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sed: Extended Commands

 
 3.7 Commands Specific to GNU 'sed'
 ==================================
 
 These commands are specific to GNU 'sed', so you must use them with care
 and only when you are sure that hindering portability is not evil.  They
 allow you to check for GNU 'sed' extensions or to do tasks that are
 required quite often, yet are unsupported by standard 'sed's.
 
 'e [COMMAND]'
      This command allows one to pipe input from a shell command into
      pattern space.  Without parameters, the 'e' command executes the
      command that is found in pattern space and replaces the pattern
      space with the output; a trailing newline is suppressed.
 
      If a parameter is specified, instead, the 'e' command interprets it
      as a command and sends its output to the output stream.  The
      command can run across multiple lines, all but the last ending with
      a back-slash.
 
      In both cases, the results are undefined if the command to be
      executed contains a NUL character.
 
      Note that, unlike the 'r' command, the output of the command will
      be printed immediately; the 'r' command instead delays the output
      to the end of the current cycle.
 
 'F'
      Print out the file name of the current input file (with a trailing
      newline).
 
 'Q [EXIT-CODE]'
      This command accepts only one address.
 
      This command is the same as 'q', but will not print the contents of
      pattern space.  Like 'q', it provides the ability to return an exit
      code to the caller.
 
      This command can be useful because the only alternative ways to
      accomplish this apparently trivial function are to use the '-n'
      option (which can unnecessarily complicate your script) or
      resorting to the following snippet, which wastes time by reading
      the whole file without any visible effect:
 
           :eat
           $d       Quit silently on the last line
           N        Read another line, silently
           g        Overwrite pattern space each time to save memory
           b eat
 
 'R FILENAME'
      Queue a line of FILENAME to be read and inserted into the output
      stream at the end of the current cycle, or when the next input line
      is read.  Note that if FILENAME cannot be read, or if its end is
      reached, no line is appended, without any error indication.
 
      As with the 'r' command, the special value '/dev/stdin' is
      supported for the file name, which reads a line from the standard
      input.
 
 'T LABEL'
      Branch to LABEL only if there have been no successful
      's'ubstitutions since the last input line was read or conditional
      branch was taken.  The LABEL may be omitted, in which case the next
      cycle is started.
 
 'v VERSION'
      This command does nothing, but makes 'sed' fail if GNU 'sed'
      extensions are not supported, simply because other versions of
      'sed' do not implement it.  In addition, you can specify the
      version of 'sed' that your script requires, such as '4.0.5'.  The
      default is '4.0' because that is the first version that implemented
      this command.
 
      This command enables all GNU extensions even if 'POSIXLY_CORRECT'
      is set in the environment.
 
 'W FILENAME'
      Write to the given filename the portion of the pattern space up to
      the first newline.  Everything said under the 'w' command about
      file handling holds here too.
 
 'z'
      This command empties the content of pattern space.  It is usually
      the same as 's/.*//', but is more efficient and works in the
      presence of invalid multibyte sequences in the input stream.  POSIX
      mandates that such sequences are _not_ matched by '.', so that
      there is no portable way to clear 'sed''s buffers in the middle of
      the script in most multibyte locales (including UTF-8 locales).
 

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