Ruby, Internet, and Programming

To write outputs to a file in a shell, we can use redirection. However, when we read a file and modify the content and redirect to the same original file, we'll get just an empty file. What's going on here? $ cat file bravo delta charlie alpha charlie $ cat file | sort | uniq > file $ cat file # empty file $ Of course, if you redirect to a different file and rename it to the original name, it works as expected.

Today I learned that in docker-compose.yml, a variable interpolation with a default can be expressed as ${VAR1:-default value} like shell-style parameter expansion. With parameter expansion, you can do more than setting a default value such as modifying the parameter. However, the supported features in docker-compose.yml are limited to two of them: to set a default variable or to make the parameter mandatory. Let's define VAR_ALPHA with a default value for the example service.

Sometimes I want to make multiple outputs from multiple processes into one single output. Let's say we have to aggregate K8s pod logs from a deployment. We could do that by redirecting each output to a file and follow the file by tail -f like below. tempfile=$(mktemp /tmp/rip-example.XXXXXXXXXX) trap "rm \"$tempfile\"" EXIT pods=$(kubectl get pods | grep Running | grep -oe "app-[a-z0-9\-]\+") for pod in $pods do kubectl logs -f "$pod" >> "$tempfile" & done tail -f "$tempfile" However, that method consumes disk spaces.