A slack clone in 5 lines of bash

The title oversells the content a bit11: Some called it disingenous dickish clickbait. I don't disagree ;-) :

Nevertheless, suc's core indeed consists of five lines of bash; and suc provides Slack, Mattermost, etc.'s core features:

This paper shows how suc implements those features. suc stays small by leveraging the consistent and composable primitives offered by modern UNIX implementations 22: in the dam case, GNU Guix .

1. Line count matters

One of my most productive days was throwing away 1000 lines of code.

– Ken Thompson, apparently

Measuring programming progress by lines of code is like measuring aircraft building progress by weight.

– Bill Gates, (probably apocryphal)

Some of the managers decided that it would be a good idea to track the progress of each individual engineer in terms of the amount of code that they wrote from week to week. […] When he got to the lines of code part, [Bill Atkinson] […] wrote in the number: -2000.


Their fundamental design flaws are completely hidden by their superficial design flaws.

– Douglas Adams

There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies.

– Tony Hoare

Despite the wide consensus among competent programmers that code is a liability, almost every widely-distributed piece of software is a complexity behemoth.

Case in point, let's examine Mattermost's line count:

cd /tmp
git clone --depth=1 https://github.com/mattermost/mattermost-server
cd mattermost-server
guix shell cloc -- cloc --quiet --timeout 0 .
github.com/AlDanial/cloc v 1.96  T=14.34 s (606.7 files/s, 139790.0 lines/s)
Language                     files          blank        comment           code
Go                            1805          96705          26782         501249
JSON                           177              5              0         492604
TypeScript                    4125          74236          24557         480491
JavaScript                     811          21494          20745          68653
SCSS                           557           9164            359          51464
HTML                            54           6108           1167          37814
JSX                             92           3473           1054          29707
SQL                            807           3553           2253          18266
Text                            11           3824              0          10638
YAML                            45            126             96           7972
SVG                             68              6             12           2586
Markdown                        73            906             88           2168
make                             8            234             68            974
GraphQL                          4             65              2            596
XML                             29             10              1            572
Bourne Shell                    18            128             17            492
CSS                              5             70              0            385
Dockerfile                       4             14              8             46
CSV                              2              0              0             25
diff                             2              1             13              9
INI                              1              2              0              7
SUM:                          8698         220124          77222        1706718

Half a million lines of Go33: user lolinder on Hacker news looked into it and saw that TypeScript is used for tests and the web client, so to avoid comparing apple to oranges, one should only count the 500.000 lines of Go here. . Just for the server !

Let's compare with suc:

cd /tmp
git clone --depth=1  https://gitlab.com/edouardklein/suc
cd suc
guix shell cloc -- cloc --quiet --timeout 0 .
github.com/AlDanial/cloc v 1.96  T=0.01 s (475.8 files/s, 8207.6 lines/s)
Language                      files          blank        comment           code
Bourne Again Shell                1              2              2             19
C                                 1              3              3             17
make                              1              3              0             14
Bourne Shell                      1              0              1              5
SUM:                              4              8              6             55

suc can implement Mattermost's core features with 0.005% of the code. This is madness !

2. suc's core loop

Behold the five lines of bash44: The actual script is longer, because there is a preamble to make it secure and a hash-based color computation to decorate the username. that do as much as half a million lines of Go:

while IFS= read -r -n $MAXWIDTH line
    \printf '\e[38;5;243m%(%FT%T%z)T \e[38;5;%dm\e[48;5;%dm%-9s\e[0m %s\e[0m\n' \
        -1 "$USER_FG_COLOR" "$USER_BG_COLOR" "$USER" "$line" >> /var/lib/suc/"$1"

This infinite loop:

  • reads a line from standard input,
  • prefixes it with:
    • the date,
    • the real user name,
  • and appends it to a file in /var/lib/suc/

Surely, you think, this cannot do. What about authentication, access control, encryption, rich text, etc. ?

suc does all that by leveraging SSH, UNIX's access control API, and UNIX's text-based modularity.

3. Authentication

The suc process can only be launched by an authenticated user 55: Usually done by calling usuc <some-channel> on the command line . Therefore, suc contains no authentication code at all. All the authentication stuff happens before suc even starts.

As with almost all UNIX servers nowadays, remote authentication is handled by ssh. Before granting them the ability to start suc, ssh requires users to prove their identity.

