Allow to pass rendered template as stdin to post-template command

[ProvoK]

As discussed in #578 with @hairyhenderson , it might be useful a flag to permit passing the rendered template as stdin to the command executed when gomplate finishes succesfully its rendering.

Eg:

gomplate -f my_template.tpl -exec-pipe -- my_stdin_reading_application

Other possible flags names:

• stdin
• command-stdin
• pipe
• ...others?

This would be a great addition since the post-process command is executed only on gomplate success, and will avoid (sometimes) unnecessary writes to file.

⚠️ This features may not be needed if gotemplate could generate output only when it's sure the template could be rendered (alias buffering the output till complete rendering). In that case, a simple shell pipe could do the work.

EDIT: changed stdin to exec-pipe

[charles-dyfis-net]

@ProvoK, ...so, is what you're looking for akin to the following?

maybe_pipe() {
  local -a cmd=( )
  local output
  while [[ $1 != -- ]]; do
    cmd+=( "$1" )
    shift
  done
  [[ $1 = -- ]] && shift
  output=$( "${cmd[@]}" ) && [[ $output ]] && "$@" <<<"$output"
}

maybe_pipe gomplate -f my-template.tpl -stdin -- my_stdin_reading_application

[hairyhenderson]

Possible flags names:

• stdin
• command-stdin
• pipe
• ...others?

How about --exec-pipe? As in "pipe the output to the post-run exec command"... 🤔

In that case, a simple shell pipe could do the work.

TBH I think this feature is still useful - there are users who don't run gomplate in an environment where any shells are installed (such as in a FROM scratch Docker container).

[ProvoK]

@hairyhenderson

How about --exec-pipe? As in "pipe the output to the post-run exec command"... thinking

Seems fine to me :D

TBH I think this feature is still useful - there are users who don't run gomplate in an environment where any shells are installed (such as in a FROM scratch Docker container).

Good call 👍

[ProvoK]

@ProvoK, ...so, is what you're looking for akin to the following?

maybe_pipe() {
  local -a cmd=( )
  local output
  while [[ $1 != -- ]]; do
    cmd+=( "$1" )
    shift
  done
  [[ $1 = -- ]] && shift
  output=$( "${cmd[@]}" ) && [[ $output ]] && "$@" <<<"$output"
}

maybe_pipe gomplate -f my-template.tpl -stdin -- my_stdin_reading_application

@charles-dyfis-net
Seems a bit complicated but it's should be equivalent to
$ rendered_template=$(gomplate -f template.tpl) && echo "$rendered_template" | my_binary
#578 (comment)

The point is.
It's very useful that gomplate can call a command on success: less useful (imho, for use cases such as the one in the comment above) that for consuming gomplate output you have to save it to a file (or use shell tricks).

[charles-dyfis-net]

I believe ifne from moreutils would do the same thing, but more efficiently than any of the above-proposed shell approaches (lauching binary after the first byte of stdin input is seen, rather than collecting all of gomplate's output):

gomplate -f my-template.tpl -stdin | ifne my_binary

[hairyhenderson]

@charles-dyfis-net ifne doesn't really help, since it still outputs when the return code is non-zero