Challenges

Coding practice and systems · Build a Unix Shell

#	Challenge title	Domain	Difficulty
01	Build a Unix Shell PREQUELS 0/6 LEVELS 0/6	Command shell	Medium
PREQUELS (0 / 6)Focused coding problems · Solve in any order Start Prequels → Prequel 1Split a Line into Words· Strings, Tokenization Easy Prequel 2Find a Program's Path· Strings, Linear Search Easy Prequel 3Track Finished Tasks· Arrays, Strings Medium Prequel 4Split a Command into Stages· Arrays, Strings Medium Prequel 5Calculate Window Statistics· Sliding Window, Arrays Hard Prequel 6Top K Frequent Items· Arrays, Strings Hard LEVELS (0 / 6)Build a working system · One level at a time Start Level 1 → Level 1Builtin Command Execution (echo, pwd, cd, exit)· Strings, Parsing Easy Level 2PATH Resolution & Binary Execution (which, run, exec)· Strings, Linear Search Medium Level 3Process Lifecycle & Zombie Reaping (sigint, spawn-bg, reap)· Hash Maps, State Machines Medium Level 4Multi-Stage Pipelines & I/O Redirection (pipe, dup, >, >>)· Strings, Queues Hard Level 5Sliding-Window Latency Profiling (profile window)· Sliding Window, Arrays Medium Level 6Command History Frequency Ranking (hot k)· Hash Maps, Sorting Medium
02	Build an HTTP ServerLocked PREQUELS 0/6 LEVELS 0/6	Web server	Hard
03	Build a Git EngineLocked PREQUELS 0/6 LEVELS 0/6	Version control	Medium
04	Build a Key-Value StoreLocked PREQUELS 0/6 LEVELS 0/6	Key-value database	Hard
05	Build an Object StoreLocked PREQUELS 0/6 LEVELS 0/6	Object storage	Hard
06	Build an LRU CacheLocked PREQUELS 0/6 LEVELS 0/6	Memory cache	Medium
07	Build a Database IndexLocked PREQUELS 0/6 LEVELS 0/6	Database index	Hard
08	Build a Container RuntimeLocked PREQUELS 0/6 LEVELS 0/7	Container runtime	Hard
09	Build a Message QueueLocked PREQUELS 0/6 LEVELS 0/6	Message queue	Hard
10	Build a Log Analytics EngineLocked PREQUELS 0/6 LEVELS 0/6	Log processing	Medium
11	Build a Rate LimiterLocked PREQUELS 0/6 LEVELS 0/6	Rate limiting	Medium
12	Build a Load BalancerLocked PREQUELS 0/6 LEVELS 0/6	Load balancing	Hard
13	Build a Task SchedulerLocked PREQUELS 0/6 LEVELS 0/6	Task scheduling	Medium
14	Build a Raft Consensus ModelLocked PREQUELS 0/6 LEVELS 0/8	Distributed consensus	Hard
15	Build a Service RegistryLocked PREQUELS 0/6 LEVELS 0/7	Service discovery	Hard
16	Build a Distributed Object StoreLocked PREQUELS 0/6 LEVELS 0/6	Distributed storage	Hard
17	Build a Search EngineLocked PREQUELS 0/6 LEVELS 0/6	Search engine	Medium
18	Build a Vector DatabaseLocked PREQUELS 0/6 LEVELS 0/7	Vector database	Medium
19	Build an LLM Inference ModelLocked PREQUELS 0/6 LEVELS 0/6	AI inference	Medium
20	Build an MCP Tool RuntimeLocked PREQUELS 0/6 LEVELS 0/6	AI tool runtime	Medium

Showing 20 of 20 verified systems challenges

Python, C++, Rust, Go, Java Supported

Build a Unix Shell

3. Process Lifecycle & Zombie Reaping (sigint, spawn-bg, reap)

Medium

A user interrupts a command and expects the terminal to remain usable. Another task may finish in the background while the user continues typing. The shell needs to remember which jobs are running and which are ready for cleanup.

What you'll build

Track background jobs and their states. Handle sigint without ending the loop, and remove only finished jobs when reap is requested. These are simulated events, not operating-system signals or processes.

Input: One event per line: sigint, spawn-bg <name>, reap, echo [words...], or exit.

