Support line lengths without

builtins/builtins.go

@@ -38,6 +38,11 @@ type Command struct {
 	Description string
 	Help        string
 	MakeFlags   func(*FlagSet) HandlerFunc
+
+	// NormalizeArgs, if non-nil, rewrites raw argument slices before pflag
+	// parsing. This allows commands to support legacy flag syntax that pflag
+	// cannot handle natively (e.g. head/tail -5 → -n 5).
+	NormalizeArgs func(args []string) []string
 }
 
 // NoFlags wraps a HandlerFunc in the MakeFlags format for commands that
@@ -57,6 +62,7 @@ func NoFlags(fn HandlerFunc) func(*FlagSet) HandlerFunc {
 func (c Command) Register() {
 	name := c.Name
 	factory := c.MakeFlags
+	normalize := c.NormalizeArgs
 	metaRegistry[name] = CommandMeta{Name: name, Description: c.Description, Help: c.Help}
 	addToRegistry(name, func(ctx context.Context, callCtx *CallContext, args []string) Result {
 		fs := pflag.NewFlagSet(name, pflag.ContinueOnError)
@@ -66,6 +72,9 @@ func (c Command) Register() {
 			// No flags declared: pass all args through unchanged.
 			return handler(ctx, callCtx, args)
 		}
+		if normalize != nil {
+			args = normalize(args)
+		}
 		if err := fs.Parse(args); err != nil {
 			callCtx.Errf("%s: %v\n", name, err)
 			return Result{Code: 1}
@@ -244,3 +253,32 @@ func Meta(name string) (CommandMeta, bool) {
 	m, ok := metaRegistry[name]
 	return m, ok
 }
+
+// NormalizeBareNumberArg rewrites legacy -N shorthand (e.g. -5) to -n N so
+// that pflag can parse it. Only a bare -<digits> token in the first argument
+// position is rewritten; -<digits> appearing later in the argument list is
+// left unchanged (matching GNU head/tail behavior where the obsolete form is
+// only accepted as the first option). Processing stops at "--".
+//
+// When the first argument is a value-taking flag (-n, -c, --lines, --bytes),
+// the second argument is its value and must not be rewritten — even if it
+// looks like -<digits> (e.g. "head -n -9223372036854775809").
+//
+// valueFlags lists the flags that consume the next argument as a value
+// (e.g. []string{"-n", "-c", "--lines", "--bytes"}).
+func NormalizeBareNumberArg(args []string, valueFlags []string) []string {
+	if len(args) == 0 {
+		return args
+	}
+	a := args[0]
+	if a == "--" {
+		return args
+	}
+	if len(a) >= 2 && a[0] == '-' && a[1] >= '0' && a[1] <= '9' {
+		out := make([]string, 0, len(args)+1)
+		out = append(out, "-n", a[1:])
+		out = append(out, args[1:]...)
+		return out
+	}
+	return args
+}

builtins/head/head.go

@@ -61,9 +61,18 @@ import (
 
 // Cmd is the head builtin command descriptor.
 var Cmd = builtins.Command{
-	Name:        "head",
-	Description: "output the first part of files",
-	MakeFlags:   registerFlags,
+	Name:          "head",
+	Description:   "output the first part of files",
+	MakeFlags:     registerFlags,
+	NormalizeArgs: normalizeArgs,
+}
+
+// normalizeArgs rewrites legacy -N shorthand (e.g. -5) to -n N so that
+// pflag can parse it. Only a bare -<digits> token in the first argument
+// position is rewritten, matching GNU head behavior where the obsolete form
+// is only accepted as the first option.
+func normalizeArgs(args []string) []string {
+	return builtins.NormalizeBareNumberArg(args, []string{"-n", "-c", "--lines", "--bytes"})
 }
 
 // MaxCount is the maximum accepted line or byte count. Values above this

builtins/tail/tail.go

@@ -86,9 +86,18 @@ import (
 
 // Cmd is the tail builtin command descriptor.
 var Cmd = builtins.Command{
-	Name:        "tail",
-	Description: "output the last part of files",
-	MakeFlags:   registerFlags,
+	Name:          "tail",
+	Description:   "output the last part of files",
+	MakeFlags:     registerFlags,
+	NormalizeArgs: normalizeArgs,
+}
+
+// normalizeArgs rewrites legacy -N shorthand (e.g. -5) to -n N so that
+// pflag can parse it. Only a bare -<digits> token in the first argument
+// position is rewritten, matching GNU tail behavior where the obsolete form
+// is only accepted as the first option.
+func normalizeArgs(args []string) []string {
+	return builtins.NormalizeBareNumberArg(args, []string{"-n", "-c", "--lines", "--bytes"})
 }
 
 // MaxCount is the maximum accepted line or byte count. Values above this

tests/scenarios/cmd/head/lines/bare_number_shorthand.yaml

@@ -0,0 +1,13 @@
+description: head -3 (bare numeric shorthand) outputs the first 3 lines, equivalent to head -n 3.
+setup:
+  files:
+    - path: file.txt
+      content: "alpha\nbeta\ngamma\ndelta\nepsilon\n"
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    head -3 file.txt
+expect:
+  stdout: "alpha\nbeta\ngamma\n"
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/head/lines/bare_number_shorthand_large.yaml

@@ -0,0 +1,8 @@
+description: head -42 works with larger bare numeric shorthand.
+input:
+  script: |+
+    printf "1\n2\n3\n" | head -42
+expect:
+  stdout: "1\n2\n3\n"
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/head/lines/bare_number_shorthand_pipe.yaml

@@ -0,0 +1,8 @@
+description: head -5 works with piped input.
+input:
+  script: |+
+    printf "1\n2\n3\n4\n5\n6\n7\n" | head -5
+expect:
+  stdout: "1\n2\n3\n4\n5\n"
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/head/lines/bare_number_shorthand_zero.yaml

@@ -0,0 +1,8 @@
+description: head -0 (bare zero shorthand) produces no output, equivalent to head -n 0.
+input:
+  script: |+
+    printf "1\n2\n3\n" | head -0
+expect:
+  stdout: ""
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/tail/lines/bare_number_shorthand.yaml

@@ -0,0 +1,13 @@
+description: tail -3 (bare numeric shorthand) outputs the last 3 lines, equivalent to tail -n 3.
+setup:
+  files:
+    - path: file.txt
+      content: "alpha\nbeta\ngamma\ndelta\nepsilon\n"
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    tail -3 file.txt
+expect:
+  stdout: "gamma\ndelta\nepsilon\n"
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/tail/lines/bare_number_shorthand_pipe.yaml

@@ -0,0 +1,8 @@
+description: tail -5 works with piped input.
+input:
+  script: |+
+    printf "1\n2\n3\n4\n5\n6\n7\n" | tail -5
+expect:
+  stdout: "3\n4\n5\n6\n7\n"
+  stderr: ""
+  exit_code: 0

tests/scenarios/cmd/tail/lines/bare_number_shorthand_zero.yaml

@@ -0,0 +1,8 @@
+description: tail -0 (bare zero shorthand) produces no output, equivalent to tail -n 0.
+input:
+  script: |+
+    printf "1\n2\n3\n" | tail -0
+expect:
+  stdout: ""
+  stderr: ""
+  exit_code: 0