Department of Computer Science

SmartBoard Documents

Linux Lab FAQ

Spring 2017 Linux Help Sessions: Meeting in Linux Lab:

  • Linux for Beginners (pdf)

  • Filesystem Structure (pdf)

  • Remotely logging Into Computer Science Subnet Via Eniac (pdf)

  • How To Determine Your Cable Modem IP Address (pdf)

  • How To: Config Windows to Remotely Log into Linux Lab (pdf)

  • Instructions for submitting course rosters to Linux account creation (revised 8/31/16) (pdf)

IPv4: almost 4.3 billion addresses
IPv6: more than 280 trillion addresses

Ubuntu 16.04 now installed on all Linux Lab computers.


Have a problem with your account or with a computer? Here's what is needed to begin troubleshooting:
  • send an email to Tom Walter at from your Hunter email account.
  • include the exact command you are trying to execute.
  • include the exact error message(s).
  • what is the name of the machine you're trying to log into?
  • include your Linux account user name and your full name as it appears in CUNYfirst.

Tom Walter
office: room 1003B North

Linux Accounts: Use it or lose it! Accounts not used for more than 90 days are subject to deletion. You must use your account at least once during the first two weeks of the fall and spring semesters to keep an account.

Don't wait until the day you have to upload your homework to try your account and then find out it has been disabled or deleted. Anticipate your needs!

1000G Lab Rules

The Computing Lab and its equipment are to be used for academic purposes. Commercial use of the Computing Lab computer systems is prohibited.

You may not bring food or drink into the Computing Lab. Violators will be told to log off and leave the lab and your account will be disabled.
Shared systems such as those connected to the Internet or World Wide Web (WWW) provide special services and require special responsibilities. You are responsible for the safeguarding of your computer account on any such system. You must follow the Department Rules for choosing your password, you should change your password frequently, and should not disclose it to anyone.
You may not share your account with any other users and you are responsible for any activity  which is associated with your account.
You may not use this equipment to transmit abusive, anonymous, unsolicited, obscene or  fraudulent messages.

You may not install software on any machine. All software installs are done by department IT staff. Unless it is an emergency, you should not save work to the local PC drives. All of your  files should be saved in your home directory.
You may not copy or distribute software without permission.

If a class is being held in the Computing Lab, and you are asked to leave, you must comply with the instructor's request. You will be able to work remotely if you are in the middle of something important.

Do not power down computers or disconnect network cables.

Problems with any software or hardware should be reported to Include the machine name, a detailed description of the problem and an exact copy of any error messages.

Accounts inactive for more than 90 days will be deactivated and deleted after 120 days.

All accounts have storage quotas. You will not be able to exceed your disk quota, the system will deny you permission to save files.


Q: When I try to remotely log onto any of the Linux lab machines from eniac I get this message: WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!

A: Linux Lab computers have been updated to Fedora 20. When you remotely login to lab computers (cslab1, cslab2, etc) you will receive a long message:


In this case, the warning can be ignored. To be able to login onto Lab machines you need to issue this command:

ssh-keygen -R    (substitute the name of the lab machine, cslab1 or cslab2, etc.)

Q: How to prevent FireFox from using up all of your disk quota.

A: Every time you use FireFox it caches files that will eventually use up all of your disk space. There's two things you need to do to prevent this.

  • From a command prompt in your home directory, issue the following commands to remove the existing cache files:
    • rm -rf ~/.mozilla/firefox/*.default/Cache/*
    • rm -rf .cache/mozilla/firefox/*
  • Now, go to this web site and follow the instructions to automatically remove cache files after a FireFox session:


Q: I forgot my password and/or user name. What do I do?

A: Send an email to from your Hunter email address and give your full name as it appears in Hunter College records and details of your request. You must either be enrolled in a Computer Science course requiring a Linux account or be a Computer Science major to be eligible for an account.


Q: I can log into my account but I get this error message:

Could not chdir to home directory /data/biocs/b/student.accounts/............

A: You have not used your account for more than 90 days and your home directory was deleted just before your account will be deleted. Send me an email with the exact error message from your Hunter email account.


Q: I'm trying to run FireFox but it tells me the program is already running.

A: When FF starts it creates a lock file. If FF terminates improperly the lock file is not removed. Then, next time you try to start FF it thinks it's already running. The lock file is a symbolic link to another file. To find the FF lock file issue the following commands from your home directory:

> cd .mozilla     (you need to change directory to the mozilla directory (that's a dot in front of the file name))

> find . -name lock -exec rm -f {} \;     (this command looks for a file named "lock" and executes the rm command)


Q: I'm exceeding my disk quota. What can I do?

A:  If you can log into your account you can issue the following command determine how much disk space you're using.

- open an X-term window
- make sure you're in your home directory. If you're not sure, issue the command: cd   <-- this command will put you in your    home directory
- issue the following command:  du -k | sort -nr > filename   <-- the output from this command will list your biggest files.

- Disk quota: you have a soft disk quota of 300MB and a hard disk quota of 350MB. You can exceed the soft disk quota limit for up to seven (7) days. If you don't reduce your disk usage below the soft limit, after 7 days you wont be able to save anything and you won't be able to log on properly. You can never exceed your hard disk quota limit.

- If you determine you're exceeding your soft disk quota limit of 300MB you'll have to remove some files. Often, Firefox creates 200+MB of cache files and other file types so that's a good place to start. Firefox files can be found in the .mozzila subdirectory. The BIG files are typically found in the firefox subdirectory. Once you're in the firefox subdirectory you have to find the BIG files.

- issue the command:  du -k > filename   <--- this is the disk use command with the "k" option that will list every file in all the subdirectories along with the size of - the file. The output is redirected to the file you specify.
- open the file using "vi" or another editor and look for the BIG files. Remove them.


Q: Can I log in remotely?

A: Yes. You can SSH to using an SSH client such as putty. Eniac is not to be used for work. After logging into eniac then choose one of the lab machines to log onto and do your work there. You can SSH to any of the lab computers (cslab1 to cslab28) using the ssh command.

Here is the full command to use:


In the Xterm window you're using to log onto enaic from your home machine enter this command at the prompt:

                            ssh cslab5  <-- you don't have to add your user name in the command. If you don't,
                                                    the command assumes you want to log into the next machine using
                                                    the current user.


Q: How do I change my password?

A: Open an Xterm window and at the prompt issue the command yppasswd and follow the instructions. Follow the password creation instructions.


Q: What are the Department of Computer Science rules for creating a good password?

  1. Passwords should be at least 10 characters long
  2. Do not use personal information. You should never use personal information as a part of your password. It is very easy for someone to guess things like your last name, pet's name, child's birth date and other similar details.
  3. Do not use real words. There are tools available to help attackers guess your password. With today's computing power, it doesn't take long to try every word in the dictionary and find your password, so it is best if you do not use real words for your password.
  4. Mix different character types. You can make a password much more secure by mixing different types of characters. Use some uppercase letters along with lowercase letters, numbers and even special characters such as '&' or '%'.
  5. Use a passphrase. Rather than trying to remember a password created using various character types which is also not a word from the dictionary, you can use a passphrase. Think up a sentence or a line from a song or poem that you like and create a password using the first letter from each word.

        For example, rather than just having a password like 'yr$1Hes', you could take a sentence such as "I like to read the Internet / Network Security web site" and convert it to a password like 'il2rtA!nsws". By substituting the number '2' for the word 'to' and using an exclamation point in place of the 'i' for 'Internet', you can use a variety of character types and create a secure password that is hard to crack, but much easier for you to remember.