Browsing to your AWS account console can be a bit of a tedious task when you just want to examine service logs. Even when using Firefox containers to ease the headache of dealing with multiple accounts it just takes a portion of time. This gets even more cumbersome when you factor in the fact that Lambda functions create log streams for each cluster of log events over time.
AWS CLI Tail
AWS CLI tail function to the rescue! You can view the full documentation here.
AWS tail function works similar to the familiar Bash tail command (in terms of output functionality at least, complex underlying mechanics aside).
All it needs to function is a log group name (and valid AWS credentials configured of course 😉).
$ aws logs tail /aws/lambda/MyLambdaFunction
2023-04-23T16:17:49.224000+00:00 2023/04/23/[$LATEST]6823287090b946d394dbdc99b48ea434 START RequestId:
...
...
You will notice that all the time windowed log streams are aggregated using this function, compared to the console dashboard you will have to either create a log query or navigate in and out of different log streams to find your treasure. This is fine if you have done it a few times and are comfortable with clicking through the dashboard to achieve this but it can feel counter intuitive to well known basics when doing similar hunting with logical log files on a host.
Follow Logs
The above is just a time bound subset of the latest logs. Let's say we are testing some integrations between development servers or using our Lambda function as the backend for an API. For example we may have some frontend making requests to the API.
We can add the --follow argument to watch all logs as they are streamed to
CloudWatch from the Lambda function.
aws logs tail /aws/lambda/MyLambdaFunction --follow
If you find the log output a bit too verbose (it will contain the name of the
log stream which can be lengthy) we can reduce the verbosity slightly by adding
the --format short argument.
aws logs tail /aws/lambda/MyLambdaFunction --follow --format short
If we want to filter particular log events we can also use the
--filter-pattern argument. Let's say we know an exception has occurred and we
want to drill into that specific event.
aws logs tail /aws/lambda/MyLambdaFunction --follow --format short \
--filter-pattern "Exception"
We can compound this further by looking further back into time if required. We
can use the --since argument with appropriate value. For example, if we want
to see all logs for the current day.
aws logs tail /aws/lambda/MyLambdaFunction --follow --format short --since "1d"
As per AWS documentation:
By default, logs will be displayed starting from ten minutes in the past
Or from the past 2 hours.
aws logs tail /aws/lambda/MyLambdaFunction --follow --format short --since "2h"
Advanced Filtering
More advanced filtering can be applied to --filter-pattern using the filter
API.
For example, if you only want to view [ERROR] events.
aws logs tail "/aws/lambda/MyLambdaFunction" --filter-pattern "ERROR"
Happy logging! 🙂