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article-athas-shell-redirections.md (2124B)
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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 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.