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Udge is an online judge for programming problems.

Submitting a couple solutions to Udge

Using Udge you can host programming problems in an HTTP site. Users can submit solution programs which are automatically scored. Scores are then reported in each user's page.


  • hosting programming contests;
  • hosting programming problems for students by teachers, lecturers or professors;
  • hosting an online judge.

The usual goal of problems hosted on Udge is to create a command line program that reads data from standard input and produces results on standard output.

Udge is not a collection of programming problems but rather the software used to host them. You should create your own problems: write the problem description in markdown and create input and output cases following Udge's Problem directory format. Udge comes with seven example problems illustrating this.


Udge is implemented in Bash and works on Linux systems with Nginx. It uses static HTML pages where possible. These pages are updated by minutely cron jobs. The only two dynamic pages are for submission and user creation.

Udge is free/libre software. It is available under the GPLv2 license unless otherwise stated in specific files. --- Copyright (C) 2015-2022 Rudy Matela --- This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See the GNU General Public License for more details.

Udge is already functional and usable but it is a work in progress. Keep in mind that Udge runs submitted programs so use with care. There is submission sandboxing if users and cron jobs are set up correctly but this feature can still be improved. Keep an eye on the logs to see what is being submitted and run.

Udge is currently used on Computer Science by Example, a collection of programming exercises.


To install and run Udge, you will need:

You probably already have the following installed, but it does not hurt to double-check:

  • coreutils --- for timeout and whatnot
  • grep
  • psmisc --- for killall and whatnot
  • time --- for measuring runtime with /usr/bin/time
  • util-linux --- for sandboxing with unshare, kill, etc.

These are the optional dependencies:

On Ubuntu or Debian, you can install all dependencies with:

apt-get install make bash python nginx fcgiwrap cron cracklib-runtime diffutils discount fakechroot procmail clitest tidy gcc ghc openjdk-11-jdk lua mono-devel nodejs ruby guile-3.0 guile-3.0-dev erlang-base racket r-base-core

On Arch Linux, with the exception of clitest, you can install all dependencies with:

pacman -S make bash python nginx fcgiwrap cronie cracklib diffutils time discount fakechroot procmail tidy gcc ghc jdk11-openjdk lua mono nodejs ruby guile erlang racket-minimal r

Installing and Configuring

TODO: provide packages for Arch Linux and Ubuntu.

First make sure you have all the dependencies installed. Then:

  1. run make install as root:

     $ sudo make install

    Depending on your Linux distribution, you may need to set HTTPD_USER:

     $ sudo make install HTTPD_USER=<user>

    The Makefile should be able to figure these automatically on Arch Linux (tested) and on Debian/Ubuntu variants (tested).

  2. (required for Ubuntu/Debian variants, recommended for other systems) add the following to /etc/fstab:

     tmpfs /run/udge   tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=755,uid=udge,gid=udge   0 0
     tmpfs /run/udge/1 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-1 0 0
     tmpfs /run/udge/2 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-2 0 0
     tmpfs /run/udge/3 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-3 0 0
     tmpfs /run/udge/4 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-4 0 0
     tmpfs /run/udge/5 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-5 0 0
     tmpfs /run/udge/6 tmpfs rw,nosuid,nodev,relatime,size=12288k,mode=775,uid=udge,gid=udge-6 0 0

    The above will isolate the file system that Udge uses for each slot making sure that a submission that accidentally fills up the disk is not able to disrupt other submissions that are running at the same time. Here we use a modest limit of 12M per slot since tmpfss reside in memory.

    If you are running Ubuntu or another system that has /run mounted as a filesystem with noexec restrictions, at least the first line with /run/udge is required. Though if you decide to use the first line alone, better use a bigger size such as 48944k.

  3. (optional) add your problems to /var/lib/udge/problem and update accordingly otherwise you will be using the default example problems. This can be done at a later point.

    After setting them up, make sure you run:

     chmod o-r /var/lib/udge/problem/{*,*/*}/out

    The above will prevent attempted solutions to read directly from solution files. (cf. examples/sandbox/sol-searcher.c)

  4. generate static HTML files:

     sudo -u udge udge-update-all-problem-htmls
     sudo -u udge udge-update-rank-html

    You will have to re-run udge-update-all-problem-htmls every time you add or edit a problem description so HTML files are updated.

  5. Add the following entry to /etc/hosts. udge

    If you have another domain name you would like to use, replace it here instead. If you already have a public DNS entry pointing to your server you may skip this step.

  6. (optional) edit the domain name on Udge's Nginx config located on /etc/nginx/sites-available/udge.

  7. (optional) start the Nginx server if you haven't done so with either:

    • systemctl start nginx; or
    • service nginx start; or
    • /etc/inid.d/nginx start; or
    • ...

    which will depend on your Linux distribution.

