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+# ed
+
+Standalone musl-libc-based Linux port of FreeBSD `ed` for Project Tick BSD/Linux Distribution.
+
+## Build
+
+```sh
+gmake -f GNUmakefile
+gmake -f GNUmakefile CC=musl-gcc
+```
+
+## Test
+
+```sh
+gmake -f GNUmakefile test
+gmake -f GNUmakefile test CC=musl-gcc
+```
+
+## Notes
+
+- Port strategy is direct Linux-native cleanup of the FreeBSD source, not a BSD ABI shim.
+- The original multi-file editor core and FreeBSD regression corpus are preserved, but the Linux build surface is standalone: `GNUmakefile`, shell test entrypoint, and musl-safe libc usage.
+- Scratch-buffer storage uses `mkstemp(3)` in `TMPDIR` or `/tmp`, then `fdopen(3)` for the editor's temp file stream.
+- Shell escapes and filter I/O (`!`, `r !cmd`, `w !cmd`) use `system(3)`, `popen(3)`, and `pclose(3)`.
+- Terminal resize handling uses Linux `ioctl(TIOCGWINSZ)` on stdin.
+- Regex handling stays on POSIX `regcomp(3)` / `regexec(3)` from the active libc.
+- FreeBSD `strlcpy(3)` usage was replaced with a local implementation so the port does not depend on glibc extensions or `libbsd`.
+
+## Linux Semantics
+
+- Supported: core editing commands, shell escapes, global commands, undo, binary buffer handling, and the upstream FreeBSD regression scripts.
+- Supported: restricted `red` mode path checks from the original source.
+- Unsupported: historic crypt mode. Startup `-x` exits with an explicit Linux error, and the interactive `x` command reports `crypt mode is not supported on Linux`.
+- Known inherited quirk: the implicit newline print command still accepts the historic `,1` address form from this `ed` lineage. The Linux test harness allows only that single upstream deviation so other parser regressions still fail hard.
+
+## What is ed?
+
+ed is an 8-bit-clean, POSIX-compliant line editor.  It should work with
+any regular expression package that conforms to the POSIX interface
+standard, such as GNU regex(3).
+
+If reliable signals are supported (e.g., POSIX sigaction(2)), it should
+compile with little trouble.  Otherwise, the macros SPL1() and SPL0()
+should be redefined to disable interrupts.
+
+The following compiler directives are recognized:
+NO_REALLOC_NULL	- if realloc(3) does not accept a NULL pointer
+BACKWARDS	- for backwards compatibility
+NEED_INSQUE	- if insque(3) is missing
+
+The file `POSIX' describes extensions to and deviations from the POSIX
+standard.
+
+The ./test directory contains regression tests for ed. The README
+file in that directory explains how to run these.
+
+For a description of the ed algorithm, see Kernighan and Plauger's book
+"Software Tools in Pascal," Addison-Wesley, 1981.
+
+## ed POSIX message
+
+
+This version of ed(1) is not strictly POSIX compliant, as described in
+the POSIX 1003.2 document.  The following is a summary of the omissions,
+extensions and possible deviations from POSIX 1003.2.
+
+OMISSIONS
+---------
+1) For backwards compatibility, the POSIX rule that says a range of
+   addresses cannot be used where only a single address is expected has
+   been relaxed.
+
+2) To support the BSD `s' command (see extension [1] below),
+   substitution patterns cannot be delimited by numbers or the characters
+   `r', `g' and `p'.  In contrast, POSIX specifies any character expect
+   space or newline can used as a delimiter.
+
+EXTENSIONS
+----------
+1) BSD commands have been implemented wherever they do not conflict with
+   the POSIX standard.  The BSD-ism's included are:
+	i) `s' (i.e., s[n][rgp]*) to repeat a previous substitution,
+	ii) `W' for appending text to an existing file,
+	iii) `wq' for exiting after a write,
+	iv) `z' for scrolling through the buffer, and
+	v) BSD line addressing syntax (i.e., `^' and `%')  is recognized.
+
+2) The POSIX interactive global commands `G' and `V' are extended to 
+   support multiple commands, including `a', `i' and `c'.  The command
+   format is the same as for the global commands `g' and `v', i.e., one
+   command per line with each line, except for the last, ending in a
+   backslash (\).
+
+3) An extension to the POSIX file commands `E', `e', `r', `W' and `w' is
+   that <file> arguments are processed for backslash escapes, i.e.,  any
+   character preceded by a backslash is interpreted literally.  If the
+   first unescaped character of a <file> argument is a bang (!), then the
+   rest of the line is interpreted as a shell command, and no escape
+   processing is performed by ed.
+
+4) For SunOS ed(1) compatibility, ed runs in restricted mode if invoked
+   as red.  This limits editing of files in the local directory only and
+   prohibits shell commands.
+
+DEVIATIONS
+----------
+1) Though ed is not a stream editor, it can be used to edit binary files.
+   To assist in binary editing, when a file containing at least one ASCII
+   NUL character is written, a newline is not appended if it did not
+   already contain one upon reading.  In particular, reading /dev/null
+   prior to writing prevents appending a newline to a binary file.
+
+   For example, to create a file with ed containing a single NUL character:
+      $ ed file
+      a
+      ^@
+      .
+      r /dev/null
+      wq
+
+    Similarly, to remove a newline from the end of binary `file':
+      $ ed file
+      r /dev/null
+      wq
+
+2) Since the behavior of `u' (undo) within a `g' (global) command list is
+   not specified by POSIX, it follows the behavior of the SunOS ed:
+   undo forces a global command list to be executed only once, rather than
+   for each line matching a global pattern.  In addition, each instance of
+   `u' within a global command undoes all previous commands (including
+   undo's) in the command list.  This seems the best way, since the
+   alternatives are either too complicated to implement or too confusing
+   to use.  
+
+   The global/undo combination is useful for masking errors that
+   would otherwise cause a script to fail.  For instance, an ed script
+   to remove any occurrences of either `censor1' or `censor2' might be
+   written as:
+   	ed - file <<EOF
+	1g/.*/u\
+	,s/censor1//g\
+	,s/censor2//g
+	...
+
+3) The `m' (move) command within a `g' command list also follows the SunOS
+   ed implementation: any moved lines are removed from the global command's
+   `active' list.
+
+4) If ed is invoked with a name argument prefixed by a bang (!), then the
+   remainder of the argument is interpreted as a shell command.  To invoke
+   ed on a file whose name starts with bang, prefix the name with a
+   backslash.