Advanced¶

Deep dive into advanced bash concepts: job control, subshells, signals, and performance optimization.

Topics¶

Job Control ¶

Managing background processes, nohup, disown, and running jobs after terminal close.

Subshells ¶

Understanding subshells, command substitution, process substitution, and their implications for variable scope.

Signals ¶

Unix signals, the trap command, handling interrupts, and graceful shutdown patterns.

Performance ¶

Optimizing bash scripts: avoiding common performance pitfalls, when to use external tools, and profiling techniques.

Prerequisites¶

This section assumes familiarity with:

Basic scripting (variables, conditionals, loops)
Functions and scope
Process concepts (PIDs, exit codes)
Basic I/O redirection

Why These Topics Matter¶

Job Control¶

Essential for:

Running long processes without tying up the terminal
Managing multiple concurrent tasks
Keeping processes alive after SSH disconnect
Development workflows with background servers

Subshells¶

Understanding subshells prevents bugs related to:

Variables not persisting after pipes
Unexpected process isolation
Performance from unnecessary forks
Proper use of $() and ()

Signals¶

Critical for:

Graceful shutdown handling
Cleanup on script interruption
Inter-process communication
Robust production scripts

Performance¶

Important for:

Scripts that process large files
Avoiding unnecessary external commands
Understanding when bash is (not) the right tool
Production script optimization

Key Concepts Preview¶

Job Control¶

# Run in background
./server.sh &

# Keep running after logout
nohup ./server.sh &
disown

# Wait for background jobs
wait

Subshells¶

# Subshell - variables don't persist
(cd /tmp && pwd)  # Back to original after

# Command substitution runs in subshell
output=$(some_command)

# Pipe creates subshell
echo "data" | while read line; do
    var="$line"  # Won't persist!
done

Signals¶

# Cleanup on exit
trap 'rm -f $tmpfile' EXIT

# Handle Ctrl+C
trap 'echo "Interrupted"; exit 1' INT

# Ignore signal
trap '' TERM

Performance¶

# Slow - external command in loop
for i in {1..1000}; do
    result=$(echo "$i * 2" | bc)
done

# Fast - bash arithmetic
for i in {1..1000}; do
    ((result = i * 2))
done

Common Patterns¶

These topics combine into powerful patterns:

Daemon Script¶

#!/usr/bin/env bash

cleanup() {
    echo "Shutting down..."
    kill $worker_pid 2>/dev/null
    rm -f "$pidfile"
}

trap cleanup EXIT INT TERM

pidfile="/var/run/mydaemon.pid"
echo $$ > "$pidfile"

while true; do
    do_work &
    worker_pid=$!
    wait $worker_pid
    sleep 60
done

Parallel Processing¶

#!/usr/bin/env bash

max_jobs=4
for file in *.txt; do
    while (( $(jobs -r -p | wc -l) >= max_jobs )); do
        sleep 0.1
    done
    process "$file" &
done
wait

Graceful Timeout¶

#!/usr/bin/env bash

timeout=30
( sleep $timeout; kill $$ 2>/dev/null ) &
timer_pid=$!

trap "kill $timer_pid 2>/dev/null" EXIT

# Long running work here
do_work

# Completed before timeout
kill $timer_pid 2>/dev/null

When to Use What¶

Situation	Approach
Long-running task	Background job + `disown`
Need output later	`nohup` with output file
Cleanup required	`trap EXIT`
Handle Ctrl+C	`trap INT`
Parallel processing	Background jobs + `wait`
Variable in pipe	Process substitution or here-string
Isolated environment	Subshell `()`