This proof can take the form

  • of a shared secret (i.e. a password),
  • of a cryptographic challenge (as is the case on the dam),
  • of the use of a One-Time-Passord (OTP) generating device,
  • or of any combination of the above (also known as Multi-Factor Authentication, MFA).

ssh also authenticates the server to the client, thus preventing Man-in-the-Middle (MitM) attacks.

Last but not least, ssh encrypts all data between the clients and the server.

A successful installation of suc therefore depends on a correct configuration of the UNIX host and its ssh server. To use suc, a user needs to exist on the system; and the ssh server needs to be configured to let her remotely log in.

Most UNIX distribution provide the useradd, passwd, etc. commands for user management (creation, deletion, assignation to one or more groups, etc.). The ssh server reads its configuration from a text file in /etc/ (typically /etc/ssh/sshd_config), and from public key files (typically in /home/<user>/.ssh/authorized_keys).

The dam server uses GNU Guix. GNU Guix differs from almost all other UNIX distributions, because it uses declarative configuration. This means that root just has to say what she wishes the configuration to be. The system then complies and reconfigures itself to match root's declaration.

For example, granting ssh access to alice on a GNU Guix system 66: configured with Beaver Labs' channel requires only the following line in the system's configuration file:

(ssh-user "alice" #:groups '("c3n" "frenchies")
                  #:keys '((plain-file "alice.pub" "SOMESSHKEY")))

User alice exists on the system only as long as the line exists in the configuration file. When the line disappears, the reconfiguration process removes user alice, and she can no longer log in.

Some big advantages of declarative configuration systems include:

  • removing the need for clean up actions when removing functionality: once it is no longer part of the declaration, it will be removed from the system automatically.
  • the ability to clone a specific configuration by just replicating the declaration; useful for back-ups, failovers, etc..

Among the disadvantages, one counts an increased difficulty for quick and dirty setups (usually for a quick test to try out a piece of software). New tools (such as e.g. guix shell) allows one to sidestep this difficulty.

In such a declarative system, suc's overhead per user is limited to a single line in the global configuration file. One cannot need less, and current chat systems need more.

4. Access control

As with authentication, suc contains no access control code whatsoever. This combination of caring about neither authentication nor access control is called security agnosticism. Security anosticism allows suc to be lean, and therefore more probably correct (and so, paradoxically, more secure) than its heavier counterparts.

On UNIX, software can afford to be security agnostic because the system provides a clean and powerful API for access control: the kernel knows about

  • users and groups,
  • processes and files.

Let's dive in.

UNIX veterans will have noticed that suc prefixes the user's messages with her real name. Indeed, files have an owner (a user), whereas processes have two owners (two users). The real one and the effective one.

Most of the time, real and effective owners are the same. suc's ownership differs: it effectively belongs to a special user also named suc; it really belongs to whoever (e.g. user alice) launched the suc command 77: UNIX exposes this capability through something called the setuid bit. .

The kernel examines the effective ownership of a process to determine said process' ability to read or write to files.

With that in mind, let's examine the content of /var/lib/suc on the dam:

ssh -i ~/.ssh/id_rsa edk@the-dam.org ls -l /var/lib/suc
total 92
-rw-r----- 1 suc c3n            44368 Apr 13 19:18 banane
-rw-r----- 1 suc forbiddenlands  6234 Apr 13 21:04 forbiddenlands
-rw-r----- 1 suc frenchies         62 Apr 21 22:23 frenchies
-rw-r----- 1 suc guixdevs           0 Apr 22 15:39 guix
-rw-r----- 1 suc iwp9            4181 Apr 21 21:46 iwp9
-rw-r----- 1 suc users          18241 Jun 30 07:14 the-dam
-rw-r----- 1 suc wb3c             188 May 10 11:56 wb3c

The files in /var/lib/suc belong to suc; only suc can read and write those files 88: If you do not understand the -rw-r----- output of the command above, I recommend section 2.4, "Permissions" of Brian W Kernighan and Rob Pike, The UNIX Programming Environment, (Prentice-Hall Englewood Cliffs, NJ, 1984). .

Any other user, such as alice, may read some of the files (e.g. banane), provided she belongs to the appropriate group (e.g. c3n).

With this configuration, suc does not need to care about access control at all. For example suc need not match a user against the list of authorized readers or writers of a channel.

Instead, usuc 99: usuc extends suc with bells and whistles, see below. will just happily always try to read or write the file. The kernel will do the matching and prevent any unauthorized access.

