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Accepted
Chris Halcrow
Asked: 2020-11-25 20:26:32 +0800 CST

What's is the purpose of a trailing '-' in a Kubernetes apply -f -

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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 -

linux kubernetes flags options

2 Answers

  1. 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.

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  2. Best Answer

    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.

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