Parallel execution patterns in Bash Scripting - Step-by-Step Execution

Concept Flow - Parallel execution patterns

Start script

↓

Launch commands in background (&)

↓

Commands run in parallel

↓

Wait for all to finish (wait)

↓

Continue with next steps

↓

Script ends

The script starts, launches multiple commands in the background to run at the same time, waits for all to finish, then continues.

Execution Sample

Bash Scripting

echo "Start"
sleep 2 &
sleep 3 &
wait
echo "Done"

Runs two sleep commands in parallel, waits for both to finish, then prints 'Done'.

Execution Table

Step	Action	Command	Background?	Output	Notes
1	Print start message	echo "Start"	No	Start	Prints immediately
2	Launch first sleep	sleep 2	Yes		Runs in background
3	Launch second sleep	sleep 3	Yes		Runs in background
4	Wait for all background jobs	wait	No		Pauses until both sleeps finish
5	Print done message	echo "Done"	No	Done	Runs after both sleeps finish

💡 All background jobs complete, wait ends, script continues

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Background jobs count	0	1	2	0	0

Key Moments - 2 Insights

Why does the script print 'Done' only after both sleep commands finish?

Because the 'wait' command pauses the script until all background jobs complete, as shown in step 4 of the execution_table.

What happens if we omit the '&' after the sleep commands?

Without '&', the commands run one after another, not in parallel, so the script waits for each sleep to finish before starting the next, unlike steps 2 and 3 where '&' runs them in background.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the script wait for background jobs to finish?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Check the 'Action' and 'Command' columns in execution_table row 4.

According to variable_tracker, how many background jobs are running after step 3?

A 0

B 1

C 2

D 3

💡 Hint

Look at 'Background jobs count' after Step 3 in variable_tracker.

If we remove the 'wait' command, what will happen to the output order?

A 'Done' prints before sleeps finish

B 'Done' prints after sleeps finish

C Script will hang forever

D Script will print nothing

💡 Hint

Refer to the role of 'wait' in execution_table step 4.

Concept Snapshot

Parallel execution in bash uses '&' to run commands in background.
Use 'wait' to pause script until all background jobs finish.
Without '&', commands run one by one.
This pattern speeds up scripts by running tasks simultaneously.

Full Transcript

This example shows how to run commands in parallel in bash. First, the script prints 'Start'. Then it launches two sleep commands with '&' so they run at the same time in the background. The script uses 'wait' to pause until both sleep commands finish. After that, it prints 'Done'. The variable tracker shows how background jobs increase when commands start and reset after wait. Key points are that '&' runs commands in parallel and 'wait' pauses until all finish. Without '&', commands run sequentially. Without 'wait', the script prints 'Done' immediately without waiting. This pattern helps run tasks faster by doing them at the same time.