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	---
	commit c8279e8b137436c61ad76e077afe0cec09ed9260
	parent 9c6672aaa881709d5e0139128423ad83000b557f
	Author: Josuah Demangeon <[email protected]>
	Date: Sun, 16 Oct 2022 02:09:47 +0200

	opus7: submit a new article from Athas

	Diffstat:
	A opus7/article-athas-shell-redirect… \| 73 +++++++++++++++++++++++++++…

	1 file changed, 73 insertions(+), 0 deletions(-)
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	diff --git a/opus7/article-athas-shell-redirections.mw b/opus7/article-athas-sh…
	@@ -0,0 +1,73 @@
	+.SH athas
	+Shell Redirections
	+.2C 60
	+.
	+.PP
	+Newcomers to the Unix shell quickly encounter handy tools such as
	+sed(1) and sort(1). This command prints the lines of the given file
	+to stdout, in sorted order:
	+.
	+.DS
	+$ sort numbers
	+.DE
	+.
	+.PP
	+Soon after, newcomers will also encounter shell redirection, by which
	+the output of these tools can conveniently be read from or stored in
	+files:
	+.
	+.DS
	+$ sort < numbers > numbers_sorted
	+.DE
	+.
	+.PP
	+Our new user, fascinated by the modularity of the Unix shell, may then
	+try the rather obvious possibility of having the input and output file
	+be the same:
	+.
	+.DS
	+$ sort < numbers > numbers
	+.DE
	+.
	+.PP
	+But disaster strikes: the file is empty! The user has lost their
	+precious collection of numbers - let's hope they had a backup. Losing
	+data this way is almost a rite of passage for Unix users, but let us
	+spell out the reason for those who have yet to hurt themselves this
	+way.
	+.
	+.PP
	+When the Unix shell evaluates a command, it starts by processing the
	+redirection operators - that's the '>' and '<' above. While '<' just
	+opens the file, '>' truncates the file in-place as it is opened for
	+reading! This means that the 'sort' process will dutifully read an
	+empty file, sort its non-existent lines, and correctly produce empty
	+output.
	+.
	+.PP
	+Some programs can be asked to write their output directly to files
	+instead of using shell redirection (sed(1) has '-i', and for sort(1)
	+we can use '-o'), but this is not a general solution, and does not
	+work for pipelines. Another solution is to use the sponge(1) tool
	+from the "moreutils" project, which stores its standard input in
	+memory before finally writing it to a file:
	+.
	+.DS
	+$ sort < numbers \| sponge numbers
	+.DE
	+.
	+.PP
	+The most interesting solution is to take advantage of subshells, the
	+shell evaluation order, and Unix file systems semantics. When we
	+delete a file in Unix, it is removed from the file system, but any
	+file descriptors referencing the file remain valid. We can exploit
	+this behaviour to delete the input file after directing the input,
	+but before redirecting the output:
	+.
	+.DS
	+$ (rm numbers && sort > numbers) < numbers
	+.DE
	+.
	+.PP
	+This approach requires no dependencies and will work in any Unix
	+shell.