  8. (optional) start and enable fcgiwrap and fcgiwrap.socket:

    • systemctl start fcgiwrap.socket
    • systemctl start fcgiwrap
    • systemctl enable fcgiwrap.socket
    • systemctl enable fcgiwrap
  9. enable Udge on Nginx and reload the configuration:

     make enable-nginx-udge-site
  10. test that everything works by typing (or your selected domain of steps 5 and 6) in your browser's address bar. If this does not work try also http://udge/ or

    You should see the problem index and the menu at the top and bottom.

  11. (optional) install and configure a certificate with certbot

    apt install certbot python-certbot-nginx  # or equivalent
    sudo certbot --nginx -d -d
  12. (optional) add an extra css file to /usr/local/lib/udge/extra.css if you would like to configure the appearance of Udge.

Udge can be customized on it's configuration file /etc/udgerc.

Pages and Routes

If the installation is working you should be able to access the following pages, each accessed by typing udge/<page> or <yourdomain>/<page>:

  • /: the index with the list of problems (
  • /submit: submission of solutions
  • /new-user: user creation
  • /<problem>: a problem description, e.g.:
    • /hello
    • /add
    • /hello-world
  • /u/<user_name>: user's page with scores for each problem and latest submissions
  • /rank: the user rank

Creating a Problem

To create a problem on Udge:

  1. Create a subdirectory to /var/lib/udge/problem called:


    The problem code should contain only digits, lowercase English letters and dashes (-).

  2. Create three files in the newly created problem folder:

    • a markdown file with your problem description

    • in: the input file. This will be used as the standard input to submitted solutions:

        ./submitted-program <in
    • out: the reference output file. This will be compared to the standard output of submitted solutions:

        ./submitted-program <in >run/out
        diff -rud run/out out

      If output matches exactly, the submission will get a score.

      Make sure you chmod o-r the out file after installing it to make sure solution programs are not able to read from it.

  3. Link to the newly created problem on the problem index by editing the following file:

  4. Update static HTML files by running:

     sudo -u udge udge-update-all-problem-htmls
  5. (testing) Access the newly created problem on:

  6. (testing) Submit a correct solution to the newly created problem and make sure it receives a full score.

  7. (testing) Submit an incorrect solution to the newly created problem and make sure it does not receive a full score.

Please see the addition problem for an example of this.

Multiple input and output pairs

To set up a problem with multiple I/O pairs, instead of creating just in and sol, create several subdirectories 1, 2, 3, ... Inside each create in and out files:

  • 1/in
  • 1/out
  • 2/in
  • 2/out
  • 3/in
  • 3/out
  • ...

Please see the add problem for an example of this.

Development Information

This section contains development information to those interested in forking or contributing to Udge's development.

Setting up a Development Environment

This section is intended for people who want to work on Udge development itself. If you just want to use Udge to host problems please see the Installing and Configuring section instead.

First make sure you have all the dependencies installed. Make sure you don't have Udge installed by make install. Use make uninstall-and-purge if needed.

The following sequence of commands can be used to set up the development environment. Run them as your regular user. You should only use root while running those preceded by sudo.

make dev-setup
sudo make dev-install
make html
sudo make start-services
sudo make enable-nginx-udge-site

You should also add udge to /etc/hosts.

If everything worked correctly, you should be able to run make test successfully.

If you like your development environment to automatically judge and update pages in the background, you should also add the contents of etc/cron.d/udge to your regular user's crontab (use crontab -e). You will have to remove references to the udge and udge-* but the contents should be otherwise the same.

Programs and Commands

Here's a complete list of programs provided with Udge:

  • cgi-bin/udge-new-user: CGI script that handles user creation;
  • cgi-bin/udge-submit: CGI script that handles submission of solution;
  • udge-add-user: adds a user creating an entry on users.
  • udge-judge: judges a solution printing results to stdout
  • udge-latest-results: shows the latest results from a user
  • udge-pick-and-judge: picks a solution at random from submissions, creates a result on results. For debug only.
  • udge-check-and-pick: checks run submissions and picks new ones. To be run from cron at each minute.
  • udge-compile-and-run <slot>: runs a submission on a specific slot. To be run from cron at each minute.
  • udge-rank: computes and prints the current user rank
  • udge-sandbox: runs a program in a sandbox. Currently sandboxing has a few limitations: forking protection can be improved; it does not chroot yet (as the chroot folders are still TBD); it is not applied to compilation (but should). These should be addressed in the future.
  • udge-submit: submits a solution to the judge using HTTP
  • udge-update-all-problem-htmls: updates all problem HTML files
  • udge-update-all-user-htmls: updates all user HTML files
  • udge-update-rank-html: updates the rank HTML file
  • udge-update-user-html: updates a single user HTML file
  • udge-user-stats: prints the stats for a given user
  • udge-delete-user: deletes a user
  • udge-passwd: changes the password of a user
  • udge-rejudge: queues a result to be rejudged
  • udge-create-submission: creates a submission in submissions.
  • udge-create-run: creates the /run/udge hierarchy. You should actually use etc/tmpfiles.d/udge.conf on real installations.
  • udge-backup: saves a backup of /var/lib/udge/... on /var/lib/udge/backups.