On the dam, everyone can start suc, whose effective owner will be the user suc, who has the right to write into any channel. By design, any user on the dam can request membership into a group by blindly writing a request to the group's channel.

Less loosely-managed communities may wish to restrict channel write access to members only. root achieves this by maintaining multiple copies of the suc binary.

Let's assume that

  • alice and bob belong to the blue group,
  • while eve and mallory belong to the red group.

root creates nobody-like1010: nobody is usually a passwordless, shellless user who owns no files, to whom ownership of a process is transferred when one wants to prevent said process from being allowed to do any damage on the system. users red and blue. She then creates two copies of suc, one for each group:

ls -l /usr/bin/suc*
total 32
-rwsr-xr-- 1 red     red      15624 Jun  4 10:51 suc_red
-rwsr-xr-- 1 blue    blue     15624 Jun  4 10:56 suc_blue

And she also creates one channel for each team:

ls -l /var/lib/suc/
total 16
-rw-r----- 1 blue     blue     11027 Jun  4 11:30 blue
-rw-r----- 1 red      red      17    Jun  4 10:53 red

One can see that:

  • alice and bob belong to group blue.
    • They can read the blue channel. Indeed the file /var/lib/suc/blue belongs to group blue and has mode -rw-r----- : the second r means that members of the owning group (here, blue), can read the file (but not write to it).
    • They cannot directly write to the file. Only user blue can.
    • They can however launch the /usr/bin/suc_blue program, because group blue owns it, and it has mode -rwsr-xr-- . The x means that members of the owning group (here, blue) can start the program.
    • This program will run with user blue as the effective owner: User blue owns the file and root has set its setuid bit (the s in the mode line says so).
    • Therefore, alice and bob, being members of the blue group, can launch the /usr/bin/suc_blue program, which being effectively owned by user blue (despite being launched by alice or bob who will be the real, but not effective owner) can write to the /var/lib/suc/blue file.
  • eve and mallory belong to group red (but not group blue).
    • They cannot read the blue channel. Indeed, people other than user blue or members of group blue have no rights on the /var/lib/suc/blue file (the end of its mode line is ---).
    • They cannot write to the blue channel directly, only user blue can.
    • They cannot start the /usr/bin/suc_blue program, because they do not belong to group blue. The only thing they can do to this file is read it (its mode line ends in r--).
    • Therefore they can neither read nor write the blue channel.

To relieve root from the cumbersome and error-prone process of setting this all up, suc provides an 80-something-lines long helper script called suc_channel.sh.

GNU Guix users can create a suc channel by adding a single line to the system's configuration file:

(suc-private-channel "red" "red")

This line takes care of creating the necessary

  • suc_red setuid binary,
  • red user
  • red group
  • red channel file.

Here, GNU Guix's declarative configuration paradigm shines again. The suc_channel.sh script may fail halfway, leaving the system in an undetermined state, whereas GNU Guix provides transactional updates: either the transition happens fully or it does not at all. The system always stays in a known clean state. One can even roll-back to a previous working state (see Multi-dimensional transactions and rollbacks, oh my!).

GNU Guix also automatically computes which groups, users, and setuid binaries should exist on the system. When root removes a private channel (e.g. red), she must assess whether the associated group (also named red), user (also red), and setuid binary (suc_red) should stay or go. That entails looking at the other channels to see if any of them is still owned by user red or group red. Again, a cumbersome and error prone task whereas on GNU Guix, root just removes the channel's line from the system declaration. The red group, red user, and suc_red binary will stay if and only if another part of the system needs them.

As an illustration, here is a full system declaration for the above example. One can hardly be simpler than that.

(begin (use-modules
        (gnu packages base)
        (guix gexp)
        (beaver system)
        (beaver packages plan9)
        (beaver functional-services))
       (-> (minimal-ovh)
           (ssh-user "alice"   #:groups '("suc" "blue") #:keys '())
           (ssh-user "bob"     #:groups '("suc" "blue") #:keys '())
           (ssh-user "eve"     #:groups '("suc" "red")  #:keys '())
           (ssh-user "mallory" #:groups '("suc" "red")  #:keys '())
           (suc-private-channel "red" "red")
           (suc-private-channel "blue" "blue")
           (suc-public-channel "purple")))

5. Fancy text

We have seen how suc is security-agnostic, relying on:

  • ssh for authentication,
  • UNIX's file and process ownership and permission model for access control.

