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1-article-athas-shell-redirections.md (2125B)
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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
21 But disaster strikes: the file is empty! The user has lost their
22 precious collection of numbers - let's hope they had a backup. Losing
23 data this way is almost a rite of passage for Unix users, but let us
24 spell out the reason for those who have yet to hurt themselves this
25 way.
26
27 When the Unix shell evaluates a command, it starts by processing the
28 redirection operators - that's the '>' and '<' above. While '<' just
29 opens the file, '>' *truncates* the file in-place as it is opened for
30 reading! This means that the 'sort' process will dutifully read an
31 empty file, sort its non-existent lines, and correctly produce empty
32 output.
33
34 Some programs can be asked to write their output directly to files
35 instead of using shell redirection (sed(1) has '-i', and for sort(1)
36 we can use '-o'), but this is not a general solution, and does not
37 work for pipelines. Another solution is to use the sponge(1) tool
38 from the "moreutils" project, which stores its standard input in
39 memory before finally writing it to a file:
40
41 $ sort < numbers | sponge numbers
42
43 The most interesting solution is to take advantage of subshells, the
44 shell evaluation order, and Unix file systems semantics. When we
45 delete a file in Unix, it is removed from the file system, but any
46 file descriptors referencing the file remain valid. We can exploit
47 this behaviour to delete the input file *after* directing the input,
48 but *before* redirecting the output:
49
50 $ (rm numbers && sort > numbers) < numbers
51
52 This approach requires no dependencies and will work in any Unix
53 shell.