Output: sigint prints ^C. reap prints REAPED: N. echo prints its words. Starting a job is silent. exit prints CLEAN_EXIT only when tracked jobs remain.

Example 1

Input:

sigint
echo ok
exit

Output:

^C
ok

Explanation: sigint prints ^C, but echo still runs afterward. An interruption does not end this shell loop.

Example 2

Input:

spawn-bg true
reap
exit

Output:

REAPED: 1

Explanation: true is immediately marked finished in this model. reap removes that one finished record.

Example 3

Input:

spawn-bg true
spawn-bg true
reap
exit

Output:

REAPED: 2

Explanation: Two finished jobs are waiting, so one cleanup removes both and reports 2.

Example 4

Input:

sigint
sigint
echo done
exit

Output:

^C
^C
done

Explanation: Repeated interruptions do not disable later commands. The loop still reaches echo done.

Example 5

Input:

spawn-bg true
exit

Output:

CLEAN_EXIT

Explanation: The finished job has not been reaped, so it is still tracked. exit reports CLEAN_EXIT before stopping.

Try a small change

Paste this into Custom Input and run your solution.

spawn-bg true
reap
reap
exit

Why does the second cleanup return zero?

Check your prediction

Expected output:

REAPED: 1
REAPED: 0

The first reap removes the finished record. Cleanup changes state; it does not repeatedly count historical completions. No jobs remain when exit is reached.

Constraints

spawn-bg true and spawn-bg false immediately create finished jobs in this model. Other names create jobs that stay running.
reap counts and removes finished jobs only. Repeating it without new finished jobs returns REAPED: 0.
sigint leaves all jobs and the command loop intact.
On exit, a remaining tracked job triggers CLEAN_EXIT even if that job is finished but not yet reaped. Stop processing after exit.

Hint

Can each job carry a finished flag? Which flags should cleanup remove?

Where This Skill Takes You

Lifecycle tracking explains why a shell must collect completed work. This is a teaching state machine, not an implementation of OS signal delivery or waitpid.

What Comes Next?

Pass one command's output into another and distinguish overwrite from append.

Level 3 of 6

Code

5 executions per level

Loading ALGO Monaco Editor...

Input:Case 1: Trap SIGINT

Output:

^C
ok

Challenges

Coding practice and systems · Build a Unix Shell

#	Challenge title	Domain	Difficulty
01	Build a Unix Shell PREQUELS 0/6 LEVELS 0/6	Command shell	Medium
PREQUELS (0 / 6)Focused coding problems · Solve in any order Start Prequels → Prequel 1Split a Line into Words· Strings, Tokenization Easy Prequel 2Find a Program's Path· Strings, Linear Search Easy Prequel 3Track Finished Tasks· Arrays, Strings Medium Prequel 4Split a Command into Stages· Arrays, Strings Medium Prequel 5Calculate Window Statistics· Sliding Window, Arrays Hard Prequel 6Top K Frequent Items· Arrays, Strings Hard LEVELS (0 / 6)Build a working system · One level at a time Start Level 1 → Level 1Builtin Command Execution (echo, pwd, cd, exit)· Strings, Parsing Easy Level 2PATH Resolution & Binary Execution (which, run, exec)· Strings, Linear Search Medium Level 3Process Lifecycle & Zombie Reaping (sigint, spawn-bg, reap)· Hash Maps, State Machines Medium Level 4Multi-Stage Pipelines & I/O Redirection (pipe, dup, >, >>)· Strings, Queues Hard Level 5Sliding-Window Latency Profiling (profile window)· Sliding Window, Arrays Medium Level 6Command History Frequency Ranking (hot k)· Hash Maps, Sorting Medium
02	Build an HTTP ServerLocked PREQUELS 0/6 LEVELS 0/6	Web server	Hard
03	Build a Git EngineLocked PREQUELS 0/6 LEVELS 0/6	Version control	Medium
04	Build a Key-Value StoreLocked PREQUELS 0/6 LEVELS 0/6	Key-value database	Hard
05	Build an Object StoreLocked PREQUELS 0/6 LEVELS 0/6	Object storage	Hard
06	Build an LRU CacheLocked PREQUELS 0/6 LEVELS 0/6	Memory cache	Medium
07	Build a Database IndexLocked PREQUELS 0/6 LEVELS 0/6	Database index	Hard
08	Build a Container RuntimeLocked PREQUELS 0/6 LEVELS 0/7	Container runtime	Hard
09	Build a Message QueueLocked PREQUELS 0/6 LEVELS 0/6	Message queue	Hard
10	Build a Log Analytics EngineLocked PREQUELS 0/6 LEVELS 0/6	Log processing	Medium
11	Build a Rate LimiterLocked PREQUELS 0/6 LEVELS 0/6	Rate limiting	Medium
12	Build a Load BalancerLocked PREQUELS 0/6 LEVELS 0/6	Load balancing	Hard
13	Build a Task SchedulerLocked PREQUELS 0/6 LEVELS 0/6	Task scheduling	Medium
14	Build a Raft Consensus ModelLocked PREQUELS 0/6 LEVELS 0/8	Distributed consensus	Hard
15	Build a Service RegistryLocked PREQUELS 0/6 LEVELS 0/7	Service discovery	Hard
16	Build a Distributed Object StoreLocked PREQUELS 0/6 LEVELS 0/6	Distributed storage	Hard
17	Build a Search EngineLocked PREQUELS 0/6 LEVELS 0/6	Search engine	Medium
18	Build a Vector DatabaseLocked PREQUELS 0/6 LEVELS 0/7	Vector database	Medium
19	Build an LLM Inference ModelLocked PREQUELS 0/6 LEVELS 0/6	AI inference	Medium
20	Build an MCP Tool RuntimeLocked PREQUELS 0/6 LEVELS 0/6	AI tool runtime	Medium

