Reading one-line lists with the Bash shell

Commands like the AWS CLI may return a list of values all in one line, where each item in the list is separated by the nearby items with spaces. Using a plain read command doesn’t really work: read will read all the values in one go into the variable. You need to change the delimiter that read uses to split the input. No need to pipe the output through Perl or other tools, read got you covered with the -d option.

In this example I get the list of the ARNs of all target groups in an AWS account, and then iterate over those ARNs to list all the instances in each target group. The ouput will also be saved into a file through the tee command:

aws elbv2 describe-target-groups \
  --query 'TargetGroups[].TargetGroupArn' \
  --output text | \
  while read -d ' ' ARN ; do \
    echo -n "$ARN: " ; \
    aws elbv2 describe-target-health \
      --target-group-arn "$ARN" \
      --query 'TargetHealthDescriptions[].Target.Id' \
      --output text ; sleep 1 ; \
  done | \
  tee tg-instances.txt

The ouput of this one liner will be in the format:

ARN: instance_ID [instance_ID...]

Things to notice:

  • the AWS CLI’s describe-target-groups command will list all target groups’ ARNs thanks to the --query option and list as many as possible on single lines, according to the shell’s output buffer capacity; the ouput is piped through a while loop;
  • the while loop uses read -d ' ' to split each line at spaces and save each item in the $ARN variable, one per cycle;
  • the echo command prints the value of $ARN followed by a colon, a space, but will not output a newline sequence due to the -n option;
  • the AWS CLI’s describe-target-health command will list all target IDs thanks to the --query option and print them out in a single line; it will also provide a newline sequence, so that the next loop will start on a new line;
  • the sleep 1 command slows down the loop, so that we don’t hammer the API to the point that they will rate limit us;
  • finally, the tee command will duplicate the output of the while loop to both the standard output and the file tg-instances.txt.

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