imported>WikiSysop |
imported>ThorstenStaerk |
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| − | Regular expressions allow you to formulate patterns to search for. Here's an example: It is easy to search for the string "Sep" in a file, you do it with
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| − | [[grep]] "Sep" file
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| − | This gives you all lines containing the string "Sep". But what do you do if you only want lines ''starting'' with "Sep", for example, to read all lines in your syslog regarding september? Then you need '''regular expressions'''. It works like this:
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| − | grep -E "^Sep" /var/log/messages
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| − | gives you all entries for september in your syslog. And there is much more you can do with regular expressions.
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| − | = Escaping =
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| − | The characters ^ and \ are seen as control-characters. ^ means "at the beginning of a line". With a backslash, you can ''escape'' these control-characters, meaning they act as body-characters again:
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| − | grep "^hallo" file
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| − | finds all occurrences of "hallo" at the beginning of a line in ''file''.
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| − | grep "\^hallo"
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| − | finds all occurrences of "^hallo" in a file
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| − | grep "\\^hallo"
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| − | finds all occurrences of "\^hallo" in a file
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| − | grep "\\\\^hallo"
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| − | finds all occurrences of "\\^hallo" in a file
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| − | And so on...
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| − | = Write regular expressions =
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| − | == Matching ==
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| − | For "finding a pattern defined by a regular expression", we speak of "matching".
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| − | === Match at the beginning of a line ===
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| − | grep "^hallo" ''file''
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| − | prints all occurrences of "hallo" at the beginning of a line in ''file''.
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| − | === Match at the end of a line ===
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| − | grep "hallo$" ''file''
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| − | prints all occurrences of "hallo" at the end of a line in ''file''.
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| − | === Match string1 OR string2 ===
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| − | grep -E "Sep|Aug" ''file''
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| − | prints all lines from ''file'' that contain "Sep" ''or'' "Aug".
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| − | === Match a group of characters ===
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| − | grep -E "L[I,1]NUX" ''file''
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| − | prints all lines from ''file'' that contain "LINUX" or "L1NUX"
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| − | === Match a range of characters ===
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| − | grep -E "foo[1-9]" ''file''
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| − | prints all lines from ''file'' that contain "foo1" or "foo2" till "foo9"
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| − | === Invert a group of characters ===
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| − | grep -E "for[^ e]" ''file''
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| − | prints all lines from ''file'' that contain "for", but not followed by a space or an e, so not "for you" or "foresee"
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| − | === Invert matches ===
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| − | grep -Ev "gettimeofday" ''file''
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| − | prints all lines from ''file'' that do NOT contain "gettimeofday". This is a grep feature.
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| − | === Match any character ===
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| − | grep -E "L.nux" ''file''
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| − | matches any character that is not a newline, e.g. Linux, Lenux and Lnux in ''file''.
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| − | === Match one or more times ===
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| − | grep -E "L[i]+nux" ''file''
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| − | Match if i is there at least once in ''file''
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| − | The + here is a quantifier. It means, that i occurs 1 or more times. It is also possible to accept 0 or more times if you replace the + by a *.
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| − | = Read regular expressions =
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| − | == * ==
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| − | An asterisk is a quantifier saying "whatever number of".
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| − | grep -E "Li*nux" file
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| − | Lnux
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| − | Linux
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| − | Liinux
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| − | Liiinux
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| − | An asterisk is placed next to an atom that can be repeated in whatever number. In the above example, the atom is the ''i'' character, but it can also be a group of characters:
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| − | grep -E "ba(na)*" file
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| − | ba
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| − | bana
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| − | banana
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| − | bananana
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