go tool — Hands-on Tasks¶

Work through these in order. Each has explicit acceptance criteria. Use Go 1.21+.

Task 1: List the tool catalog¶

Discover what go tool ships on your machine.

Acceptance criteria - [ ] go tool (no arguments) prints a list of tools. - [ ] ls "$(go env GOTOOLDIR)" shows the same names as real files. - [ ] You can name the directory in the form $GOROOT/pkg/tool/<GOOS_GOARCH>/.

Task 2: Inspect symbols with nm¶

Build a small binary and look inside.

// hello.go
package main
import "fmt"
func greet(name string) { fmt.Println("hi,", name) }
func main() { greet("world") }

Acceptance criteria - [ ] go build -o hello hello.go produces ./hello. - [ ] go tool nm hello | grep main.greet shows a T (text) symbol. - [ ] go tool nm -size hello | sort -k2 -nr | head lists the biggest symbols in the binary.

Task 3: Disassemble one function with objdump¶

Use a regex to focus on a single function.

Acceptance criteria - [ ] go tool objdump -s '^main\.greet$' hello prints the disassembly only for main.greet. - [ ] You can identify a CALL instruction in the output. - [ ] Re-running with go build -gcflags='all=-N -l' -o hello hello.go produces longer, less-optimized assembly.

Task 4: Check a build ID¶

Verify the identity of an artifact.

Acceptance criteria - [ ] go tool buildid hello prints a non-empty ID string. - [ ] Rebuilding without changes (and same flags) keeps the ID stable: go build -o hello hello.go && go tool buildid hello matches the previous value. - [ ] Adding -ldflags='-X main.x=now' changes the build ID.

Task 5: Generate coverage and view it in a browser¶

Run a test, then open the HTML report.

// math.go
package mathx
func Add(a, b int) int { return a + b }
func Sub(a, b int) int { return a - b }

// math_test.go
package mathx
import "testing"
func TestAdd(t *testing.T) { if Add(2, 3) != 5 { t.Fail() } }

Acceptance criteria - [ ] go test -coverprofile=cover.out ./... creates cover.out. - [ ] go tool cover -func=cover.out prints a per-function table with a total: line. - [ ] go tool cover -html=cover.out -o coverage.html produces an HTML file that opens in a browser and shows Sub in red (uncovered).

Task 6: List supported targets¶

Use dist list to enumerate cross-compilation targets.

Acceptance criteria - [ ] go tool dist list | head prints GOOS/GOARCH pairs. - [ ] go tool dist list -json | jq 'length' returns an integer (target count) — verifying machine-readable output works. - [ ] You can confirm linux/arm64 and darwin/arm64 are in the list.

Task 7: Use test2json in a script¶

Capture structured test events for a downstream tool.

Acceptance criteria - [ ] go test -json ./... > events.jsonl produces one JSON object per line. - [ ] jq -r 'select(.Action=="fail") | .Test' events.jsonl prints the names of failed tests (zero if all pass). - [ ] go test -v ./... | go tool test2json > events2.jsonl produces the same shape — confirming test2json is what -json uses under the hood.

Task 8: Run pprof on a profile¶

Generate and analyze a CPU profile.

// bench_test.go
package mathx
import "testing"
func BenchmarkAdd(b *testing.B) { for i := 0; i < b.N; i++ { _ = Add(i, i) } }

Acceptance criteria - [ ] go test -cpuprofile=cpu.out -bench=. ./... produces cpu.out. - [ ] go tool pprof -top cpu.out prints a non-empty top list. - [ ] go tool pprof -http=:0 cpu.out opens a browser UI (an arbitrary free port is chosen) and you can navigate the flame graph.

Task 9: Open a trace¶

Visualize the runtime tracer.

Acceptance criteria - [ ] go test -trace=trace.out -run=TestAdd ./... produces trace.out. - [ ] go tool trace trace.out starts a local HTTP server and prints its URL. - [ ] You can navigate to "View trace" and see at least one goroutine timeline.