.). The first and only line of your file should be the e-mail address you want your mail to be forwarded to.

    Note that this only forwards new, incoming mail that is sent to you. It won’t transfer existing emails from your CS account to the account you are forwarding your email to.

    To undo this forwarding, simply remove the .forward file: rm /mnt/mail.cs.brandeis.edu/`whoami`/.forward

    See the Mail Settings page for more information.

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    How do I use Emacs?

    To start Emacs, type in emacs at the command prompt and press Enter. Emacs has a pretty good tutorial which can be accessed within Emacs by pressing Ctrl+ht. You can quit Emacs with Ctrl+xCtrl+c.

    Alternatively, gnu.org has an online tutorial for Emacs you might find useful.

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    How do I use Vim?

    Run the Vim tutorial with vimtutor, which takes approximately 25-30 minutes to complete.

    To exit vim, type :q and then hit ENTER.

    If you would prefer to avoid the effort of learning Vim, the nano editor is significantly simpler and more user-friendly for beginners.

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    How do I use the RHEL operating system (the operating system on all of the public workstations)?

    RHEL (Red Hat Enterprise Linux) is a modified version of the Fedora Linux distibution. This guide, written by the people at Canonical (the creators of the Ubuntu Linux operating system), provides a very brief history of how Linux came to be. It also gives an overview of how to use the command line. Please be aware that, while this tutorial covers command line tools that can be used on almost any OS in the Linux family, the workstations administered by the Systems Office run RHEL – not Ubuntu.

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    How much space do I have on my CS account?

    See the usage policy.

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    How often are my web pages backed up?

    Since your web pages are stored in ~/.www, they are backed up with your home directory.

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    How often are the files in my home directory backed up?

    Each directory within your home directory contains a hidden file called .snapshots. This file cannot be seen using ls -al, but you can cd into it. The .snapshots folder contains hourly, daily, and weekly backups of its parent directory. Hourly backups are kept for the past six hours, dailies for the past two days, and weeklies for the past two weeks.

    The .snapshots directory is synced to a server in another building on campus daily. This backup is stored in ~/.snapshot/snapmirror.*.

    And no, there is NO way to undelete a file once you’ve erased it using rm.

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    I forgot my password, or it expired. What can I do?

    Passwords must be changed at least once every 365 days or sooner. If your account password is not changed within that time, it will expire.

    If you have forgotten your password, or it has expired, please visit the accounts webpage to reset it. If you encounter any difficulties, stop by the guru office with a photo ID (we accept IDs issued by: Brandeis University, the state of Massachusetts, and the United States Federal Government), and ask a guru to change your password. The System Operators can change your password, but they DO NOT know what your password is.

    Please note that, as email is an inherently insecure form of communication, we will not perform password resets via email. As such, please do not ask for an email-communicated reset, and please do not send us Personally Identifying Information (eg: US Social Security Number, Brandeis Student ID Number, etc) via email. We cannot accept such information as proof of identity, and you only place yourself at risk of fraud (“Identity Theft”) by sending it over the public internet in the clear.

    Faculty and staff who require emergency password resets but are unable to come to Volen 125 in person (eg: if traveling away from the University) should contact the guru office for assistance.

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    I have a friend who doesn't have a CS account. Can I let him/her use mine?

    Nope. The only thing you can achieve is to lose your account as well. People caught sharing accounts will have their account suspended and, in case of repeated offense – disabled.

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    I need to leave the Vertica Lounge. Do I have to logout?

    You don’t have to logout if you’re leaving for a short period of time, but you do need to lock your screen before leaving it. To do so, you can set your screen saver to activate automatically and require a password to unlock (good), and/or lock your screen manually before leaving it (better).

    To configure your screen saver:

    Because someone else could potentially take over your session after you leave your workstation but before your screen saver has time to activate, we strongly suggest that you lock your screen manually before walking away from it. This can be done by pressing Ctrl-Cmd-Q or by selecting Apple Menu → Lock Screen.

    Note that you should only use screen saver for a short period of time (10 minutes or less). Please be sure to save your files before using screen saver. Very often people walk in to the lounge and log out of an account, destroying any unsaved work.

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    I think someone has my password. What do I do?

    Change your password immediately and contact the Systems Office.

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    I tried to log in and received the message "Your account has expired; please contact your system administrator." What does this mean? What should I do?

