Parallel execution patterns in Bash Scripting - Mini Project: Build & Apply

Parallel execution patterns

📖 Scenario: You are managing a server where you need to run multiple tasks at the same time to save time. Running tasks one after another takes too long. You want to learn how to run commands in parallel using Bash scripting.

🎯 Goal: Build a Bash script that runs three simple commands in parallel and waits for all of them to finish before showing a message.

📋 What You'll Learn

Create three simple commands that each sleep for a few seconds

Run these commands in parallel using background execution

Use a variable to count how many commands are running

Wait for all commands to finish before printing a completion message

💡 Why This Matters

🌍 Real World

Running multiple tasks at the same time saves time on servers and automation scripts. For example, downloading files or processing data in parallel.

💼 Career

System administrators and DevOps engineers often use parallel execution in Bash scripts to improve efficiency and manage multiple processes.

1

Create three simple commands

Create three variables called cmd1, cmd2, and cmd3. Set cmd1 to sleep 3, cmd2 to sleep 4, and cmd3 to sleep 2.

Bash Scripting

# Your code here

Hint

Use simple assignment like cmd1="sleep 3" to store commands as strings.

2

Add a counter for running commands

Create a variable called running and set it to 0. This will count how many commands are running in parallel.

Bash Scripting

cmd1="sleep 3"
cmd2="sleep 4"
cmd3="sleep 2"
# Your code here

Hint

Initialize the counter with running=0.

3

Run commands in parallel and update counter

Use $cmd1 &, $cmd2 &, and $cmd3 & to run the commands in the background. After each command, increase running by 1 using running=$((running + 1)).

Bash Scripting

cmd1="sleep 3"
cmd2="sleep 4"
cmd3="sleep 2"
running=0
# Your code here

Hint

Run each command with & at the end to run in background. Increase running after each.

4

Wait for all commands and print completion message

Use the wait command to wait for all background commands to finish. Then print echo "All $running commands finished." to show the message.

Bash Scripting

cmd1="sleep 3"
cmd2="sleep 4"
cmd3="sleep 2"
running=0

$cmd1 &
running=$((running + 1))

$cmd2 &
running=$((running + 1))

$cmd3 &
running=$((running + 1))
# Your code here

Hint

Use wait to pause until all background jobs finish, then print the message.

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