Showing 20 of 20 verified systems challenges

Python, C++, Rust, Go, Java Supported

Build a Unix Shell

3. Process Lifecycle & Zombie Reaping (sigint, spawn-bg, reap)

Medium

What you'll build

Input: One event per line: sigint, spawn-bg <name>, reap, echo [words...], or exit.

Output: sigint prints ^C. reap prints REAPED: N. echo prints its words. Starting a job is silent. exit prints CLEAN_EXIT only when tracked jobs remain.

Example 1

Input:

sigint
echo ok
exit

Output:

^C
ok

Explanation: sigint prints ^C, but echo still runs afterward. An interruption does not end this shell loop.

Example 2

Input:

spawn-bg true
reap
exit

Output:

REAPED: 1

Explanation: true is immediately marked finished in this model. reap removes that one finished record.

Example 3

Input:

spawn-bg true
spawn-bg true
reap
exit

Output:

REAPED: 2

Explanation: Two finished jobs are waiting, so one cleanup removes both and reports 2.

Example 4

Input:

sigint
sigint
echo done
exit

Output:

^C
^C
done

Explanation: Repeated interruptions do not disable later commands. The loop still reaches echo done.

Example 5

Input:

spawn-bg true
exit

Output:

CLEAN_EXIT

Explanation: The finished job has not been reaped, so it is still tracked. exit reports CLEAN_EXIT before stopping.

Try a small change

Paste this into Custom Input and run your solution.

spawn-bg true
reap
reap
exit

Why does the second cleanup return zero?

Check your prediction

Expected output:

REAPED: 1
REAPED: 0

The first reap removes the finished record. Cleanup changes state; it does not repeatedly count historical completions. No jobs remain when exit is reached.

Constraints

spawn-bg true and spawn-bg false immediately create finished jobs in this model. Other names create jobs that stay running.
reap counts and removes finished jobs only. Repeating it without new finished jobs returns REAPED: 0.
sigint leaves all jobs and the command loop intact.
On exit, a remaining tracked job triggers CLEAN_EXIT even if that job is finished but not yet reaped. Stop processing after exit.

Hint

Can each job carry a finished flag? Which flags should cleanup remove?

Where This Skill Takes You

Lifecycle tracking explains why a shell must collect completed work. This is a teaching state machine, not an implementation of OS signal delivery or waitpid.

What Comes Next?

Pass one command's output into another and distinguish overwrite from append.

Level 3 of 6

Code

5 executions per level

Loading ALGO Monaco Editor...

Input:Case 1: Trap SIGINT

Output:

^C
ok