Debug — Elixir Debugging Workflow

Quick Reference

Interactive Debugging

# IEx.pry — drops into interactive session at this point
require IEx; IEx.pry()

# Run tests with pry support
iex -S mix test test/overmind/mission_test.exs:42

Tracing with IO.inspect

# Inline inspect (returns the value, safe in pipelines)
value |> IO.inspect(label: "after_transform")

# Inspect in pipelines
data
|> parse()
|> IO.inspect(label: "parsed")
|> validate()
|> IO.inspect(label: "validated")

dbg() — Pipe Debugging (Elixir 1.14+)

# Shows each step of the pipeline
data
|> parse()
|> validate()
|> transform()
|> dbg()

Targeted Test Runs

# Single file
mix test test/overmind/mission_test.exs

# Single test by line number
mix test test/overmind/mission_test.exs:42

# With trace (verbose)
mix test --trace test/overmind/mission_test.exs

Daemon Debugging

# Check mission logs
overmind logs <id>

# Get mission details (os_pid, status, command)
overmind info <id>

# Check socket
ls -la ~/.overmind/overmind.sock

# Raw socket command
echo '{"command":"ps"}' | nc -U ~/.overmind/overmind.sock

Process & ETS Inspection

# Start Observer (GUI)
:observer.start()

# ETS table contents
:ets.tab2list(:overmind_missions)

# ETS lookup specific key
:ets.lookup(:overmind_missions, id)

# Process info
Process.info(pid)
Process.info(pid, [:current_function, :message_queue_len, :status])

Port Debugging

# Get os_pid from overmind
overmind info <id>  # returns os_pid field

# Check if process is running
ps -p <os_pid>

# Check what the process has open
lsof -p <os_pid>

# Send signal
kill -0 <os_pid>  # check if alive (no signal sent)

Common Issues

Symptom	Check	Likely Cause
Mission stuck in :running	overmind info <id>	Port not exiting, stall detection not configured
{:error, :not_found}	:ets.tab2list(:overmind_missions)	Wrong ID, or ETS entry cleaned up
Socket connection refused	ls -la ~/.overmind/overmind.sock	Daemon not running, stale socket
Test timeout	mix test --trace	Waiting on assert_receive for message that never arrives
Dialyzer warning	mix dialyzer	Typespec mismatch, check return types

Rules

• Never commit debug code — no IEx.pry, IO.inspect with labels, or dbg() in committed code
• Remove after use — clean up all debug instrumentation before moving on
• Use labels — always label IO.inspect calls to distinguish multiple inspection points
• Prefer dbg() — for pipeline debugging, dbg() is cleaner than chained IO.inspect