I was passed the following, which does a dry run of the creation of a kubernetes secret from an appsettings.json file, which is then piped to a kubernetes apply. I think that the -f will take the filename of the --dry-run created secret and use that when applying the secret, but I'm wondering if the trailing - has any significance or if it's just a typo?
kubectl create secret generic test --from-file=appsettings.json --dry-run -oyaml | kubectl apply -f -
The - is a parameter to the -f option, which means to accept input from standard input instead of a named file. Hundreds of UNIX/Linux commands have options like this.
The - character can be understood as a placeholder for the output of the command which is piped ( using | character ). By using it, we instruct very specifically the subsequent command ( to which the output is piped ), where the standard output of the first command ( it's execution result ) should be placed, in other words how it should be taken or parsed.
So rather than piping the result of:
kubectl create secret generic test --from-file=appsettings.json --dry-run -oyaml
(which happens to be a yaml manifest)
directly to:
kubectl apply -f
which doesn't know what to do with such input (as it expects a file after -f flag), we indicate very precisely where it fits:
kubectl apply -f -
In this case, we instruct kubectl apply command that the piped output from the previous command should be taken instead of a file, which is expected after providing -f flag.
