Within an AWS CLI command, add the ISO 8601 datetime

Question

As a fun side project, I'm building a serverless Todo application on AWS. I do a lot in the terminal, but my knowledge is basic.

The command to add something into my DynamoDB table via the AWS CLI (v2.3.4) is this:

aws dynamodb put-item \
    --table-name tasks \
    --item \
        '{"task_id": {"S": "3495353e-726f-4e0e-b290-8014c03be971"}, "user_id": {"S": "aae30f8e-aabe-4e38-918f-0f5a2223f589"}, "created_at": {"S": "2022-09-09T12:51:05Z"}, "content": {"S": "Clean car"}, "is_done": {"BOOL": false}}' \
    --profile personal

Notice that for created_at I'm manually typing in the ISO-8601 date as a string.

Now I know that on linux, in order to get the UTC datetime in the ISO-8601 format I need to run:

date -u +"%Y-%m-%dT%H:%M:%SZ"

My question is, how do I fit that into my DynamoDB put-item command so that I automatically/dynamically get the created_at from my linux system.

What I have tried:

I tried to simply plunk the date command into my DynamoDB command where the created_at value would go like this:

aws dynamodb put-item \
    --table-name tasks \
    --item \
        '{"task_id": {"S": "3495353e-726f-4e0e-b290-8014c03be971"}, "user_id": {"S": "aae30f8e-aabe-4e38-918f-0f5a2223f589"}, "created_at": {"S": date -u +"%Y-%m-%dT%H:%M:%SZ"}, "content": {"S": "Clean car"}, "is_done": {"BOOL": false}}' \
    --profile personal

But that doesn't work. The command errors out and it returns with:

Error parsing parameter '--item': Invalid JSON: Expecting value: line 1 column 138 (char 137)
JSON received: {"task_id": {"S": "3495353e-726f-4e0e-b290-8014c03be971"}, "user_id": {"S": "aae30f8e-aabe-4e38-918f-0f5a2223f589"}, "created_at": {"S": date -u +"%Y-%m-%dT%H:%M:%SZ"}, "content": {"S": "Clean car"}, "is_done": {"BOOL": false}}

Update:

I just tried the $(command) method as suggested by Marcus, and I still get the invalid JSON error.

aws dynamodb put-item \
    --table-name tasks \
    --item \
        '{"task_id": {"S": "3495353e-726f-4e0e-b290-8014c03be971"}, "user_id": {"S": "aae30f8e-aabe-4e38-918f-0f5a2223f589"}, "created_at": {"S": "$(date -u +"%Y-%m-%dT%H:%M:%SZ")"}, "content": {"S": "Clean car"}, "is_done": {"BOOL": false}}' \
    --profile personal

Answer 1

You could use jq to insert the date into the json object:

aws dynamodb put-item \
    --table-name tasks \
    --item "$(
      jq -c '.created_at.S=(now|todate)' << 'EOF'
        {
          "task_id": {
            "S": "3495353e-726f-4e0e-b290-8014c03be971"
          },
          "user_id": {
            "S": "aae30f8e-aabe-4e38-918f-0f5a2223f589"
          },
          "created_at": {"S":""},
          "content": {
            "S": "Clean car"
          },
          "is_done": {
            "BOOL": false
          }
        }
EOF
     )" --profile personal

Also note that you can format dates in zsh without having to invoke date:

${(%):-%D{%FT%TZ}} expands to the current time in that format, though you'd have to set TZ=UTC0 to get actual Zulu time. There's also a strftime builtin in the zsh/datetime module.

Answer 2

As the error message highlights, the JSON is invalid.

So, as to be expected, the command you wanted to be executed never was – how was the shell supposed to know which parts of the command line you wanted to pass unaltered, and which one you want executed?

If you want to substitute a command's output, use $(command), e.g., $(date -u +"%Y-%m-%dT%H:%M:%SZ")