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92 lines
3.7 KiB
Plaintext
92 lines
3.7 KiB
Plaintext
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This version of ed(1) is not strictly POSIX compliant, as described in
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the POSIX 1003.2 document. The following is a summary of the omissions,
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extensions and possible deviations from POSIX 1003.2.
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OMISSIONS
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---------
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1) For backwards compatibility, the POSIX rule that says a range of
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addresses cannot be used where only a single address is expected has
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been relaxed.
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2) To support the BSD `s' command (see extension [1] below),
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substitution patterns cannot be delimited by numbers or the characters
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`r', `g' and `p'. In contrast, POSIX specifies any character expect
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space or newline can used as a delimiter.
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EXTENSIONS
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----------
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1) BSD commands have been implemented wherever they do not conflict with
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the POSIX standard. The BSD-ism's included are:
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i) `s' (i.e., s[n][rgp]*) to repeat a previous substitution,
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ii) `W' for appending text to an existing file,
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iii) `wq' for exiting after a write,
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iv) `z' for scrolling through the buffer, and
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v) BSD line addressing syntax (i.e., `^' and `%') is recognized.
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2) The POSIX interactive global commands `G' and `V' are extended to
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support multiple commands, including `a', `i' and `c'. The command
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format is the same as for the global commands `g' and `v', i.e., one
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command per line with each line, except for the last, ending in a
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backslash (\).
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3) An extension to the POSIX file commands `E', `e', `r', `W' and `w' is
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that <file> arguments are processed for backslash escapes, i.e., any
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character preceded by a backslash is interpreted literally. If the
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first unescaped character of a <file> argument is a bang (!), then the
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rest of the line is interpreted as a shell command, and no escape
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processing is performed by ed.
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4) For SunOS ed(1) compatibility, ed runs in restricted mode if invoked
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as red. This limits editing of files in the local directory only and
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prohibits shell commands.
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DEVIATIONS
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----------
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1) Though ed is not a stream editor, it can be used to edit binary files.
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To assist in binary editing, when a file containing at least one ASCII
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NUL character is written, a newline is not appended if it did not
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already contain one upon reading. In particular, reading /dev/null
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prior to writing prevents appending a newline to a binary file.
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For example, to create a file with ed containing a single NUL character:
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$ ed file
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a
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^@
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.
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r /dev/null
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wq
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Similarly, to remove a newline from the end of binary `file':
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$ ed file
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r /dev/null
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wq
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2) Since the behavior of `u' (undo) within a `g' (global) command list is
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not specified by POSIX, it follows the behavior of the SunOS ed:
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undo forces a global command list to be executed only once, rather than
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for each line matching a global pattern. In addition, each instance of
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`u' within a global command undoes all previous commands (including
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undo's) in the command list. This seems the best way, since the
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alternatives are either too complicated to implement or too confusing
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to use.
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The global/undo combination is useful for masking errors that
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would otherwise cause a script to fail. For instance, an ed script
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to remove any occurrences of either `censor1' or `censor2' might be
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written as:
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ed - file <<EOF
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1g/.*/u\
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,s/censor1//g\
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,s/censor2//g
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...
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3) The `m' (move) command within a `g' command list also follows the SunOS
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ed implementation: any moved lines are removed from the global command's
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`active' list.
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4) If ed is invoked with a name argument prefixed by a bang (!), then the
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remainder of the argument is interpreted as a shell command. To invoke
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ed on a file whose name starts with bang, prefix the name with a
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backslash.
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