From fb15ea8b7d37fd015257db5f6346d20c63415f91 Mon Sep 17 00:00:00 2001
From: Ivar Vong <ivar@ivarvong.com>
Date: Thu, 2 Jul 2026 17:28:33 -0400
Subject: [PATCH] fix: printf %b support and no-progress guard against
 format-recycle hangs
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printf recycles its format while arguments remain, but a directive the
format regex doesn't recognize consumes nothing — so printf "%b" "x"
(or any unknown directive with an argument) looped forever, hanging the
host process. %b is now a real directive sharing echo -e's escape
dialect, and a recycle pass that consumes no arguments stops after one
iteration instead of recursing. Unknown directives pass through
literally where bash errors; the divergence is noted in the tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FpxHS5iGm9ktgpnL9QZu9x
---
 lib/just_bash/commands/echo.ex   |  6 +++++-
 lib/just_bash/commands/printf.ex | 16 ++++++++++++++--
 test/commands/utilities_test.exs | 30 ++++++++++++++++++++++++++++++
 3 files changed, 49 insertions(+), 3 deletions(-)

diff --git a/lib/just_bash/commands/echo.ex b/lib/just_bash/commands/echo.ex
index 8d0c683..f79a08e 100644
--- a/lib/just_bash/commands/echo.ex
+++ b/lib/just_bash/commands/echo.ex
@@ -35,7 +35,11 @@ defmodule JustBash.Commands.Echo do
 
   defp parse_flags(rest, flags), do: {flags, rest}
 
-  defp interpret_escapes(str) do
+  # Also used by printf's %b directive, which shares echo -e's escape
+  # dialect (\0NNN octals, unlike $'...' ANSI-C quoting's \NNN).
+  @doc false
+  @spec interpret_escapes(String.t()) :: binary()
+  def interpret_escapes(str) do
     do_interpret_escapes(str, "")
   end
 
diff --git a/lib/just_bash/commands/printf.ex b/lib/just_bash/commands/printf.ex
index f970a94..a3e90b1 100644
--- a/lib/just_bash/commands/printf.ex
+++ b/lib/just_bash/commands/printf.ex
@@ -3,8 +3,9 @@ defmodule JustBash.Commands.Printf do
   @behaviour JustBash.Commands.Command
 
   alias JustBash.Commands.Command
+  alias JustBash.Commands.Echo
 
-  @format_regex ~r/%(-)?(0)?(\d+)?(?:\.(\d+))?([sdxXofec%])/
+  @format_regex ~r/%(-)?(0)?(\d+)?(?:\.(\d+))?([sdxXofecb%])/
 
   @impl true
   def names, do: ["printf"]
@@ -42,7 +43,14 @@ defmodule JustBash.Commands.Printf do
 
   defp recycle_format(format, args, acc) do
     {result, remaining} = apply_formats_with_remaining(format, args)
-    recycle_format(format, remaining, [result | acc])
+
+    if length(remaining) < length(args) do
+      recycle_format(format, remaining, [result | acc])
+    else
+      # A pass that consumed no arguments (an unrecognized directive) must
+      # not recycle the format forever.
+      recycle_format(format, [], [result | acc])
+    end
   end
 
   defp apply_formats_with_remaining(format, args) do
@@ -97,6 +105,10 @@ defmodule JustBash.Commands.Printf do
     {padded, []}
   end
 
+  defp do_format("b", arg, _precision) do
+    Echo.interpret_escapes(arg)
+  end
+
   defp do_format("s", arg, precision) do
     case precision do
       "" -> arg
diff --git a/test/commands/utilities_test.exs b/test/commands/utilities_test.exs
index 02f0b6a..ac26725 100644
--- a/test/commands/utilities_test.exs
+++ b/test/commands/utilities_test.exs
@@ -136,6 +136,36 @@ defmodule JustBash.Commands.UtilitiesTest do
       assert result.stdout == "a\tb"
     end
 
+    test "printf %b expands escape sequences in the argument" do
+      bash = JustBash.new()
+      {result, _} = JustBash.exec(bash, ~S[printf "%b" "A\tB\n"])
+      assert result.stdout == "A\tB\n"
+      assert result.exit_code == 0
+    end
+
+    @tag timeout: 5_000
+    test "printf %b with hex escapes emits raw bytes and terminates" do
+      bash = JustBash.new()
+      {result, _} = JustBash.exec(bash, ~S[printf "%b" "A\xffB\n"])
+      assert result.stdout == <<?A, 0xFF, ?B, ?\n>>
+    end
+
+    test "printf %b recycles the format across arguments" do
+      bash = JustBash.new()
+      {result, _} = JustBash.exec(bash, ~S[printf "%b\n" "a\tb" "c"])
+      assert result.stdout == "a\tb\nc\n"
+    end
+
+    # Bash errors on unknown directives; we pass them through literally.
+    # Either way, a directive that consumes no argument must not recycle
+    # the format forever.
+    @tag timeout: 5_000
+    test "printf with an unrecognized directive terminates instead of looping" do
+      bash = JustBash.new()
+      {result, _} = JustBash.exec(bash, ~S[printf "%v\n" x])
+      assert result.stdout == "%v\n"
+    end
+
     test "printf with %x hex format" do
       bash = JustBash.new()
       {result, _} = JustBash.exec(bash, "printf '%x' 255")