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#1
01-26-2011
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Grepping for hex characters - explanation?
Hello, Yesterday I was looking for a way to grep for a tab in the shell, and found this solution in several places: Code:
grep $'[\x09]a[\x09]' # Grep for the letter 'a' between two tabs I'm fine with most of this, but I don't understand what the $ (dollar sign) before the first quote does. It doesn't work without, but I couldn't find any explanation in the grep man or info pages. The only mention of $ there is as a meta-character that matches the end of a regular expression. Can someone explain and/or point me to other documentation where I can read it up? Thanks! |
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#2
01-26-2011
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The $ part actually happens in the shell. Code:
$ echo $'[\x09]a[\x09]' | hexdump -C 00000000 5b 09 5d 61 5b 09 5d 0a |[.]a[.].| 00000008 $ So it's not a special option to grep, it's actually an expression that feeds these characters into grep's expression raw. That's interesting. I've occasionally used that syntax for making tabs like $'\t' but didn't know you could put whole strings in that. |
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#3
01-26-2011
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My take is the $ is literal, but regex keep morphing under my feet: Regular-Expressions.info - Regex Tutorial, Examples and Reference - Regexp Patterns I usually just use the tab key or ctrl-V ctrl-whatever, in ' ', or $(echo a|tr 'a' '\027') (but my tr only does octal). But I use ksh, this is a bash-ism: Code:
$ bash <<! echo $'\x61' ! a $ Last edited by DGPickett; 01-26-2011 at 05:32 PM.. |
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#4
01-26-2011
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Quote:
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#5
01-26-2011
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Quote:
Code:
$ echo -e "hello world\n\n" hello world $ echo "hello world\n\n" hello world\n\n $ The string is fed into echo as is, leaving \n as two characters, \ and n. When you give echo -e you tell it to understand and translate that sort of escape sequence. but if you tell echo this: Code:
$ echo $'hello world\n\n' hello world $ ...echo doesn't have to translate. The argument is translated before the command is run, by the shell. The characters generated by the escape sequence get fed straight into it. |
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#6
01-27-2011
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This construction can be used in bash and ksh93:
man ksh: Quote:
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| The Following User Says Thank You to Scrutinizer For This Useful Post: | ||
mregine (01-27-2011) | ||
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#7
01-27-2011
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Thanks to both of you. I've learned something and I even know where to look for more :-)
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