mdw@git.distorted.org.uk Git - sgt/utils/blob - reservoir/reservoir.but

   1 \cfg{man-identity}{reservoir}{1}{2008-03-22}{Simon Tatham}{Simon Tatham}
   2
   3 \title Man page for \cw{reservoir}
   4
   5 \U NAME
   6
   7 \cw{reservoir} - delay stage in a pipeline
   8
   9 \U SYNOPSIS
  10
  11 \c reservoir [ -o filename | -O filename ]
  12 \e bbbbbbbbb   bb iiiiiiii   bb iiiiiiii
  13
  14 \U DESCRIPTION
  15
  16 \cw{reservoir}'s function is to read from its standard input until
  17 it sees end-of-file, then to write everything it has seen to its
  18 standard output.
  19
  20 It behaves exactly like \cw{cat} with no arguments, except that it
  21 writes none of its outgoing data until all of its input has arrived.
  22
  23 \U OPTIONS
  24
  25 \dt \cw{-O} \e{filename}
  26
  27 \dd Causes the output to be written to \e{filename} rather than to
  28 standard output. \e{filename} is not opened until after
  29 \cw{reservoir} detects end of file on its input.
  30
  31 \dt \cw{-o} \e{filename}
  32
  33 \dd Exactly like \cw{-O}, but with one special case: if there is no
  34 output at all to be written, \cw{reservoir} will not open the output
  35 file for writing at all. Hence, if the process which is supposed to
  36 generate the output completely fails to run, \e{filename} will not
  37 be overwritten.
  38
  39 \U EXAMPLES
  40
  41 If you have a program which filters its input in some way (for
  42 example, a base-64 decoder, or a \cw{tr}(1) command performing
  43 rot13), and you wish to copy a small amount of data into that
  44 program using a terminal emulator's paste function, it can be
  45 inconvenient to have the output interspersed with the echoed input
  46 so that you cannot select and copy the output as a whole.
  47
  48 For example:
  49
  50 \c $ tr a-zA-Z n-za-mN-ZA-M
  51 \e   bbbbbbbbbbbbbbbbbbbbbb
  52 \c Hello, world.
  53 \e bbbbbbbbbbbbb
  54 \c Uryyb, jbeyq.
  55 \c This is a test.
  56 \e bbbbbbbbbbbbbbb
  57 \c Guvf vf n grfg.
  58
  59 If your terminal emulator pastes the text line by line, then to copy
  60 the transformed output requires you to separately select each line
  61 of the output. If the terminal pastes in larger chunks, you may not
  62 see the problem quite so quickly, but it will still appear
  63 eventually.
  64
  65 You can solve this using \cw{reservoir}:
  66
  67 \c $ tr a-zA-Z n-za-mN-ZA-M | reservoir
  68 \e   bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
  69 \c Hello, world.
  70 \e bbbbbbbbbbbbb
  71 \c This is a test.
  72 \e bbbbbbbbbbbbbbb
  73 \c (now the user presses ^D)
  74 \e iiiiiiiiiiiiiiiiiiiiiiiii
  75 \c Uryyb, jbeyq.
  76 \c Guvf vf n grfg.
  77
  78 A common reason why you might want to buffer data in a pipeline is
  79 in order to transform a file in place. For example, you cannot write
  80
  81 \c $ tr a-zA-Z n-za-mN-ZA-M < temp.txt > temp.txt
  82
  83 because the output redirection will destroy the contents of the file
  84 before its original contents can be read. \cw{reservoir} can help,
  85 because it does not begin writing output until after the input has
  86 all been read.
  87
  88 You still cannot use output redirection, because the presence of the
  89 \cw{>} operator on your command line will cause the output file to
  90 be truncated to zero length \e{before} running \cw{reservoir}, so
  91 there is nothing \cw{reservoir} can do about this. Instead, you can
  92 use the \cw{-o} option provided by \cw{reservoir}:
  93
  94 \c $ tr a-zA-Z n-za-mN-ZA-M < temp.txt | reservoir -o temp.txt
  95
  96 Now \cw{reservoir} will not open \cw{temp.txt} for output until
  97 \e{after} the rest of the pipeline has finished reading data from it.
  98
  99 (This is not a reliable means of editing files in place. If
 100 something goes wrong half way through writing the output, part of
 101 your data will be lost, although the default behaviour of \cw{-o}
 102 will at least avoid overwriting the file if something goes wrong
 103 \e{before} the output begins to be written. Also, the file is not
 104 replaced atomically. This method is very convenient in non-critical
 105 situations, such as when the target file is backed up in source
 106 control, but is not recommended for critical or automated use.)
 107
 108 Another use for \cw{-o} is for requesting a list of files using
 109 \c{find}(1) or \c{ls}(1), without the output file appearing in the
 110 list:
 111
 112 \c $ find . -type f | reservoir -o filelist
 113
 114 \U LICENCE
 115
 116 \cw{reservoir} is free software, distributed under the MIT licence. Type
 117 \cw{reservoir --licence} to see the full licence text.
 118
 119 \versionid $Id$