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1 <stránka |
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2 xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana" |
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3 xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro"> |
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4 |
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5 <nadpis>Formatting fstab</nadpis> |
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6 <perex>implementing a simple relpipe-out-fstab filter using -in-fstab, -out-nullbyte, xargs and Perl</perex> |
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7 <m:pořadí-příkladu>00300</m:pořadí-příkladu> |
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8 |
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9 <text xmlns="http://www.w3.org/1999/xhtml"> |
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10 |
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11 <p> |
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12 As we have seen before, we can convert <code>/etc/fstab</code> (or <code>mtab</code>) |
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13 to e.g. an XML or a nice and colorful table using <m:name/>. |
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14 But we can also convert these data back to the <code>fstab</code> format. And do it with proper indentation/padding. |
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15 Fstab has a simple format where values are separated by one or more whitespace characters. |
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16 But without proper indentation, these files look a bit obfuscated and hard to read (however, they are valid). |
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17 </p> |
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18 |
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19 <m:pre jazyk="text" src="examples/relpipe-out-fstab.txt"/> |
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20 |
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21 <p> |
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22 So let's build a pipeline that reformats the <code>fstab</code> and makes it more readable. |
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23 </p> |
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24 |
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25 <m:pre jazyk="bash">relpipe-in-fstab | relpipe-out-fstab > reformatted-fstab.txt</m:pre> |
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26 |
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27 <p> |
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28 We can hack together a script called <code>relpipe-out-fstab</code> that accepts relational data and produces <code>fstab</code> data. |
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29 Later this will be probably implemented as a regular tool, but for now, it is just an example of a ad-hoc shell script: |
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30 </p> |
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31 |
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32 <m:pre jazyk="bash" src="examples/relpipe-out-fstab.sh" odkaz="ano"/> |
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33 |
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34 <p> |
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35 In the first part, we prepend a single record (<code>relpipe-in-cli</code>) before the data coming from STDIN (<code>cat</code>). |
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36 Then, we use <code>relpipe-out-nullbyte</code> to convert relational data to values separated by a null-byte. |
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37 This command processes only attribute values (skips relation and attribute names). |
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38 Then we used <code>xargs</code> to read the null-separated values and execute a Perl command for each record (pass to it a same number of arguments, as we have attributes: <code>--max-args=7</code>). |
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39 Perl does the actual formatting: adds padding and does some little tunning (merges two attributes and replaces empty values with <em>none</em>). |
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40 </p> |
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41 |
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42 <p>This is formatted version of the <code>fstab</code> above:</p> |
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43 |
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44 <m:pre jazyk="text" src="examples/relpipe-out-fstab.formatted.txt"/> |
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45 |
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46 <p> |
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47 And using following command we can verify, that the files differ only in comments and whitespace: |
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48 </p> |
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49 |
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50 <pre>relpipe-in-fstab | relpipe-out-fstab | diff -w /etc/fstab -</pre> |
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51 |
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52 <p> |
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53 Another check (should print same hashes): |
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54 </p> |
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55 |
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56 <pre><![CDATA[relpipe-in-fstab | sha512sum |
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57 relpipe-in-fstab | relpipe-out-fstab | relpipe-in-fstab | sha512sum]]></pre> |
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58 |
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59 <p> |
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60 Regular implementation of <code>relpipe-out-fstab</code> will probably keep the comments |
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61 (it needs also one more attribute and small change in <code>relpipe-in-fstab</code>). |
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62 </p> |
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63 |
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64 <p> |
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65 For just mere <code>fstab</code> reformatting, this approach is a bit overengineering. |
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66 We could skip the whole relational thing and do just something like this: |
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67 </p> |
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68 |
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69 <m:pre jazyk="bash">cat /etc/fstab | grep -v '^#' | sed -E 's/\s+/\n/g' | tr \\n \\0 | xargs -0 -n7 ...</m:pre> |
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70 |
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71 <p> |
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72 plus prepend the comment (or do everything in Perl). |
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73 But this example is intended as a demostration, how we can |
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74 1) prepend some additional data before the data from STDIN |
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75 2) use <m:name/> and traditional tools like <code>xargs</code> or <code>perl</code> together. |
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76 And BTW we have implemented a (simple but working) <em>relpipe output filter</em> – and did it without any serious programming, just put some existing commands together :-) |
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77 </p> |
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78 |
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79 <blockquote> |
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80 <p> |
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81 There is more Unix-nature in one line of shell script than there is in ten thousand lines of C. |
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82 <m:podČarou>see <a href="http://www.catb.org/~esr/writings/unix-koans/ten-thousand.html">Master Foo and the Ten Thousand Lines</a></m:podČarou> |
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83 </p> |
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84 </blockquote> |
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85 |
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86 </text> |
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87 |
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88 </stránka> |