This occasionally comes in handy, so I'll add it to utils.

[sgt/utils] / reservoir / reservoir.but

\cfg{man-identity}{reservoir}{1}{2005-05-13}{Simon Tatham}{Simon Tatham}

\title Man page for \cw{reservoir}

\U NAME

\cw{reservoir} - delay stage in a pipeline

\U SYNOPSIS

\c reservoir [ -o filename ]
\e bbbbbbbbb   bb iiiiiiii

\U DESCRIPTION

\cw{reservoir}'s function is to read from its standard input until
it sees end-of-file, then to write everything it has seen to its
standard output.

It behaves exactly like \cw{cat} with no arguments, except that it
writes none of its outgoing data until all of its input has arrived.

\U OPTIONS

\dt \cw{-o} \e{filename}

\dd Causes the output to be written to \e{filename} rather than to
standard output. \e{filename} is not opened until after
\cw{reservoir} detects end of file on its input.

\U EXAMPLES

If you have a program which filters its input in some way (for
example, a base-64 decoder, or a \cw{tr}(1) command performing
rot13), and you wish to copy a small amount of data into that
program using a terminal emulator's paste function, it can be
inconvenient to have the output interspersed with the echoed input
so that you cannot select and copy the output as a whole.

For example:

\c $ tr a-zA-Z n-za-mN-ZA-M
\e   bbbbbbbbbbbbbbbbbbbbbb
\c Hello, world.
\e bbbbbbbbbbbbb
\c Uryyb, jbeyq.
\c This is a test.
\e bbbbbbbbbbbbbbb
\c Guvf vf n grfg.

If your terminal emulator pastes the text line by line, then to copy
the transformed output requires you to separately select each line
of the output. If the terminal pastes in larger chunks, you may not
see the problem quite so quickly, but it will still appear
eventually.

You can solve this using \cw{reservoir}:

\c $ tr a-zA-Z n-za-mN-ZA-M | reservoir
\e   bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
\c Hello, world.
\e bbbbbbbbbbbbb
\c This is a test.
\e bbbbbbbbbbbbbbb
\c (now the user presses ^D)
\e iiiiiiiiiiiiiiiiiiiiiiiii
\c Uryyb, jbeyq.
\c Guvf vf n grfg.

A common reason why you might want to buffer data in a pipeline is
in order to transform a file in place. For example, you cannot write

\c $ tr a-zA-Z n-za-mN-ZA-M < temp.txt > temp.txt

because the output redirection will destroy the contents of the file
before its original contents can be read. \cw{reservoir} can help,
because it does not begin writing output until after the input has
all been read.

You still cannot use output redirection, because the presence of the
\cw{>} operator on your command line will cause the output file to
be truncated to zero length \e{before} running \cw{reservoir}, so
there is nothing \cw{reservoir} can do about this. Instead, you can
use the \cw{-o} option provided by \cw{reservoir}:

\c $ tr a-zA-Z n-za-mN-ZA-M < temp.txt | reservoir -o temp.txt

Now \cw{reservoir} will not open \cw{temp.txt} for output until
\e{after} the rest of the pipeline has finished reading data from it.

(This is not a reliable means of editing files in place. If
something goes wrong half way through writing the output, part of
your data will be lost. Also, the file is not replaced atomically.
This method is very convenient in non-critical situations, but is
not recommended for critical or automated use.)

\U LICENCE

\cw{reservoir} is free software, distributed under the MIT licence. Type
\cw{reservoir --licence} to see the full licence text.

\versionid $Id$