CLI scripting basics in AWS - Step-by-Step Execution

Process Flow - CLI scripting basics

Start Script

↓

Write CLI Command

↓

Run Command in Terminal

↓

Command Executes

↓

Output Received

↓

Use Output or Next Command

↓

Repeat or End Script

This flow shows how a CLI script runs commands one by one, gets output, and uses it for next steps or finishes.

Execution Sample

AWS

aws s3 ls
aws s3 mb s3://my-bucket-example
aws s3 cp file.txt s3://my-bucket-example/

This script lists S3 buckets, creates a new bucket, then uploads a file to it.

Process Table

Step	Command	Action	Output	Next Step
1	aws s3 ls	List all S3 buckets	Shows existing buckets	Proceed to create bucket
2	aws s3 mb s3://my-bucket-example	Make new bucket	Bucket created confirmation	Proceed to upload file
3	aws s3 cp file.txt s3://my-bucket-example/	Copy file to bucket	File upload success message	End script

💡 All commands executed successfully, script ends.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
bucket_name	undefined	undefined	my-bucket-example	my-bucket-example
file_uploaded	false	false	false	true

Key Moments - 2 Insights

Why do we run commands one after another instead of all at once?

Commands run step-by-step to ensure each finishes successfully before the next starts, as shown in execution_table rows 1 to 3.

What happens if the bucket already exists when we try to create it?

The create bucket command will fail or show an error, stopping the script or requiring error handling, which is not shown in this simple flow.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after step 1?

A Bucket created confirmation

B Shows existing buckets

C File upload success message

D Error message

💡 Hint

Check the Output column in row 1 of the execution_table.

At which step does the file get uploaded to the bucket?

A Step 1

B Step 2

C Step 3

D No file upload in script

💡 Hint

Look at the Command and Action columns in the execution_table.

If the bucket name changes, which variable in variable_tracker updates?

A bucket_name

B file_uploaded

C Start

D Output

💡 Hint

Refer to the variable_tracker rows and see which variable holds the bucket name.

Concept Snapshot

CLI scripting basics:
- Write commands line by line
- Run commands in order
- Each command outputs result
- Use output for next commands
- Scripts automate cloud tasks

Full Transcript

This lesson shows how CLI scripts run commands one after another. First, a command is written and run in the terminal. The command executes and returns output. The script uses this output or moves to the next command. For example, listing S3 buckets, creating a bucket, then uploading a file. Variables like bucket name and file upload status change as commands run. Running commands step-by-step ensures success before continuing. If errors occur, scripts may stop or need handling. This basic flow helps automate cloud tasks using CLI.

Practice

1. What does the AWS CLI command aws s3 ls do?

easy

A. Deletes all S3 buckets in your AWS account

B. Creates a new S3 bucket

C. Lists all S3 buckets in your AWS account

D. Uploads a file to an S3 bucket

Solution

1. Step 1: Understand the command structure

   The command uses aws s3 which relates to the S3 service, and ls which means list.

2. Step 2: Interpret the command action

   Listing in S3 context means showing all buckets or objects. Without extra parameters, it lists all buckets.

3. Final Answer:

   Lists all S3 buckets in your AWS account -> Option C

4. Quick Check:

   Command aws s3 ls lists buckets [OK]

Hint: Remember: 'ls' means list, so it shows existing resources [OK]

Common Mistakes:

• Confusing 'ls' with 'rm' (delete)
• Thinking it creates resources
• Assuming it uploads files

2. Which of the following is the correct syntax to create an EC2 instance using AWS CLI?

easy

A. aws ec2 run-instances --image-id ami-12345678 --count 1

B. aws ec2 create-instance --image-id ami-12345678 --count 1

C. aws ec2 start-instance --image-id ami-12345678 --count 1

D. aws ec2 launch-instance --image-id ami-12345678 --count 1

Solution

1. Step 1: Identify the correct AWS CLI command for EC2 instance creation

   The official command to create EC2 instances is run-instances.

2. Step 2: Check the syntax correctness

   aws ec2 run-instances --image-id ami-12345678 --count 1 uses aws ec2 run-instances --image-id ami-12345678 --count 1, which is the correct syntax to launch one instance.

3. Final Answer:

   aws ec2 run-instances --image-id ami-12345678 --count 1 -> Option A

4. Quick Check:

   EC2 instance creation uses 'run-instances' command [OK]

Hint: Use 'run-instances' to launch EC2, not 'create-instance' [OK]

Common Mistakes:

• Using 'create-instance' which is invalid
• Confusing 'start-instance' with creation
• Using 'launch-instance' which is not a valid command

3. What will be the output of this AWS CLI command?

aws s3api list-buckets --query 'Buckets[].Name' --output json

medium

A. A JSON array of bucket names

B. A list of bucket creation dates

C. An error because of wrong syntax

D. A plain text list of bucket names

Solution

1. Step 1: Understand the command components

   The command uses list-buckets to get all buckets, with a query to extract only the bucket names.

2. Step 2: Analyze the output format

   The --output json option formats the result as JSON, so the output is a JSON array of bucket names.

3. Final Answer:

   A JSON array of bucket names -> Option A

4. Quick Check:

   Query filters names, output json formats as JSON array [OK]

Hint: Query filters data, output json formats it as JSON [OK]

Common Mistakes:

• Expecting plain text instead of JSON
• Confusing bucket names with creation dates
• Assuming syntax error due to query

4. You run this command to delete an S3 bucket:

aws s3 rb s3://my-bucket

But it fails with an error. What is the most likely cause?

medium

A. You need to use aws s3 rm instead

B. The bucket is not empty

C. The bucket name is invalid

D. The AWS CLI is not installed

Solution

1. Step 1: Understand the command purpose

   aws s3 rb removes (deletes) an S3 bucket, but only if it is empty.

2. Step 2: Identify the common error cause

   If the bucket contains files, the command fails. You must delete all objects first.

3. Final Answer:

   The bucket is not empty -> Option B

4. Quick Check:

   Bucket must be empty before removal [OK]

Hint: Empty bucket before deleting with 'rb' command [OK]

Common Mistakes:

• Trying to delete non-empty bucket directly
• Confusing 'rm' (remove objects) with 'rb' (remove bucket)
• Assuming AWS CLI is not installed without checking

5. You want to write a script that lists all EC2 instances and then stops each one. Which AWS CLI commands should you combine in the script?

hard

A. Use aws ec2 show-instances to list, then aws ec2 power-off-instances to stop

B. Use aws ec2 list-instances to list, then aws ec2 terminate-instances to stop

C. Use aws ec2 get-instances to list, then aws ec2 shutdown-instances to stop

D. Use aws ec2 describe-instances to list, then aws ec2 stop-instances --instance-ids to stop

Solution

1. Step 1: Identify the correct command to list EC2 instances

   The correct command to list instances is describe-instances.

2. Step 2: Identify the correct command to stop instances

   To stop instances, use stop-instances with the instance IDs.

3. Step 3: Combine commands in script logic

   First list instances to get IDs, then stop them using those IDs.

4. Final Answer:

   Use aws ec2 describe-instances to list, then aws ec2 stop-instances --instance-ids to stop -> Option D

5. Quick Check:

   List with describe, stop with stop-instances [OK]

Hint: List with 'describe-instances', stop with 'stop-instances' [OK]

Common Mistakes:

• Using non-existent commands like 'list-instances'
• Confusing 'terminate' with 'stop' (termination deletes)
• Using wrong commands that cause errors