    If either your account OR your password has expired, our Linux machines will deny access with the message “Your account has expired; please contact your system administrator.” As such, if you receive this message, it means one of two things. Either:

    1. Your account has expired, or
    2. Your password has expired.

    If your account has expired, contact us and let us know what CS class(es) you are in this semester or, if you are a declared CS major, who your advisor is.

    If your password has expired, see our password reset instructions.

    If you are unsure whether it is your account, password, or both which have expired, contact us and ask.

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    It's 9 pm on a Sunday night, and I really really really need help from a guru. What do I do?

    Do a Google search to see if you can find the answer to your question elsewhere. If you can’t, contact the Gurus. Calling the Gurus at home will only get them angry, and as such they will probably not want to help you.

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    There's something wrong with a program/computer/printer. What do I do?

    Email Guru with as much factual information as you can find about the problem. Please be sure to include a description of the problem (both what you expect to happen, and what is in fact happening), where and when it happened, and what computer you were using (you can find a computer’s name using the hostname command). The GNU Emacs manual has an excellent description of what to include in a bug report:

    When you decide that there is a bug, it is important to report it and to report it in a way which is useful. What is most useful is an exact description of what commands you type, starting with the shell command to run Emacs, until the problem happens.

    The most important principle in reporting a bug is to report facts. Hypotheses and verbal descriptions are no substitute for the detailed raw data. Reporting the facts is straightforward, but many people strain to posit explanations and report them instead of the facts. If the explanations are based on guesses about how Emacs is implemented, they will be useless; meanwhile, lacking the facts, we will have no real information about the bug.

    For example, suppose that you type C-x C-f /glorp/baz.ugh RET, visiting a file which (you know) happens to be rather large, and Emacs displayed ‘I feel pretty today’. The best way to report the bug is with a sentence like the preceding one, because it gives all the facts.

    A bad way would be to assume that the problem is due to the size of the file and say, “I visited a large file, and Emacs displayed ‘I feel pretty today’.” This is what we mean by “guessing explanations.” The problem is just as likely to be due to the fact that there is a ‘z’ in the file name. If this is so, then when we got your report, we would try out the problem with some “large file,” probably with no ‘z’ in its name, and not see any problem. There is no way in the world that we could guess that we should try visiting a file with a ‘z’ in its name.

    Alternatively, the problem might be due to the fact that the file starts with exactly 25 spaces. For this reason, you should make sure that you inform us of the exact contents of any file that is needed to reproduce the bug. What if the problem only occurs when you have typed the C-x C-a command previously? This is why we ask you to give the exact sequence of characters you typed since starting the Emacs session.

    You should not even say “visit a file” instead of C-x C-f unless you know that it makes no difference which visiting command is used. Similarly, rather than saying “if I have three characters on the line,” say “after I type RET A B C RET C-p,” if that is the way you entered the text.

    So please don’t guess any explanations when you report a bug. If you want to actually debug the problem, and report explanations that are more than guesses, that is useful–but please include the facts as well.

    Source: GNU Emacs manual

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    What is SCP, and how do I use it?

    SCP stands for “Secure Copy.” It is similar to SFTP. It is used to copy files from one computer to another. SCP comes installed with Linux and with Mac OS X. You can install SCP on Windows using PuTTY.

    To use SCP, use scp <source-file> <destination-directory>. You can copy directories recursively with the -r flag.

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    What is SFTP, and how do I use it?

    SFTP stands for “Secure File Transfer Protocol,” and it is the successor to FTP. It is used to transfer files between computers. SFTP comes installed with Linux and with Mac OS X. You can install SFTP on Windows using PuTTY.

    To use SFTP, you can use either a command line interface, or a GUI. For the command line, you’re probably better off using SCP, but to see how to use SFTP, use man sftp.

    For the GUI, you have several options, depending on what system you are running. If you are running Windows, you can use the SSHSecureShell program distributed by LTS, or you can use PuTTY, WinSCP, or FileZilla, all of which are free and open source. If you are running Mac OS X, you can use Cyberduck, which is free and very good, or you can use Transmit, which costs money. If you are running Gnome on Linux, you can use the Nautilus file browser (File→Connect to Server…, select SSH from Service Type).

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    What is SSH, and how do I use it?

    For information regarding SSH and how it can be used to access CS systems, see the intro guide.

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    What is a CS Account?

