Parallel execution patterns in Bash Scripting

Introduction

Running tasks at the same time saves time and makes your scripts faster.

You want to download multiple files at once.

You need to run several commands that do not depend on each other.

You want to speed up processing by using multiple CPU cores.

You want to start background jobs and continue working without waiting.

You want to run tests or scripts in parallel to save time.

Syntax

Bash Scripting

command1 &
command2 &
wait

The ampersand (&) runs the command in the background.

The wait command pauses the script until all background jobs finish.

Examples

Runs two sleep commands at the same time, then waits for both to finish before printing.

Bash Scripting

sleep 3 &
sleep 5 &
wait
echo "All done"

Downloads two files in parallel, then waits for both downloads to complete.

Bash Scripting

curl http://example.com/file1 &
curl http://example.com/file2 &
wait

Starts three tasks in parallel with random sleep times, then waits for all to finish.

Bash Scripting

for i in {1..3}; do
  echo "Task $i started"
  sleep $((RANDOM % 3 + 1)) &
done
wait
echo "All tasks finished"

Sample Program

This script starts three sleep commands at the same time. It waits until all finish, then prints a message.

Bash Scripting

#!/bin/bash

echo "Starting tasks..."

sleep 2 &
sleep 4 &
sleep 3 &

wait
echo "All tasks completed!"

Important Notes

Use & to run commands in the background so your script can do other things.

Use wait to pause until all background tasks finish before moving on.

Be careful with too many parallel jobs; it can slow your system down.

Summary

Parallel execution runs multiple commands at the same time.

Use & to start background jobs and wait to pause until they finish.

This helps save time when tasks do not depend on each other.

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