Implement shell output format (#1645)

* fix typo in a comment

* implement shell output format

* fix a typo

* add two test cases, have source uses ascii only

* add integration tests and documentation

* add fixes after code revieew

pkg/yqlib/encoder_shellvariables_test.go

@@ -0,0 +1,93 @@
+package yqlib
+
+import (
+	"bufio"
+	"bytes"
+	"strings"
+	"testing"
+
+	"github.com/mikefarah/yq/v4/test"
+)
+
+func assertEncodesTo(t *testing.T, yaml string, shellvars string) {
+	var output bytes.Buffer
+	writer := bufio.NewWriter(&output)
+
+	var encoder = NewShellVariablesEncoder()
+	inputs, err := readDocuments(strings.NewReader(yaml), "test.yml", 0, NewYamlDecoder(ConfiguredYamlPreferences))
+	if err != nil {
+		panic(err)
+	}
+	node := inputs.Front().Value.(*CandidateNode).Node
+	err = encoder.Encode(writer, node)
+	if err != nil {
+		panic(err)
+	}
+	writer.Flush()
+
+	test.AssertResult(t, shellvars, strings.TrimSuffix(output.String(), "\n"))
+}
+
+func TestShellVariablesEncoderNonquoting(t *testing.T) {
+	assertEncodesTo(t, "a: alice", "a=alice")
+}
+
+func TestShellVariablesEncoderQuoting(t *testing.T) {
+	assertEncodesTo(t, "a: Lewis Carroll", "a='Lewis Carroll'")
+}
+
+func TestShellVariablesEncoderQuotesQuoting(t *testing.T) {
+	assertEncodesTo(t, "a: Lewis Carroll's Alice", "a='Lewis Carroll'\"'\"'s Alice'")
+}
+
+func TestShellVariablesEncoderStripComments(t *testing.T) {
+	assertEncodesTo(t, "a: Alice # comment", "a=Alice")
+}
+
+func TestShellVariablesEncoderMap(t *testing.T) {
+	assertEncodesTo(t, "a:\n b: Lewis\n c: Carroll", "a_b=Lewis\na_c=Carroll")
+}
+
+func TestShellVariablesEncoderArray_Unwrapped(t *testing.T) {
+	assertEncodesTo(t, "a: [{n: Alice}, {n: Bob}]", "a_0_n=Alice\na_1_n=Bob")
+}
+
+func TestShellVariablesEncoderKeyNonPrintable(t *testing.T) {
+	assertEncodesTo(t, `"be\all": ring!`, "bell='ring!'")
+}
+
+func TestShellVariablesEncoderKeyPrintableNonAlphaNumeric(t *testing.T) {
+	assertEncodesTo(t, `"b-e l=l": ring!`, "b_e_l_l='ring!'")
+}
+
+func TestShellVariablesEncoderKeyPrintableNonAscii(t *testing.T) {
+	assertEncodesTo(t, `"b\u00e9ll": ring!`, "bell='ring!'")
+}
+
+func TestShellVariablesEncoderRootKeyStartingWithDigit(t *testing.T) {
+	assertEncodesTo(t, "1a: onea", "_1a=onea")
+}
+
+func TestShellVariablesEncoderRootKeyStartingWithUnderscore(t *testing.T) {
+	assertEncodesTo(t, "_key: value", "_key=value")
+}
+
+func TestShellVariablesEncoderChildStartingWithUnderscore(t *testing.T) {
+	assertEncodesTo(t, "root:\n _child: value", "root__child=value")
+}
+
+func TestShellVariablesEncoderEmptyValue(t *testing.T) {
+	assertEncodesTo(t, "empty:", "empty=")
+}
+
+func TestShellVariablesEncoderEmptyArray(t *testing.T) {
+	assertEncodesTo(t, "empty: []", "")
+}
+
+func TestShellVariablesEncoderEmptyMap(t *testing.T) {
+	assertEncodesTo(t, "empty: {}", "")
+}
+
+func TestShellVariablesEncoderScalarNode(t *testing.T) {
+	assertEncodesTo(t, "some string", "value='some string'")
+}