Linux Signals

A signal is a software interrupt sent to a process by the kernel or another process. It's the primary mechanism for async process notification.

Track how many knowledge checks you clear as you go:

Common Signals

SIGTERM vs SIGKILL

flowchart LR
    KILL_15["kill -15 (SIGTERM)"] --> HANDLER["process signal handler runs<br/>flush, cleanup, graceful shutdown"]
    HANDLER --> EXIT["process exits cleanly"]

    KILL_9["kill -9 (SIGKILL)"] --> KERNEL["kernel removes process<br/>NO handler runs"]
    KERNEL --> DEAD["process gone immediately<br/>in-flight I/O dropped<br/>files not flushed"]

Rule: Always try SIGTERM first. Give 30s. Then SIGKILL.

kill -15 <pid>      # SIGTERM — ask politely
sleep 30
kill -9 <pid>       # SIGKILL — force if still alive

# One-liner grace period
kill -15 <pid> && sleep 30 && kill -9 <pid> 2>/dev/null

Why SIGKILL can't be caught: SIGKILL and SIGSTOP are handled entirely by the kernel scheduler — the process never gets CPU time to run a handler. This is intentional: it guarantees a way to always terminate a hung process.

Signal Handling Internals

sequenceDiagram
    participant K as Kernel
    participant P as Process

    K->>P: deliver signal (set bit in pending mask)
    Note over P: signal is PENDING
    P->>K: syscall returns (or preemption)
    K->>K: check pending & ~blocked mask
    K->>P: redirect execution to signal handler
    P->>P: handler runs in user space
    P->>K: sigreturn() — restore original context
    P->>P: continues from where it was

When is a signal delivered? A signal becomes pending when sent. It's delivered when the process next transitions from kernel space to user space (syscall return or interrupt return). A sleeping process (in select, read, etc.) is woken up early — the syscall returns EINTR.

Signal Masks

A process can block signals (add to the signal mask). Blocked signals stay pending until unblocked.

# View current signal masks for a process
cat /proc/1234/status | grep Sig
# SigBlk: 0000000000000000  ← blocked signals (bitmask, each bit = signal number)
# SigIgn: 0000000000001000  ← ignored (bit 12 = SIGPIPE ignored)
# SigCgt: 0000000180000000  ← caught (has handler installed)
# SigPnd: 0000000000000000  ← pending (sent but not yet delivered)

# Decode bitmask: bit N = signal N+1
# 0x1000 = bit 12 = signal 13 = SIGPIPE
python3 -c "mask=0x1000; [print(i+1) for i in range(64) if mask & (1<<i)]"

In Go:

// Go runtime installs handlers for SIGSEGV, SIGBUS, SIGFPE
// and blocks SIGPROF for its own GC/scheduler use.
// To handle SIGTERM:
ch := make(chan os.Signal, 1)
signal.Notify(ch, syscall.SIGTERM, syscall.SIGINT)
<-ch  // block until signal
// do cleanup

SIGCHLD and Zombie Processes

When a child process exits, the kernel sends SIGCHLD to the parent. The parent must call wait() or waitpid() to collect the exit status — only then is the zombie reaped.

stateDiagram-v2
    [*] --> Running: fork()
    Running --> Zombie: process exits (all memory freed except PID entry)
    Zombie --> [*]: parent calls wait() — entry removed from process table
    Running --> Zombie: SIGCHLD sent to parent

# Default: SIGCHLD is ignored → parent never calls wait() → zombies accumulate
# Fix: explicitly handle SIGCHLD
# In shell scripts:
trap 'wait' SIGCHLD

# In C: use SA_NOCLDWAIT flag or signal(SIGCHLD, SIG_DFL) with waitpid
# In Go: os/exec.Cmd.Wait() handles this automatically

Graceful Shutdown Pattern (Go)

func main() {
    srv := &http.Server{Addr: ":8080"}

    go srv.ListenAndServe()

    quit := make(chan os.Signal, 1)
    signal.Notify(quit, syscall.SIGTERM, syscall.SIGINT)
    <-quit

    ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
    defer cancel()
    srv.Shutdown(ctx)  // waits for in-flight requests, then stops
}

Kubernetes graceful shutdown sequence:

1. Pod gets SIGTERM (from terminationGracePeriodSeconds countdown start)
2. App starts draining (stop accepting, finish in-flight)
3. After terminationGracePeriodSeconds (default 30s) → SIGKILL
4. Set preStop hook if you need extra time before SIGTERM

Step through the same sequence:

Sending Signals

kill -TERM <pid>         # by PID
kill -9 <pid>            # SIGKILL
killall -TERM nginx       # by name (all matching)
pkill -TERM -u www-data   # by user
pkill -f "python app.py"  # match full command line

# Send to process group (all children too)
kill -TERM -<pgid>

# Check what signals a process handles
kill -0 <pid>            # test if process exists (no signal sent)

Signal Tracing with strace

# Watch signals being received
strace -e signal -p 1234

# Output example:
# --- SIGTERM {si_signo=SIGTERM, si_code=SI_USER, si_pid=5678} ---
# rt_sigreturn()  = 0

Signal Forwarding and PID 1 in Containers

This is one of the most common sources of "pod won't terminate gracefully" bugs in production.

The PID 1 problem

In a container, PID 1 has special responsibilities:

1. It receives all signals sent by the container runtime (Docker, containerd)
2. It must reap zombie child processes (call waitpid)
3. If PID 1 exits, the entire container exits

When you write a Dockerfile with shell-form CMD, /bin/sh becomes PID 1:

sequenceDiagram
    participant CR as Container runtime
    participant P1 as PID 1 (/bin/sh -c ...)
    participant P2 as PID 2 (java -jar app.jar)

    CR->>P1: SIGTERM
    Note over P1: shell doesn't forward signals to children by default
    Note over P2: never receives SIGTERM

The shell receives SIGTERM but doesn't forward it to the child by default. The child (your app) never gets SIGTERM. Kubernetes waits terminationGracePeriodSeconds, then sends SIGKILL to PID 1 (the shell), which kills the entire process group. Your app gets SIGKILL with no chance to flush, drain connections, or save state.

Shell form vs exec form

# SHELL FORM — do not use for the main process
# /bin/sh -c is PID 1; your app is a child
CMD java -jar app.jar
CMD ["sh", "-c", "java -jar app.jar"]

# EXEC FORM — your app IS PID 1, receives signals directly
CMD ["java", "-jar", "app.jar"]
ENTRYPOINT ["java", "-jar", "app.jar"]

# How to check: inspect the image
docker inspect <image> | jq '.[0].Config.Cmd'
# Exec form: ["java","-jar","app.jar"]
# Shell form: ["/bin/sh","-c","java -jar app.jar"]

Shell wrapper with exec (when you need a startup script)

Sometimes you need a wrapper script for env var expansion, secret injection, or pre-flight checks. Use exec to replace the shell:

#!/bin/sh
# entrypoint.sh

# Do pre-flight setup
export DB_URL="postgresql://${DB_HOST}:5432/${DB_NAME}"
echo "Starting app, DB_URL=$DB_URL"

# exec REPLACES the shell with the app process
# app becomes PID 1 and receives signals directly
exec java \
  -Xmx${JAVA_MAX_HEAP:-512m} \
  -jar /app/app.jar \
  "$@"          # pass through any CMD arguments

COPY entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]
CMD ["--config", "/etc/app/config.yaml"]

Without exec, the shell stays as PID 1 and your app is still a child.

tini — minimal init for containers

tini is a tiny but correct init that:

• Registers signal handlers and forwards all signals to its child
• Reaps zombie processes (SIGCHLD + waitpid)
• Is transparent to the application

# Option 1: Install tini directly
FROM debian:bookworm-slim
RUN apt-get update && apt-get install -y tini && rm -rf /var/lib/apt/lists/*
ENTRYPOINT ["/usr/bin/tini", "--"]
CMD ["java", "-jar", "app.jar"]

# Option 2: Copy tini binary from official image
FROM tini:latest AS tini
FROM debian:bookworm-slim
COPY --from=tini /tini /tini
ENTRYPOINT ["/tini", "--"]
CMD ["java", "-jar", "app.jar"]

