63 lines
3.2 KiB
Markdown
63 lines
3.2 KiB
Markdown
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# The `grep` command
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The `grep` filter searches a file for a particular pattern of characters, and displays all lines that contain that pattern.
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grep stands for globally search for regular expression and print out. The pattern that is searched in the file is referred to as the regular expression.
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### Examples:
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1. To search the contents of the destination.txt file for a string("KeY") case insensitively.
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```
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grep -i "KeY" destination.txt
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```
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2. Displaying the count of number of matches
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```
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grep -c "key" destination.txt
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```
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3. We can search multiple files and only display the files that contains the given string/pattern.
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```
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grep -l "key" destination1.txt destination2.txt destination3.xt destination4.txt
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```
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4. To show the line number of file with the line matched.
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```
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grep -n "key" destination.txt
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```
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5. If you want to grep the monitored log files, you can add the `--line-buffered` to search them in real time.
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```
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tail -f destination.txt | grep --line-buffered "key"
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```
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### Syntax:
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The general syntax for the grep command is as follows:
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```
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grep [options] pattern [files]
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```
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### Additional Flags and their Functionalities:
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| **Short Flag** | **Long Flag** | **Description** |
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| :------------- | :--------------------- | :---------------------------------------------------------------------------------------------- |
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| `-c` | `--count` | print a count of matching lines for each input file |
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| `-h` | `--no-filename` | Display the matched lines, but do not display the filenames |
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| `-i` | `--ignore-case` | Ignores, case for matching |
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| `-l` | `--files-with-matches` | Displays list of a filenames only. |
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| `-n` | `--line-number` | Display the matched lines and their line numbers. |
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| `-v` | `--invert-match` | This prints out all the lines that do not matches the pattern |
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| `-e` | `--regexp=` | Specifies expression with this option. Can use multiple times |
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| `-f` | `--file=` | Takes patterns from file, one per line. |
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| `-F` | `--fixed-strings=` | Interpret patterns as fixed strings, not regular expressions. |
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| `-E` | `--extended-regexp` | Treats pattern as an extended regular expression (ERE) |
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| `-w` | `--word-regexp` | Match whole word |
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| `-o` | `--only-matching` | Print only the matched parts of a matching line, with each such part on a separate output line. |
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| | `--line-buffered` | Force output to be line buffered. |
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