    A CS account, or Computer Science account, is an account that you can use to log into and use computers maintained by the Brandeis Computer Science Department. It is required to get access to the public workstations.

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    What is the Vertica Lounge?

    The Vertica Lounge (Volen 104) is a community workspace for computer science students. Many TAs for intro-level CS courses hold office hours here. The Lounge contains several Mac workstations, which are available to be used for homework and projects. Anyone with a CS account can use any of the public workstations at any time, either in Volen or by connecting to these computers remotely using SSH.

    All declared COSI majors and minors as well as all the non-declared students enrolled in COSI courses get 247 swipe access to Volen Lobby, Vertica Lounge, and G-zhang Lobby (for bathroom access). If you fall into one of the above categories and are unable to swipe-in, please notify the Computer Science Office (compsci@brandeis.edu) with the number on your ID card, the door you tried to swipe into, and the date and time of the attempt.

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    What's the best editor, Vim or Emacs?

    No comment. You’ll find both installed on our machines.

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    Where can I plug my laptop into the network?

    In the Vertica Lounge there are ethernet ports available for laptops. To use them you will need to set your TCP/IP connection to DHCP, so that you will be automatically given an IP address (this is default on most machines).

    You can also connect to eduroam, the campus-wide wireless network. For more information see here.

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    Where do I put webpages? How can people see them?

    Put webpage files in the .www directory in your home directory. (If this directory does not yet exist, just create it: mkdir ~/.www)

    To access your home page from a web browser, go to http://www.cs.brandeis.edu/~<user>, where <user> is your CS account username.

    Note: In order for the httpd daemon to serve any files located in your web directory, it must be able to access them and read them. This does not only mean that your web directory and any content within it must be world readable, which most users realize. It also means that to permit the httpd daemon to access your .www directory, you must check the file permissions to ensure that your home directory is world traversable (though it need not be world readable). Directory traversability is set by the file executability bit. Therefore, the least destructive way to make your directory world traversable is to run chmod +x ~

    Historically, public webpages used to be served out of /web, rather than ~/.www. If you believe that you still have old data under /web that you would like to access, please contact Guru.

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    Which dynamic languages am I allowed to use on my personal webpage?

    Due to security considerations, PHP and CGI scripts are not allowed on the default CS web server. HTML, CSS, and JavaScript are all permitted and are able to be served from your ~/.www directory.

    Students interested in doing dynamic web development can contact us to apply for an account at csbrandeis.org.

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    Why has my IP address been blocked by an ssh server?

    Brandeis Computer Science provides a number of public workstations that are available via ssh.

    Because many CS students & faculty work from off-campus, some of these ssh servers are available directly from the internet.

    Unfortunately, the internet is a sometimes hostile place, filled with attackers who constantly try to break into any servers they can find, including our public ssh servers.

    As such, we have implemented automated software that tracks failed authentication attempts, and temporarily blocks IP addresses after too many failures.

    This means that if you enter your username or password incorrectly too many times, your IP address will be temporarily blocked.

    If this happens to you, please double check your username and password, wait a little while, and try again.

    If your IP address receives too many temporary bans, it may be blocked permanently.

    The above applies to both on-campus and off-campus IP addresses.

    If you need to look up your CS Account name or reset your CS passphrase, you can do so at https://accounts.cs.brandeis.edu/

    For further help, contact the Guru Office by email guru@cs.brandeis.edu or phone 781-736-2740

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    Why is the machine I'm on so slow?

    You may have a process that is taking up a lot of CPU usage. To see if that’s the case, type ps at the command prompt. You will see something like the following:

          PID TTY        TIME CMD
        13391 pts/3  00:00:00 tcsh
        13470 pts/3  00:00:01 emacs
        13974 pts/3  00:00:00 ps
    

    To get rid of a process, type kill -1 PID where PID is the number in the PID column. For instance, to get rid of emacs in the above example, the user would type kill -1 13470. This should clean the process up nicely. If after a short while, ps still shows that process, type kill -9 13470.

    On Linux you can also use the graphical utility ksysguard, and on macOS you can check the CPU tab of the Activity Monitor application.

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    Why shouldn't I shut down the iMacs in Vertica?

    The iMacs in Vertica are public workstations — if you shut them off and someone is logged in with a long-running task, it would be killed! That would be bad and upset the person. Therefore, please leave them turned on and connected to the network, and be sure to log out once you’re done!

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