document and remove artifactID

post-processor/checksum/post-processor.go

@@ -31,10 +31,9 @@ type PostProcessor struct {
 }
 
 type outputPathTemplate struct {
-	BuildName   string
-	BuilderType string
-	ArtifactID  string
-	HashType    string
+	BuildName    string
+	BuilderType  string
+	ChecksumType string
 }
 
 func getHash(t string) hash.Hash {
@@ -80,7 +79,7 @@ func (p *PostProcessor) Configure(raws ...interface{}) error {
 	}
 
 	if p.config.OutputPath == "" {
-		p.config.OutputPath = "packer_{{.BuildName}}_{{.BuilderType}}_{{.HashType}}.checksum"
+		p.config.OutputPath = "packer_{{.BuildName}}_{{.BuilderType}}_{{.ChecksumType}}.checksum"
 	}
 
 	if err = interpolate.Validate(p.config.OutputPath, &p.config.ctx); err != nil {
@@ -103,12 +102,11 @@ func (p *PostProcessor) PostProcess(ui packer.Ui, artifact packer.Artifact) (pac
 	opTpl := &outputPathTemplate{
 		BuildName:   p.config.PackerBuildName,
 		BuilderType: p.config.PackerBuilderType,
-		ArtifactID:  artifact.Id(),
 	}
 
 	for _, ct := range p.config.ChecksumTypes {
 		h = getHash(ct)
-		opTpl.HashType = ct
+		opTpl.ChecksumType = ct
 		p.config.ctx.Data = &opTpl
 
 		for _, art := range files {

website/source/docs/post-processors/checksum.html.md

@@ -44,4 +44,13 @@ Optional parameters:
 
 - `checksum_types` (array of strings) - An array of strings of checksum types
 to compute. Allowed values are md5, sha1, sha224, sha256, sha384, sha512.
-- `output` (string) - Specify filename to store checksums.
+- `output` (string) - Specify filename to store checksums. This defaults to
+    `packer_{{.BuildName}}_{{.BuilderType}}_{{.ChecksumType}}.checksum`. For
+    example, if you had a builder named `database`, you might see the file
+    written as `packer_database_docker_md5.checksum`. The following variables are
+    available to use in the output template:
+
+    * `BuildName`: The name of the builder that produced the artifact.
+    * `BuilderType`: The type of builder used to produce the artifact.
+    * `ChecksumType`: The type of checksums the file contains. This should be
+        used if you have more than one value in `checksum_types`.