It seems like you're trying to set the AWS region and retrieve the latest Linux AMI using a bash script, but you're encountering an error with the substring expression. The error message `-bash: -1: substring expression

Question

It seems like you're trying to set the AWS region and retrieve the latest Linux AMI using a bash script, but you're encountering an error with the substring expression. The error message `-bash: -1: substring expression < 0` is indicating that the substring expression `${AZ::-1}` is not working as expected.

The `${AZ::-1}` is a bash substring expression that is supposed to remove the last character from the variable `AZ`. The error message suggests that the `AZ` variable is empty or not set properly.

Here's a step-by-step guide to debug and fix the issue:

1. Check if the `AZ` variable is set properly. You can do this by echoing the `AZ` variable after it's set:

```bash
AZ=`curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone`
echo $AZ
```

2. If the `AZ` variable is empty, there might be an issue with the curl command. Make sure that the instance has the necessary permissions to access the metadata URL and the URL is correct.

3. If the `AZ` variable is not empty, but the substring expression is still not working, you can try an alternative way to remove the last character from the `AZ` variable:

```bash
AZ=`curl -s http://169.254.169.254/latest/meta-data/placement/availability-zone`
AZ=${AZ%?}
```

4. After fixing the `AZ` variable, you can set the `AWS_DEFAULT_REGION` variable and retrieve the latest Linux AMI:

```bash
export AWS_DEFAULT_REGION=$AZ
AMI=$(aws ssm get-parameters --names /aws/service/ami-amazon-linux-latest/amzn2-ami-hvm-x86_64-gp2 --query 'Parameters[0].[Value]' --output text)
echo $AMI
```

5. If you're still encountering issues, make sure that the AWS CLI is installed and configured properly, and the instance has the necessary permissions to call the `ssm get-parameters` command.

Knowee AI · Accepted Answer