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1-article-athas-shell-redirections.md - 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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--- |
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1-article-athas-shell-redirections.md (2125B) |
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--- |
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1 # Shell Redirections by athas |
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2 |
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3 Newcomers to the Unix shell quickly encounter handy tools such as |
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4 sed(1) and sort(1). This command prints the lines of the given file |
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5 to stdout, in sorted order: |
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6 |
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7 $ sort numbers |
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8 |
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9 Soon after, newcomers will also encounter shell redirection, by which |
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10 the output of these tools can conveniently be read from or stored in |
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11 files: |
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12 |
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13 $ sort < numbers > numbers_sorted |
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14 |
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15 Our new user, fascinated by the modularity of the Unix shell, may then |
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16 try the rather obvious possibility of having the input and output file |
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17 be the same: |
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18 |
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19 $ sort < numbers > numbers |
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20 |
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21 But disaster strikes: the file is empty! The user has lost their |
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22 precious collection of numbers - let's hope they had a backup. Losing |
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23 data this way is almost a rite of passage for Unix users, but let us |
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24 spell out the reason for those who have yet to hurt themselves this |
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25 way. |
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26 |
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27 When the Unix shell evaluates a command, it starts by processing the |
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28 redirection operators - that's the '>' and '<' above. While '<' just |
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29 opens the file, '>' *truncates* the file in-place as it is opened for |
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30 reading! This means that the 'sort' process will dutifully read an |
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31 empty file, sort its non-existent lines, and correctly produce empty |
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32 output. |
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33 |
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34 Some programs can be asked to write their output directly to files |
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35 instead of using shell redirection (sed(1) has '-i', and for sort(1) |
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36 we can use '-o'), but this is not a general solution, and does not |
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37 work for pipelines. Another solution is to use the sponge(1) tool |
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38 from the "moreutils" project, which stores its standard input in |
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39 memory before finally writing it to a file: |
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40 |
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41 $ sort < numbers | sponge numbers |
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42 |
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43 The most interesting solution is to take advantage of subshells, the |
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44 shell evaluation order, and Unix file systems semantics. When we |
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45 delete a file in Unix, it is removed from the file system, but any |
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46 file descriptors referencing the file remain valid. We can exploit |
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47 this behaviour to delete the input file *after* directing the input, |
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48 but *before* redirecting the output: |
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49 |
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50 $ (rm numbers && sort > numbers) < numbers |
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51 |
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52 This approach requires no dependencies and will work in any Unix |
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53 shell. |
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