Let's now dive into the featureful side of things by first looking at some bells and whistles: rich text.

Most chat applications nowadays piggyback on an HTML engine to render the chat's text. For example mattermost's client uses Electron. There go another few tens of thousand of lines of code.

On the one hand, this adds tremendous complexity and increases the attack surface of the application. On the other hand it lets the chat display elements in a complex layout, or embed interactive widgets within the messages (such as emoji reactions), etc.

suc uses one file per channel. This text file is meant to be displayed to the user with a command-line tool such as tail or cat.

Before everything got shoehorned into an HTML rendering engine, people managed to display rich text, boxes, and even primitive graphics on their terminals. These capabilities more-or-less coalesced into something called ANSI escape codes1111: This is a very deep rabbit hole. If you want to dive in, I've found good starting points to be: Nick Black's "Hacking the Planet (with Notcurses). A Guide to TUIs and Character Graphics", the relevant section of the VT100 manual, or this list of animations. . Almost all terminal emulators support those. Together with proper UTF-8 support, they allow for the colorful, emoji-filled experience of your average corporate slack channel, with ~5% of the memory footprint.

If you paid attention to the 5 lines of bash that suc consists of, you have noticed that while suc writes into the channel file, it does not read from it.

This job befalls to usuc. Why two separate binaries ? Because suc is a privileged binary, which runs under the powerful effective ownership of whoever can write to a channel. One must be careful to keep the logic and external dependencies of suc to a bare minimum to minimize the attack surface, and avoid any complex logic where bugs like to hide.

usuc, conversely, runs with both effective and real owners set to the calling user. It can go crazy with the features, as whatever happens can not impact the channel file, except through suc, whose logic is so simple there should not be any bugs in it.

Here is as of <2023-06-29 Thu> the code for usuc:

set -euo pipefail

# Autowrap self in rlwrap
if [ -z "${RLWRAP:-}" ]
    RLWRAP=1 rlwrap "$0" "$@"
    exit 0

chan_owner=$(ls -l /var/lib/suc/"$1" | cut -d' ' -f 3)
if [ "$chan_owner" != suc ]
# Tail the channel
tail -f -n 20 /var/lib/suc/"$1"&
while true
    read -r line || exit 0
    if [ "${line::1}" == ":" ]
        echo '*runs* `' "${line:1}" '`' | pygmentize -l md -f 256 | "$SUC" "$1"
        bash -c "${line:1}" | "$SUC" "$1"
        echo "$line" | pygmentize -l md -f 256 | "$SUC" "$1"


  • makes sure to prefix its own call with rlwrap, which provides history and line editing capabilities,
  • selects the correct setuid suc binary to run depending on who owns the channel file,
  • calls tail -f, displaying the last 20 lines of the channel and then anything that get subsequently written to it,
  • check whether the line typed by the user starts with ":" (see the next section),
  • pipe anything the user typed through pygmentize.

Pygmentize is a nifty Python module for syntax coloring. Here it runs expecting markdown on its standard input, and outputting ANSI color coded text on its standard output. That way, a user can use markup syntax like **bold**, and get bold output. suc gets markdown support in a single line of code.

6. Chat commands

Other tools can, like pygmentize, output ANSI-styled text. One of those is e.g. gum.

To invoke gum directly from the chat interface, one just has to start a message with :. usuc will catch that and will not pipe the text to suc like it would for a normal message. It will instead run the command, and pipe its output to suc.

One can therefore type:

: gum style --border=rounded --bold --foreground=#F00 "Hello World !"

as a suc message and see something that looks like the following appear in the channel:

│Hello World !│

Any command that exists in the namespace of the user who called usuc can run that way. Its output will appear in the chat.

We use that on the dam to roll dice when we play table-top role playing games:

: roll 2d6
2023-04-13T21:04:57+00:00 gm        *runs* ` roll 2d6 `
2023-04-13T21:04:58+00:00 gm        [6, 2]

Again, it all happens in the namespace of the user. Any user can customize her environment to keep useful chat macros on hand, without any impact on the other users.

7. Piping text to suc

Instead of using usuc's command-calling facility, one can pipe right into suc the output of any command, from one's shell.

For example if you want to pretty-print a piece of source code to a relevant channel, you can invoke bat:

bat --force-colorization --paging=never --style=full toto.c | suc greybeards

and you will get a syntactically-colored listing of your code in the channel.

