🌀 Extended Subprocesses and Subshells

Understanding process creation, subshells, and job control is essential for writing efficient shell scripts.

🧭 Process Creation Basics

When you run a command, the shell:

1. Forks — Creates a copy of itself (child process)
2. Execs — Replaces child's memory with the command
3. Waits — Parent waits for child to complete (unless backgrounded)

Parent (bash)
  ├── fork()
  │     └── Child (bash)
  │           └── exec(ls) → ls process
  └── wait()

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🧪 Subshells

A subshell is a child process that inherits the parent's environment but runs independently.

Creating Subshells

# Method 1: Parentheses
(
    cd /tmp
    pwd    # Shows /tmp
)
pwd        # Shows original directory

# Method 2: Pipeline (each stage runs in subshell)
echo "hello" | tr 'a-z' 'A-Z'   # Both echo and tr run in subshells

# Method 3: Command substitution
result=$(date)   # date runs in subshell

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🧠 Subshell vs Background Job

Aspect	Subshell (...)	Background &
Runs concurrently	✅	✅
Inherits vars	✅	✅
Modifies parent	❌	❌
Waits for finish	✅ (implicit)	❌ (must use wait)
Can be piped	✅	❌

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🧪 Grouping Commands

Use subshells to group operations:

(
    cd /project
    git pull
    make build
    make test
) > build.log 2>&1

All output redirected together. Changes inside don't affect parent.

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🧠 Background Jobs

Launch tasks in background:

long_task &
PID=$!
echo "Started task with PID $PID"

# Do other work
sleep 5

# Wait for completion
wait $PID
echo "Task completed"

List background jobs:

jobs -l
# Output:
# [1]+ 12345 Running    long_task &

Bring to foreground:

fg %1    # Bring job 1 to foreground

Send signal to background job:

kill %1   # Send TERM to job 1

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🧪 Process Substitution (Advanced)

Treat command output as a file:

diff <(sort file1.txt) <(sort file2.txt)

Works like:

diff /dev/fd/63 /dev/fd/62   # Where fd/63 and fd/62 are pipes

Also for input:

while IFS= read -r line; do
    echo "Processing: $line"
done < <(find . -name "*.log")

⚠️ Bash/Zsh feature — not POSIX.

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🧠 Managing Parallel Jobs

Limit concurrent background jobs:

max_jobs=4
current_jobs=0

for file in *.dat; do
    process_file "$file" &
    current_jobs=$((current_jobs + 1))

    if [ $current_jobs -ge $max_jobs ]; then
        wait   # Wait for all background jobs
        current_jobs=0
    fi
done

wait   # Final wait for remaining jobs

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🧪 Daemonizing a Process

Convert script to background daemon:

daemonize() {
    # Fork first time
    (
        # Fork second time (double-fork pattern)
        (
            # Close stdin, stdout, stderr
            exec 0</dev/null
            exec 1>/dev/null
            exec 2>/dev/null

            # Change to root directory
            cd /

            # Run actual daemon logic
            while true; do
                do_work
                sleep 60
            done
        ) &
        disown   # Detach from parent shell
    )
}

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🧾 Portability Notes

Feature	POSIX	Bash	Zsh
Subshells (...)	✅	✅	✅
Background &	✅	✅	✅
$! (last PID)	✅	✅	✅
jobs, fg, bg	✅	✅	✅
Process substitution	❌	✅	✅
disown	❌	✅	✅

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🧾 Summary

• Subshells isolate side effects — use (...) for grouping.
• Background jobs (&) enable concurrency — manage with wait.
• Process substitution is powerful but not portable.
• Use double-fork pattern for proper daemonization.
• Limit parallel jobs to avoid resource exhaustion.

👉 Continue to: Pipelines and Process Substitution