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@nilium nilium/main.go
Last active Jun 14, 2019

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Example of parsing environment variables and CLI flags using the flag package
package main
import (
"flag"
"log"
"os"
"strings"
"unicode"
)
func main() {
// f communicates program name and default log output target (stderr in this case).
f := flag.NewFlagSet("buildit", flag.ContinueOnError)
// Bind flags for configuration
id := f.Int("backend-id", 100, "Backend agent `ID`.")
// Parse CLI flags
if err := f.Parse(os.Args[1:]); err == flag.ErrHelp {
os.Exit(2)
} else if err != nil {
log.Printf("Error parsing CLI config: %v", err)
os.Exit(1)
}
// Load environment variables for unset CLI flags
if err := LoadEnvironmentConfig(f, os.LookupEnv); err != nil {
log.Printf("Error parsing environment config: %v", err)
os.Exit(1)
}
log.Printf("ID = %v", *id) // DEBUG
}
// EnvFunc is an os.LookupEnv-compatible function type.
// An implementation looks up a value by name and return both the value (if any) and whether the
// name was known.
type EnvFunc func(name string) (value string, ok bool)
// LoadEnvironmentConfig assigns environment variables as values for unset flags defined in f.
// Flags are set in lexical order.
//
// If lookup is nil, os.LookupEnv is used instead.
//
// If any errors occur in setting a flag, the first error is returned.
func LoadEnvironmentConfig(f *flag.FlagSet, lookup EnvFunc) error {
if lookup == nil {
lookup = os.LookupEnv
}
known := map[string]struct{}{}
f.Visit(func(v *flag.Flag) {
known[v.Name] = struct{}{}
})
var err error
prefix := f.Name() + "_"
f.VisitAll(func(v *flag.Flag) {
if _, ok := known[v.Name]; ok {
return
}
name := envVarName(prefix + v.Name)
val, ok := lookup(name)
if !ok {
return
}
// Take only the first error
if ferr := f.Set(v.Name, val); err == nil && ferr != nil {
err = ferr
}
})
return err
}
// envVarName accepts a string and returns an enviornment variable name of the form NAME_FOO.
//
// The resulting name does not contain leading underscores or numbers. Repeated underscores are
// trimmed. The name may end in an underscore.
//
// Characters that are not in the range [A-Za-z0-9_] are replaced with underscores.
func envVarName(name string) string {
var last rune = -1
return strings.Map(func(r rune) rune {
r = unicode.ToUpper(r)
alpha := r >= 'A' && r <= 'Z'
numeric := r >= '0' && r <= '9'
if !(alpha || numeric) {
r = '_'
}
if (last == -1 && !alpha) || (last == '_' && r == '_') {
return -1
}
last = r
return r
}, name)
}
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