# Option 3: Docker built-in --init flag (adds tini automatically)
docker run --init myimage
# Or in compose:
# services:
#   app:
#     init: true

sequenceDiagram
    participant CR as Container runtime
    participant T as PID 1 (tini)
    participant P as PID 2 (java -jar app.jar)

    CR->>T: SIGTERM
    T->>P: forwards SIGTERM
    Note over P: shutdown hooks run — flush, drain, exit cleanly
    P-->>T: exits
    T-->>CR: tini exits, container terminates

Step through the same flow:

dumb-init — alternative to tini

RUN wget -O /usr/bin/dumb-init \
  https://github.com/Yelp/dumb-init/releases/download/v1.2.5/dumb-init_1.2.5_x86_64 \
  && chmod +x /usr/bin/dumb-init
ENTRYPOINT ["/usr/bin/dumb-init", "--"]
CMD ["python", "-m", "myapp"]

# dumb-init with setsid (creates new process group):
ENTRYPOINT ["/usr/bin/dumb-init", "--setsid", "--"]
# Useful when your app spawns children that also need to receive signals

tini vs dumb-init: Both do the same job. tini is included in Docker Engine and is the more widely recommended option. dumb-init is simpler (single Go binary). Choose based on what's already in your base image.

Docker STOPSIGNAL

By default, docker stop / kubectl delete pod sends SIGTERM to PID 1. You can override this:

# Send SIGQUIT instead of SIGTERM for graceful nginx shutdown
# nginx uses SIGQUIT for graceful drain (finish in-flight requests)
# SIGTERM to nginx causes immediate close without draining
STOPSIGNAL SIGQUIT

# Common per-app signals:
# nginx:        SIGQUIT (graceful), SIGTERM (fast stop)
# gunicorn:     SIGTERM (graceful), SIGQUIT (graceful with timeout)
# unicorn:      SIGUSR1 (reopen logs), SIGWINCH (graceful worker stop)
# PostgreSQL:   SIGTERM (smart shutdown), SIGINT (fast shutdown)

# Override in Kubernetes pod spec (takes precedence over Dockerfile STOPSIGNAL)
spec:
  containers:
    - name: nginx
      lifecycle:
        preStop:
          exec:
            command: ["/bin/sh", "-c", "nginx -s quit; while killall -0 nginx; do sleep 1; done"]
      # SIGQUIT will be sent after preStop completes

Verifying graceful shutdown works

# 1. Check what PID 1 is in your container
kubectl exec <pod> -- ps -p 1
# Should NOT be "sh" or "bash"

# 2. Check signal handlers registered by the process
kubectl exec <pod> -- cat /proc/1/status | grep Sig
# SigCgt (caught signals) should include bit 15 (SIGTERM = bit 14, 0-indexed)
python3 -c "mask=0x...; print([i+1 for i in range(64) if mask&(1<<i)])"

# 3. Test graceful shutdown manually
kubectl exec <pod> -- kill -TERM 1   # send SIGTERM to PID 1
kubectl logs <pod> -f                # watch for graceful shutdown logs
# Should see: "received shutdown signal, draining..." type messages
# Should NOT see: sudden cut-off with no cleanup messages

# 4. Measure actual shutdown time vs terminationGracePeriodSeconds
kubectl get pod <pod> -o json | jq '.spec.terminationGracePeriodSeconds'
# If your app takes 10s to drain, this must be > 10s (recommend 2-3x actual drain time)

Complete graceful shutdown checklist

Dockerfile:
  ✓ CMD in exec form (not shell form)
  ✓ ENTRYPOINT is app or tini/dumb-init (not /bin/sh -c)
  ✓ STOPSIGNAL set if app uses non-SIGTERM signal

Kubernetes:
  ✓ terminationGracePeriodSeconds ≥ preStop duration + actual drain time
  ✓ preStop: sleep 5 (gives kube-proxy time to drain the pod from endpoints)
  ✓ readinessProbe fails fast on shutdown start (pod removed from endpoints faster)

Application:
  ✓ SIGTERM handler registered
  ✓ Handler: stop accepting new requests
  ✓ Handler: wait for in-flight requests to complete
  ✓ Handler: flush buffers (logs, metrics, traces)
  ✓ Handler: close DB connections
  ✓ Handler: exit(0)