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I'm console newbie. As I know, matching multiple patterns like this:

aaa|bbb

But, | character is pipe on console, so how can I specify those multiple patterns for grep?

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7 Answers

up vote 3 down vote accepted

You can escape the pipe and put the pattern in quotes:

grep "aaa\|bbb"

or use -E:

grep -E "aaa|bbb"

or

grep -E aaa\|bbb
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The backslash in the first example is unnecessary; the shell removes it and grep never sees it, but the shell would pass the same info to grep if you omitted the backslash. – Jonathan Leffler Jun 27 '10 at 2:53
@Jonathan: Not so. Try this with and without the backslash. Only the one with the backslash will succeed. echo -e 'aaa\nbbb\nccc' | grep "aaa\|bbb" – Dennis Williamson Jun 27 '10 at 3:30
@Jonathan: You're better to read about escaping and quotation rule in ΤΖΩΤΖΙΟΥ's answer. – Eonil Jun 27 '10 at 8:42
Different shells ... and the question is meant to be about grep; oh well. Stuff that worked one way 20 years ago doesn't seem to work the same now - and I can't tell whether I'm misremembering or whether the shells have quietly changed behind my back. – Jonathan Leffler Jun 27 '10 at 20:02
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up vote 3 down vote

Put "" between your pattern, like egrep "toto|name"

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up vote 2 down vote

Or you go

grep -ie aaa -ie bbb filename

to grep for aaa or bbb in filename, case insensitively.

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Useful information. Thanks! – Eonil Jun 27 '10 at 8:39
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up vote 1 down vote

Lastly, you can put your patterns into a file, and use the -f flag. So grep -f patternlist.txt files. Where patternlist.txt is simply:

aaa
bbb
Make sure there are no empty lines, though.

--Christopher Karel

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up vote 1 down vote

Enclose your pattern in single quotes:

grep -E 'aaa|bbb'

Should your pattern include an apostrophe, enclose it in double quotes:

grep -E "its|it's"

If it contains both single and double quotes, enclose it in double quotes and prefix with a backslash the characters ", $, ` and \:

grep -E "its|it's|letter \"e\"|pay \$20|\`a'|C:\\\\AUTOEXEC\\.BAT"

The final example of C:\AUTOEXEC.BAT takes into account that backslash is special both to the shell and to the regular expression syntax of egrep.

PS: The bash quoting page is a must-read.

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Nice guidance for escaping and quotation. Thanks! – Eonil Jun 27 '10 at 8:41
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up vote 0 down vote

i wonder why this won't work

grep -v 'DST=255.255.255.255\|DST=192.168.1.255'

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egrep "aaa|bbb|ccc" <file>

will find all rows with either aaa OR bbb OR ccc in the file

egrep -i "aaa|bbb|ccc" <file>

will find aaa OR aaA OR aAa OR Aaa OR aAA OR AAa OR AAA OR ... and the same for every other string enumerated between the pipe | characters.

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