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GNU ed is an 8-bit clean, more or less POSIX-compliant implementation of
the standard Unix line editor. These days, full-screen editors have
rendered `ed' mostly of historical interest. Nonetheless, it appeals to
a handful of aging programmers who still believe that "Small is Beautiful".

Extensions to and deviations from the POSIX standard are described below.

See the file INSTALL for compilation and installation instructions.

Try "ed --help" for usage instructions.

Report bugs to <bug-ed@gnu.org>.

Ed home page: http://www.gnu.org/software/ed/ed.html

For a description of the ed algorithm, see Kernighan and Plauger's book
"Software Tools in Pascal," Addison-Wesley, 1981.


GNU ed(1) is not strictly POSIX compliant, as described in the
POSIX 1003.1-2004 document.  The following is a summary of omissions
and extensions to, and deviations from, the POSIX standard.

OMISSIONS
---------
  * Locale(3) is not supported.

EXTENSIONS
----------
  * Though GNU 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 GNU 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

  * BSD commands have been implemented wherever they do not conflict with
    the POSIX standard.  The BSD-ism's included are:
      * `s' (i.e., s[n][rgp]*) to repeat a previous substitution,
      * `W' for appending text to an existing file,
      * `wq' for exiting after a write, and
      * `z' for scrolling through the buffer.

  * 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 (\).

  * The file commands `E', `e', `r', `W' and `w'  process a <file>
    argument 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 GNU ed.

  * For SunOS ed(1) compatibility, GNU ed runs in restricted mode if invoked
    as red.  This limits editing of files in the local directory only and
    prohibits shell commands.

DEVIATIONS
----------
  * 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.

  * To support the BSD `s' command (see EXTENSIONS above),
    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.

  * Since the behavior of `u' (undo) within a `g' (global) command list is
    not specified by POSIX, GNU ed 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 addtion, 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 `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.

  * If GNU 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
    (quoted) backslash.

  * For backwards compatibility, errors in piped scripts do not force ed
    to exit.  POSIX only specifies ed's response for input via regular
    files (including here documents) or tty's.


TESTSUITE
---------
The files in the `testsuite' directory with suffixes `.t', `.d', `.r',
`.pr' and `.err' are used for testing ed. To run the tests, configure
the package and type `make check' from the build directory. The tests do
not exhaustively verify POSIX compliance nor do they verify correct
8-bit or long line support.

The test file suffixes have the following meanings:
.t    Template - a list of ed commands from which an ed script is
      constructed
.d    Data - read by an ed script
.r    Result - the expected output after processing data via an ed
      script.
.pr   Result from a piped ed script.
.err  Error - invalid ed commands that should generate an error

The output of the tests is written to the files errs.ck, pipes.ck and
scripts.ck.  At the end of the tests, these files are grep'ed for error
messages, which look like:

	*** The script u.ed exited abnormally ***
or:
	*** Output u.o of script u.ed is incorrect ***

The POSIX requirement that an address range not be used where at most
a single address is expected has been relaxed in this version of ed.
Therefore, the  following script templates are disabled:
=-err.t.posix
a1-err.posix
i1-err.posix
k1-err.posix
r1-err.posix

To use these, remove the .posix suffix and run the tests as described
above.
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