A file-based Database

Udge stores information about problems, users, submissions and results in plain text files:

  • /var/lib/udge/users: directory with user information (credentials)
  • /var/lib/udge/problem: I/O test cases and markdown files
  • /var/lib/udge/submissions: submissions that are still to be judged
  • /var/lib/udge/results: results of judging submissions
  • /var/lib/udge/html: the static HTML pages (problems and users)
  • /var/lib/udge/slot/<slot>: submission slot
  • /run/udge/<slot>: temporary files

This section describes the structure of each of these directories.

Udge diagram

Submissions first arrive on the /submissions folder then moved to one of the slots /slot/<slot> and are finally moved into the /results folder.

User Directory --- /var/lib/udge/users

The user directory stores main user information and credentials.

  • bin/udge-add-user creates entries in this directory.
  • cgi-bin/udge-new-user uses bin/udge-add-user to create entries here.
  • cgi-bin/udge-submit checks credentials in this directory.

User information is stored in plain files under the users directory. Each user is described as a directory with it's name which should be composed only of English lowercase letters, dashes (-) and underscores (_).

Emails, password hashes and salts are stored each in its own file with a single line:


For example:


User passwords are not stored into the system, just their hashes after salting. There's a different random salt for each user.

Problem Directory --- /var/lib/udge/problem

The problem directory contains test scripts, inputs and solutions for each of the problem.

  • bin/udge-judge reads this directory

Each problem has a directory, /var/lib/udge/problem/<problem>. Inside it:

  • 1/in: test input 1
  • 1/out: reference output 1
  • 1/af: (optional) files to be present in the working directory after the run
  • 1/bf: (optional) files to be present in the working directory before the run
  • 1/*.txt: (optional) additional files to the bf folder
  • 1/time-limit: (optional) the time limit in seconds (1 if not present)
  • 1/check-out: (optional) script to check the output file
  • 1/check-exit-code: (optional) script to check the exit code
  • 2/in: test input 2
  • 2/out: reference output 2
  • 2/...: test set 2
  • 3/...: test set 3
  • ...

If there is only one test set, you are allowed to let in and out reside plainly without a subdir.

Submissions Directory --- /var/lib/udge/submissions

The submissions directory contains submissions that are yet to be scored.

  • cgi-bin/udge-submit creates entries in this directory
  • bin/udge-pick moves entries away from this directory
  • bin/udge-check-and-pick moves entries away from this directory

It contain files in the following format:


For example:


Results Directory --- /var/lib/udge/results

The results directory contains the results of evaluated solutions.

  • bin/udge-check creates entries in this directory
  • bin/udge-check-and-pick creates entries in this directory
  • bin/udge-update-all-user-htmls reads from this directory
  • bin/udge-update-user-html reads from this directory

Results contain a folder for each user which in turn contains a folder for each problem which contains the best result for a problem along with a folder for each of the submissions, like so:


For example:


Slots --- /var/lib/udge/slot/<slot>

The slot directories contain entries that are ready to be run.

  • bin/udge-pick moves entries into this directory
  • bin/udge-check moves entries away from this directory
  • bin/udge-check-and-pick moves entries away and into this directory

It contains files in the following format:


For example:


There can be only one submission per slot and submissions in each slot should be run in different users. By default there are six slots named 1, 2, 3, 4, 5 and 6. The users for each of these slots are: udge-1, udge-2, udge-3, udge-4, udge-5 and udge-6.

The lock folder acts as a lock, while it exists udge-pick will never place a submission in this slot.

Run folders --- /run/udge/<slot>

For each slot, there also exist a folder on /run/udge in the following format:

/run/udge/<slot>/lock/run-/...(see below)...
  • udge-compile-and-test <slot> tries to acquire the lock folder, if it can, it will create the run- folder with the above format. Once it is done run- is moved into run.
  • udge-check tries to acquire the check folder, it if can, it will copy lock/run into check/run then it will check its contents producing a score. After creating the result and time files, it will move the submission from slot into results.

Test & Examples directory

The examples/ directory contains examples of use for several programs and commands shipped with Udge. These double as tests using the clitest tool.

Example solutions to example problems are also stored in this directory under examples/<problem>:

  • examples/add/0-ce.c: example solution to add in C -- 0/6 -- compile error;
  • examples/add/0-re.c: example solution to add in C -- 0/6 -- runtime error;
  • examples/hello/hello.c: example solution to hello in C -- 6/6;
  • examples/add/ example solution to add in Python -- 6/6;
  • examples/add/4-octals.c: example solution to add in C -- 4/6 -- incorrect output.

Other similar software

Udge is not the first and only online judge system out there. Here are some alternatives:


An online judge for programming problems



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