Complex chat system like Mattermost, Slack, etc. offer many integrations, that is, ways to interact with other software.

suc is text-based ; integrating it with other tools feels natural in a UNIX environment. For example consider the following bash one-liner:

make test > testlog || (suc devops < testlog ; exit 1)

This code will run the tests of a software project, and send the logs to the devops channel on failure.

With the necessary boilerplate, this oneliner fits into the git hook update of a git repo:

set -euxo pipefail

GIT_DIR=$(realpath "$GIT_DIR")
cd "$(mktemp -d)"
git clone "$GIT_DIR" .
git checkout "$newrev"

make test > test_log || (suc devops < test_log ;  exit 1)
exit 0

And voilà ! You get a git/suc integration in 11 lines of bash. Any push to the repo will trigger the test, reject the update on failure, and ring the DevOps team so they can solve the problem.

8. Reading from a suc channel

suc users continually update a text file (the channel). By calling tail -f on that text file, you can process the new lines as they arrive.

For example, to get notified when a new message gets posted in a channel, just run:

tail -n0 -f /var/lib/suc/some-chan | (while true;
                                      do read -r line;
                                         notify-send "$line";

Too many notifications ? Reduce the noise by grepping for keywords:

tail -n0 -f /var/lib/suc/some-chan | \
    stdbuf -i0 -o0 grep -E "(myname|build failure|fire)" | \
    (while true; do read -r line; notify-send "$line"; done)

Don't want to open as many windows as channels you follow ? Coalesce them all in a single feed:

tail -f /var/lib/suc/*

Or use the more powerful lnav (a log file viewer), which will

  • remember where you left off,
  • set bookmarks,
  • assign a color to each channel,
  • parse the date, username, or any custom field that may appear in the text,
  • let you filter the messages,
  • run SQL queries on the messages.

Try to do that with Slack…

9. Bots

If you can write and read to a suc channel, you can do both at once. Chat systems often host bots and semi-automated "assistants". These provide a text-based interface to e.g. tickets, continuous integration, corporate directory, server logs, etc. Have a look below at the code of a bot that convert into meters any length given in feet:

feet_to_meters (){
    echo -e "$feetexpr \n m" | units | grep -Eo "\* [0-9.]*" | tr -d '*'

tail -n0 -f /var/lib/suc/"$1" | \
    stdbuf -i0 -o0 grep -v "metric_bot"  | \
    stdbuf -i0 -o0 grep -Eo "[0-9]+[[:blank:]]*(feet|ft)" | \
    (while true;
     do read -r line;
        echo "[metric_bot] $line is $(feet_to_meters "$line") meters." | suc "$1"
2023-06-30T11:20:47+02:00 edouard   The plane flew at 33000 ft.
2023-06-30T11:20:47+02:00 bots      [metric_bot] 33000 ft is  10058.4 meters.

10. Prior art

I compared suc with Slack, Mattermost, Discord: the behemoths. A fairer comparison would include IRC, talk, write.

  • IRC has less feature than suc, e.g. you need a bouncer to read past chat history.
  • I couldn't get talk, ytalk, or even wall to work on a modern linux distro. If somebody can, I'd link a tutorial here !

11. Conclusion

suc piggybacks on SSH for authentication and on UNIX for access control and composability. It provides almost all the features offered by Mattermost, Slack, etc. with such a ridiculously small fraction of the code that one wonders why such complex systems even exist.

Using text files as the base for suc channels lets user leverage UNIX tools for reading (tail, bat, lnav, less, grep, etc.), writing (gum, bat, pygmentize, etc.), or semi-automated extension with bots, hooks, and scripts.

Tools can be written in any language, as long as they read and write text.

12. Advertisement

If you want to play with suc but don't want to bother with installing it, or if you don't have any friends to share a suc instance with, come and join us at the dam ! For a measly 10€/year, you can enjoy sharing suc on a GNU Guix server with people from all over the world.

If you would like your own instance of suc, don't hesitate and rent a VPS from Guix hosting ! For 100€/year, you get a GNU Guix VPS. Adding suc is just one line of configuration away. There are no usage-based restrictions, your data stays yours, and you can use your VPS to provide other services as well.

13. Changelog

13.1. <2024-04-15 Mon>

Changed the old while true suc shell script with the current while read.

13.2. <2023-07-17 Mon>

  • Edited mattermost's line count to better reflect reality,
  • added a prior art section,
  • acknowledged the disingenous dickishness of the clickbait.