gomplate

$ # at its most basic, gomplate can be used with environment variables...
$ echo 'Hello, {{ .Env.USER }}' | gomplate
Hello, hairyhenderson

$ # but that's kind of boring. gomplate has tons of functions to do useful stuff, too
$ gomplate -i 'the answer is: {{ mul 6 7 }}'
the answer is: 42

$ # and, since gomplate uses Go's templating syntax, you can do fun things like:
$ gomplate -i '{{ range seq 5 1 }}{{ . }} {{ if eq . 1 }}{{ "blastoff" | toUpper }}{{ end }}{{ end }}'
5 4 3 2 1 BLASTOFF

$ # the real fun comes when you use datasources!
$ cat ./config.yaml
foo:
  bar:
    baz: qux
$ gomplate -d config=./config.yaml -i 'the value we want is: {{ (datasource "config").foo.bar.baz }}'
the value we want is: qux

$ # datasources are defined by URLs, and gomplate is not limited to just file-based datasources:
$ gomplate -d ip=https://ipinfo.io -i 'country code: {{ (ds "ip").country }}'
country code: CA

$ # standard input can be used as a datasource too:
$ echo '{"cities":["London", "Johannesburg", "Windhoek"]}' | gomplate -d city=stdin://in.json -i '{{ range (ds "city").cities }}{{.}}, {{end}}'
London, Johannesburg, Windhoek, 

$ # and here's something a little more complicated:
$ export CITIES='city: [London, Johannesburg, Windhoek]'
$ cat in.tmpl
{{ range $i, $city := (ds "cities").city -}}
{{ add 1 $i }}: {{ include "weather" (print $city "?0") }}
{{ end }}
$ gomplate -d 'cities=env://CITIES?type=application/yaml' -d 'weather=https://wttr.in/?0' -H 'weather=User-Agent: curl' -f in.tmpl
1: Weather report: London

    \  /       Partly cloudy
  _ /"".-.     4-7 °C
    \_(   ).   ↑ 20 km/h
    /(___(__)  10 km
               0.0 mm

2: Weather report: Johannesburg

    \  /       Partly cloudy
  _ /"".-.     15 °C
    \_(   ).   ↘ 0 km/h
    /(___(__)  10 km
               2.2 mm

3: Weather report: Windhoek

    \  /       Partly cloudy
  _ /"".-.     20 °C
    \_(   ).   ↑ 6 km/h
    /(___(__)  20 km
               0.0 mm