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.

Practice

1. What does adding & at the end of a command in a bash script do?

easy

A. Runs the command in the background, allowing the script to continue immediately

B. Stops the script until the command finishes

C. Redirects the command output to a file

D. Runs the command with higher priority

Solution

1. Step 1: Understand the role of & in bash

   The & symbol placed at the end of a command tells bash to run that command in the background.

2. Step 2: Effect on script execution

   Running a command in the background means the script does not wait for it to finish and continues to the next command immediately.

3. Final Answer:

   Runs the command in the background, allowing the script to continue immediately -> Option A

4. Quick Check:

   Background execution = & at end [OK]

Hint: Remember & means run in background, no waiting [OK]

Common Mistakes:

• Thinking & pauses the script until command finishes
• Confusing & with output redirection
• Assuming & changes command priority

2. Which of the following is the correct syntax to run two commands in parallel in bash?

easy

A. command1 & command2 &

B. command1 && command2 &

C. command1 && command2

D. command1 & command2

Solution

1. Step 1: Identify how to run commands in parallel

   Appending & to a command runs it in the background. To run two commands in parallel, both must have &.

2. Step 2: Analyze options

   command1 & command2 & runs both commands with &, so both run in background simultaneously. command1 && command2 & uses && which means run second only if first succeeds, then runs second in background (but first runs in foreground). command1 && command2 uses && without background. command1 & command2 runs command1 in background but command2 in foreground, so the script blocks until command2 finishes.

3. Final Answer:

   command1 & command2 & -> Option A

4. Quick Check:

   Both commands need & to run parallel [OK]

Hint: Use & after each command to run both in parallel [OK]

Common Mistakes:

• Using && which runs commands sequentially
• Missing & on second command
• Confusing & with &&

3. What will be the output order of the following script?

echo "Start"
sleep 2 &
echo "Middle"
sleep 1 &
echo "End"
wait

medium

A. Start\nMiddle\nEnd\n(wait blocks output until sleeps finish)

B. Start\n(wait)\nMiddle\nEnd

C. Start\nMiddle\nEnd (then waits for sleeps to finish)

D. Start\nEnd\nMiddle

Solution

1. Step 1: Understand background jobs and output

   The sleep commands run in background due to &, so they do not block the script. The echo commands run immediately in order.

2. Step 2: Analyze output order

   The script prints "Start", then starts sleep 2 in background, then prints "Middle", starts sleep 1 in background, then prints "End". The wait pauses script exit until sleeps finish but does not affect output order.

3. Final Answer:

   Start\nMiddle\nEnd (then waits for sleeps to finish) -> Option C

4. Quick Check:

   Background jobs don't block echo output [OK]

Hint: Background jobs run silently; echo prints immediately [OK]

Common Mistakes:

• Assuming wait delays echo output
• Thinking sleep blocks echo
• Confusing order of background jobs

4. Identify the error in this script snippet that tries to run two commands in parallel and wait for both:

command1 &
command2
wait

medium

A. command1 & syntax is incorrect

B. wait command is missing arguments

C. Both commands run sequentially, no parallelism

D. command2 is not run in background, so wait only waits for command1

Solution

1. Step 1: Check background execution of commands

   Only command1 has & to run in background. command2 runs in foreground, blocking until it finishes.

2. Step 2: Effect on wait

   The wait command waits for all background jobs to finish. Since only command1 is background, wait waits only for it. command2 runs before wait.

3. Final Answer:

   command2 is not run in background, so wait only waits for command1 -> Option D

4. Quick Check:

   All parallel commands need & before wait [OK]

Hint: Add & to all parallel commands before wait [OK]

Common Mistakes:

• Forgetting & on second command
• Thinking wait needs arguments
• Assuming wait waits for foreground commands

5. You want to run 3 independent scripts script1.sh, script2.sh, and script3.sh in parallel and then print "All done" only after all finish. Which is the best way to do this?

hard

A. ./script1.sh && ./script2.sh && ./script3.sh && echo "All done"

B. ./script1.sh &
./script2.sh &
./script3.sh &
wait
echo "All done"

C. ./script1.sh & ./script2.sh & ./script3.sh & echo "All done"

D. ./script1.sh &
./script2.sh &
./script3.sh
echo "All done"

Solution

1. Step 1: Run all scripts in background

   Adding & after each script runs them in parallel without waiting.

2. Step 2: Use wait to pause until all finish

   The wait command pauses the script until all background jobs complete, ensuring synchronization.

3. Step 3: Print message after all finish

   After wait, echo "All done" runs, confirming all scripts finished.

4. Final Answer:

   ./script1.sh &
   ./script2.sh &
   ./script3.sh &
   wait
   echo "All done"

   -> Option B

5. Quick Check:

   Background + wait + echo after = correct pattern [OK]

Hint: Use & for parallel, then wait before next step [OK]

Common Mistakes:

• Using && runs scripts sequentially
• Missing wait causes echo to run too early
• Putting echo on same line with & causes immediate print