GopherProxy

	article-athas-shell-redirections.md - tgtimes - The Gopher Times
	git clone git://bitreich.org/tgtimes git://enlrupgkhuxnvlhsf6lc3fziv5h2hhfrinws…
	Log
	Files
	Refs
	Tags
	README
	---
	article-athas-shell-redirections.md (2124B)
	---
	1 # Shell Redirections by athas
	2
	3 Newcomers to the Unix shell quickly encounter handy tools such as
	4 sed(1) and sort(1). This command prints the lines of the given file
	5 to stdout, in sorted order:
	6
	7 $ sort numbers
	8
	9 Soon after, newcomers will also encounter shell redirection, by which
	10 the output of these tools can conveniently be read from or stored in
	11 files:
	12
	13 $ sort < numbers > numbers_sorted
	14
	15 Our new user, fascinated by the modularity of the Unix shell, may then
	16 try the rather obvious possibility of having the input and output file
	17 be the same:
	18
	19 $ sort < numbers > numbers
	20 But disaster strikes: the file is empty! The user has lost their
	21 precious collection of numbers - let's hope they had a backup. Losing
	22 data this way is almost a rite of passage for Unix users, but let us
	23 spell out the reason for those who have yet to hurt themselves this
	24 way.
	25
	26 When the Unix shell evaluates a command, it starts by processing the
	27 redirection operators - that's the '>' and '<' above. While '<' just
	28 opens the file, '>' truncates the file in-place as it is opened for
	29 reading! This means that the 'sort' process will dutifully read an
	30 empty file, sort its non-existent lines, and correctly produce empty
	31 output.
	32
	33 Some programs can be asked to write their output directly to files
	34 instead of using shell redirection (sed(1) has '-i', and for sort(1)
	35 we can use '-o'), but this is not a general solution, and does not
	36 work for pipelines. Another solution is to use the sponge(1) tool
	37 from the "moreutils" project, which stores its standard input in
	38 memory before finally writing it to a file:
	39
	40 $ sort < numbers \| sponge numbers
	41
	42 The most interesting solution is to take advantage of subshells, the
	43 shell evaluation order, and Unix file systems semantics. When we
	44 delete a file in Unix, it is removed from the file system, but any
	45 file descriptors referencing the file remain valid. We can exploit
	46 this behaviour to delete the input file after directing the input,
	47 but before redirecting the output:
	48
	49 $ (rm numbers && sort > numbers) < numbers
	50
	51 This approach requires no dependencies and will work in any Unix
	52 shell.