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article-athas-shell-redirections.mw - tgtimes - The Gopher Times |
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git clone git://bitreich.org/tgtimes git://enlrupgkhuxnvlhsf6lc3fziv5h2hhfrinws65d7roiv6bfj7d652fid.onion/tgtimes (git://bitreich.org) |
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README |
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article-athas-shell-redirections.mw (2204B) |
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--- |
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1 .SH athas |
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2 Shell Redirections |
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3 . |
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4 .PP |
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5 Newcomers to the Unix shell quickly encounter handy tools such as |
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6 sed(1) and sort(1). This command prints the lines of the given file |
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7 to stdout, in sorted order: |
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8 . |
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9 .DS |
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10 $ sort numbers |
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11 .DE |
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12 . |
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13 .PP |
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14 Soon after, newcomers will also encounter shell redirection, by which |
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15 the output of these tools can conveniently be read from or stored in |
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16 files: |
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17 . |
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18 .DS |
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19 $ sort < numbers > numbers_sorted |
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20 .DE |
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21 . |
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22 .PP |
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23 Our new user, fascinated by the modularity of the Unix shell, may then |
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24 try the rather obvious possibility of having the input and output file |
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25 be the same: |
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26 . |
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27 .DS |
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28 $ sort < numbers > numbers |
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29 .DE |
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30 . |
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31 .PP |
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32 But disaster strikes: the file is empty! The user has lost their |
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33 precious collection of numbers - let's hope they had a backup. Losing |
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34 data this way is almost a rite of passage for Unix users, but let us |
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35 spell out the reason for those who have yet to hurt themselves this |
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36 way. |
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37 . |
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38 .PP |
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39 When the Unix shell evaluates a command, it starts by processing the |
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40 redirection operators - that's the '>' and '<' above. While '<' just |
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41 opens the file, '>' *truncates* the file in-place as it is opened for |
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42 reading! This means that the 'sort' process will dutifully read an |
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43 empty file, sort its non-existent lines, and correctly produce empty |
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44 output. |
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45 . |
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46 .PP |
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47 Some programs can be asked to write their output directly to files |
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48 instead of using shell redirection (sed(1) has '-i', and for sort(1) |
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49 we can use '-o'), but this is not a general solution, and does not |
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50 work for pipelines. Another solution is to use the sponge(1) tool |
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51 from the "moreutils" project, which stores its standard input in |
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52 memory before finally writing it to a file: |
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53 . |
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54 .DS |
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55 $ sort < numbers | sponge numbers |
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56 .DE |
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57 . |
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58 .PP |
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59 The most interesting solution is to take advantage of subshells, the |
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60 shell evaluation order, and Unix file systems semantics. When we |
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61 delete a file in Unix, it is removed from the file system, but any |
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62 file descriptors referencing the file remain valid. We can exploit |
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63 this behaviour to delete the input file *after* directing the input, |
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64 but *before* redirecting the output: |
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65 . |
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66 .DS |
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67 $ (rm numbers && sort > numbers) < numbers |
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68 .DE |
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69 . |
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70 .PP |
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71 This approach requires no dependencies and will work in any Unix |
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72 shell. |
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