add assignments name=value and unset

- name=value (one or more) without a command sets variables of the shell
- name=value before a command sets them only in its environment (for
  builtins only while they run)
- an assignment needs an unquoted name and =; the value is not split
  and not expanded as wildcard or brace pattern, ~ is expanded after =
  and :
- the status of an assignment is the one of its last $(...)
- new builtin unset

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

assign_test.go

@@ -0,0 +1,109 @@
+package main
+
+import (
+	"os"
+	"testing"
+)
+
+func TestTokenize_Assignments(t *testing.T) {
+	t.Setenv("HOME", "/home/u")
+	tests := []struct {
+		input  string
+		word   string
+		assign bool
+	}{
+		{"x=1", "x=1", true},
+		{"_a1=", "_a1=", true},
+		{`x="a b"`, "x=a b", true},
+		{"x=a=b", "x=a=b", true},
+		{"P=~/bin:~/go/bin", "P=/home/u/bin:/home/u/go/bin", true},
+		{"x=a~b", "x=a~b", true},
+		{"x=~", "x=/home/u", true},
+		// Not an assignment: quoted or escaped name or =, invalid name
+		{`"x=1"`, "x=1", false},
+		{`x"=1"`, "x=1", false},
+		{`x\=1`, "x=1", false},
+		{"1x=2", "1x=2", false},
+		{"=x", "=x", false},
+		{"a-b=1", "a-b=1", false},
+	}
+	for _, tc := range tests {
+		tokens, err := tokenize(tc.input)
+		if err != nil || len(tokens) != 1 {
+			t.Errorf("tokenize(%q) = %+v, %v", tc.input, tokens, err)
+			continue
+		}
+		if tokens[0].word != tc.word || tokens[0].assign != tc.assign {
+			t.Errorf("tokenize(%q) = %q (assign %v), want %q (assign %v)",
+				tc.input, tokens[0].word, tokens[0].assign, tc.word, tc.assign)
+		}
+	}
+
+	// The output of $(...) is not split in an assignment
+	tokens, _ := tokenize("x=$(echo a b) y")
+	if len(tokens) != 2 || tokens[0].word != "x=a b" || !tokens[0].assign {
+		t.Errorf("x=$(echo a b): %+v", tokens)
+	}
+}
+
+func TestExec_Assignments(t *testing.T) {
+	for _, name := range []string{"GOSH_A", "GOSH_B", "GOSH_C"} {
+		os.Unsetenv(name)
+		defer os.Unsetenv(name)
+	}
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"GOSH_A=1; echo $GOSH_A > out", "1\n"},
+		{"GOSH_A=1 GOSH_B=2; echo $GOSH_A$GOSH_B > out", "12\n"},
+		{`GOSH_A="a  b"; echo "$GOSH_A" > out`, "a  b\n"},
+		{"GOSH_A=$(printf 'a b\\nc'); echo \"$GOSH_A\" > out", "a b\nc\n"},
+		// No wildcard or brace expansion in the value
+		{"touch x.go; GOSH_A=*.go; echo \"$GOSH_A\" > out", "*.go\n"},
+		{"GOSH_A={a,b}; echo $GOSH_A > out", "{a,b}\n"},
+		// ... but as an argument, assignment-like words are normal words
+		{"echo GOSH_A={a,b} > out", "GOSH_A=a GOSH_A=b\n"},
+		// Before a command, only in its environment
+		{"GOSH_C=temp sh -c 'echo $GOSH_C' > out; echo ${GOSH_C:-unset} >> out", "temp\nunset\n"},
+		{"GOSH_A=old; GOSH_A=new sh -c 'echo $GOSH_A' > out; echo $GOSH_A >> out", "new\nold\n"},
+		// ... and before a builtin only while it runs
+		{"GOSH_C=temp pwd > /dev/null; echo ${GOSH_C:-unset} > out", "unset\n"},
+		// In a pipeline or with a command they do not change the shell
+		{"GOSH_C=x | cat; echo ${GOSH_C:-unset} > out", "unset\n"},
+		// Redirections are applied
+		{"GOSH_A=r > out", ""},
+		// In loops
+		{"for i in 1 2 3; do GOSH_A=$i; done; echo $GOSH_A > out", "3\n"},
+		{"GOSH_B=0; while [ $GOSH_B -lt 3 ]; do echo $GOSH_B >> out; GOSH_B=$(expr $GOSH_B + 1); done", "0\n1\n2\n"},
+		// Not assignments
+		{`echo "GOSH_A=1" GOSH_B=2 > out`, "GOSH_A=1 GOSH_B=2\n"},
+		// unset
+		{"GOSH_A=1; unset GOSH_A; echo ${GOSH_A:-gone} > out", "gone\n"},
+	}
+	for _, tc := range tests {
+		if got := runIn(t, tc.input); got != tc.want {
+			t.Errorf("%q: out = %q, want %q", tc.input, got, tc.want)
+		}
+	}
+
+	// The status of an assignment is the one of its last $(...)
+	for input, want := range map[string]int{
+		"GOSH_A=1":                       0,
+		"GOSH_A=$(false)":                1,
+		"GOSH_A=$(sh -c 'exit 3')":       3,
+		"GOSH_A=$(false) GOSH_B=$(true)": 0,
+	} {
+		runIn(t, input)
+		if lastStatus != want {
+			t.Errorf("%q: $? = %d, want %d", input, lastStatus, want)
+		}
+	}
+
+	if err := execInput("unset 1x"); err == nil || err.Error() != "unset: `1x': not a valid identifier" {
+		t.Errorf("unset 1x: %v", err)
+	}
+	if err := execInput(`"GOSH_A=1"`); err == nil || exitStatus(err) != 127 {
+		t.Errorf(`"GOSH_A=1" must be a command: %v`, err)
+	}
+}