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	article-athas-shell-redirections.mw (2204B)
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	1 .SH athas
	2 Shell Redirections
	3 .
	4 .PP
	5 Newcomers to the Unix shell quickly encounter handy tools such as
	6 sed(1) and sort(1). This command prints the lines of the given file
	7 to stdout, in sorted order:
	8 .
	9 .DS
	10 $ sort numbers
	11 .DE
	12 .
	13 .PP
	14 Soon after, newcomers will also encounter shell redirection, by which
	15 the output of these tools can conveniently be read from or stored in
	16 files:
	17 .
	18 .DS
	19 $ sort < numbers > numbers_sorted
	20 .DE
	21 .
	22 .PP
	23 Our new user, fascinated by the modularity of the Unix shell, may then
	24 try the rather obvious possibility of having the input and output file
	25 be the same:
	26 .
	27 .DS
	28 $ sort < numbers > numbers
	29 .DE
	30 .
	31 .PP
	32 But disaster strikes: the file is empty! The user has lost their
	33 precious collection of numbers - let's hope they had a backup. Losing
	34 data this way is almost a rite of passage for Unix users, but let us
	35 spell out the reason for those who have yet to hurt themselves this
	36 way.
	37 .
	38 .PP
	39 When the Unix shell evaluates a command, it starts by processing the
	40 redirection operators - that's the '>' and '<' above. While '<' just
	41 opens the file, '>' truncates the file in-place as it is opened for
	42 reading! This means that the 'sort' process will dutifully read an
	43 empty file, sort its non-existent lines, and correctly produce empty
	44 output.
	45 .
	46 .PP
	47 Some programs can be asked to write their output directly to files
	48 instead of using shell redirection (sed(1) has '-i', and for sort(1)
	49 we can use '-o'), but this is not a general solution, and does not
	50 work for pipelines. Another solution is to use the sponge(1) tool
	51 from the "moreutils" project, which stores its standard input in
	52 memory before finally writing it to a file:
	53 .
	54 .DS
	55 $ sort < numbers \| sponge numbers
	56 .DE
	57 .
	58 .PP
	59 The most interesting solution is to take advantage of subshells, the
	60 shell evaluation order, and Unix file systems semantics. When we
	61 delete a file in Unix, it is removed from the file system, but any
	62 file descriptors referencing the file remain valid. We can exploit
	63 this behaviour to delete the input file after directing the input,
	64 but before redirecting the output:
	65 .
	66 .DS
	67 $ (rm numbers && sort > numbers) < numbers
	68 .DE
	69 .
	70 .PP
	71 This approach requires no dependencies and will work in any Unix
	72 shell.