102 lines
2.5 KiB
Markdown
102 lines
2.5 KiB
Markdown
---
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title: GNU grep
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category: CLI
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updated: 2021-08-23
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---
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### Usage
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{: .-prime}
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```bash
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grep <options> pattern <file...>
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```
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### Matching options
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```bash
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-e, --regexp=PATTERN
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-f, --file=FILE
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-i, --ignore-case
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-v, --invert-match
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-w, --word-regexp
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-x, --line-regexp
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```
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### Pattern options
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```bash
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-F, --fixed-strings # list of fixed strings
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-G, --basic-regexp # basic regular expression (default)
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-E, --extended-regexp # extended regular expression
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-P, --perl-regexp # perl compatible regular expression
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```
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### Expressions
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#### Basic Regular Expressions (BRE)
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In BRE, these characters have a special meaning unless they are escaped with a backslash:
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`^ $ . * [ ] \`
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However, these characters do not have any special meaning unless they are escaped with a backslash:
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`? + { } | ( )`
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#### Extended Regular Expressions (ERE)
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ERE gives all of these characters a special meaning unless they are escaped with a backslash:
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`^ $ . * + ? [ ] ( ) | { }`
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#### Perl Compatible Regular Expressions (PCRE)
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PCRE has even more options such as additional anchors and character classes, lookahead/lookbehind, conditional expressions, comments, and more. See the [regexp cheatsheet](/regexp).
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### Output Options
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```bash
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-c, --count # print the count of matching lines. suppresses normal output
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--color[=WHEN] # applies color to the matches. WHEN is never, always, or auto
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-m, --max-count=NUM # stop reading after max count is reached
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-o, --only-matching # only print the matched part of a line
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-q, --quiet, --silent
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-s, --no-messages # suppress error messages about nonexistent or unreadable files
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```
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### Context Options
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```bash
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-B NUM, --before-context=NUM # print NUM lines before a match
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-A NUM, --after-context=NUM # print NUM lines after a match
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-C NUM, -NUM, --context=NUM # print NUM lines before and after a match
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```
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### Examples
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```bash
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# Case insensitive: match any line in foo.txt
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# that contains "bar"
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grep -i bar foo.txt
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# match any line in bar.txt that contains
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# either "foo" or "oof"
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grep -E "foo|oof" bar.txt
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# match anything that resembles a URL in
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# foo.txt and only print out the match
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grep -oE "https?://((\w+[_-]?)+\.?)+" foo.txt
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# can also be used with pipes:
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# match any line that contains "export" in
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# .bash_profile, pipe to another grep that
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# matches any of the first set of matches
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# containing "PATH"
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grep "export" .bash_profile | grep "PATH"
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# follow the tail of server.log, pipe to grep
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# and print out any line that contains "error"
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# and include 5 lines of context
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tail -f server.log | grep -iC 5 error
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```
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