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      1 $NetBSD: POSIX,v 1.11 2022/05/20 07:47:16 andvar Exp $
      2 
      3 This version of ed(1) is not strictly POSIX compliant, as described in
      4 the POSIX 1003.2 document.  The following is a summary of the omissions,
      5 extensions and possible deviations from POSIX 1003.2.
      6 
      7 OMISSIONS
      8 ---------
      9 1) Locale(3) is not supported yet.
     10 
     11 2) For backwards compatibility, the POSIX rule that says a range of
     12    addresses cannot be used where only a single address is expected has
     13    been relaxed.
     14 
     15 3) To support the BSD `s' command (see extension [1] below),
     16    substitution patterns cannot be delimited by numbers or the characters
     17    `r', `g' and `p'.  In contrast, POSIX specifies any character except
     18    space or newline can be used as a delimiter.
     19 
     20 EXTENSIONS
     21 ----------
     22 1) BSD commands have been implemented wherever they do not conflict with
     23    the POSIX standard.  The BSD-ism's included are:
     24 	i) `s' (i.e., s[n][rgp]*) to repeat a previous substitution,
     25 	ii) `W' for appending text to an existing file,
     26 	iii) `wq' for exiting after a write,
     27 	iv) `z' for scrolling through the buffer, and
     28 	v) BSD line addressing syntax (i.e., `^' and `%') is recognized.
     29 
     30 2) If crypt(3) is available, files can be read and written using DES
     31    encryption.  The `x' command prompts the user to enter a key used for
     32    encrypting/decrypting subsequent reads and writes.  If only a newline
     33    is entered as the key, then encryption is disabled.  Otherwise, a key
     34    is read in the same manner as a password entry.  The key remains in
     35    effect until encryption is disabled.  For more information on the
     36    encryption algorithm, see the bdes(1) man page.  Encryption/decryption
     37    should be fully compatible with SunOS des(1).
     38 
     39 3) The POSIX interactive global commands `G' and `V' are extended to 
     40    support multiple commands, including `a', `i' and `c'.  The command
     41    format is the same as for the global commands `g' and `v', i.e., one
     42    command per line with each line, except for the last, ending in a
     43    backslash (\).
     44 
     45 4) An extension to the POSIX file commands `E', `e', `r', `W' and `w' is
     46    that <file> arguments are processed for backslash escapes, i.e.,  any
     47    character preceded by a backslash is interpreted literally.  If the
     48    first unescaped character of a <file> argument is a bang (!), then the
     49    rest of the line is interpreted as a shell command, and no escape
     50    processing is performed by ed.
     51 
     52 5) For SunOS ed(1) compatibility, ed runs in restricted mode if invoked
     53    as red.  This limits editing of files in the local directory only and
     54    prohibits shell commands.
     55 
     56 DEVIATIONS
     57 ----------
     58 1) Though ed is not a stream editor, it can be used to edit binary files.
     59    To assist in binary editing, when a file containing at least one ASCII
     60    NUL character is written, a newline is not appended if it did not
     61    already contain one upon reading.  In particular, reading /dev/null
     62    prior to writing prevents appending a newline to a binary file.
     63 
     64    For example, to create a file with ed containing a single NUL character:
     65       $ ed file
     66       a
     67       ^@
     68       .
     69       r /dev/null
     70       wq
     71 
     72     Similarly, to remove a newline from the end of binary `file':
     73       $ ed file
     74       r /dev/null
     75       wq
     76 
     77 2) Since the behavior of `u' (undo) within a `g' (global) command list is
     78    not specified by POSIX, it follows the behavior of the SunOS ed:
     79    undo forces a global command list to be executed only once, rather than
     80    for each line matching a global pattern.  In addition, each instance of
     81    `u' within a global command undoes all previous commands (including
     82    undo's) in the command list.  This seems the best way, since the
     83    alternatives are either too complicated to implement or too confusing
     84    to use.
     85 
     86    The global/undo combination is useful for masking errors that
     87    would otherwise cause a script to fail.  For instance, an ed script
     88    to remove any occurrences of either `censor1' or `censor2' might be
     89    written as:
     90    	ed - file <<EOF
     91 	1g/.*/u\
     92 	,s/censor1//g\
     93 	,s/censor2//g
     94 	...
     95 
     96 3) The `m' (move) command within a `g' command list also follows the SunOS
     97    ed implementation: any moved lines are removed from the global command's
     98    `active' list.
     99 
    100 4) If ed is invoked with a name argument prefixed by a bang (!), then the
    101    remainder of the argument is interpreted as a shell command.  To invoke
    102    ed on a file whose name starts with bang, prefix the name with a
    103    backslash.
    104