Hello Sirs,

I have a question about using backslash with grep. For searching -5 pattern in the below file.

[root@localhost script-practice]# cat phone.list
Smith, Terry 7-7989
Adams, Fran 2-3876
StClair, Pat 4-6122
Brown, Robin 1-3745
Stair, Chris 5-5972
Benson, Sam 4-5587

In the shell programming book by Ray Swartz, it is mentioned that to perform a grep operation for -5 pattern search we need to use \(backslash) before - to pass it to the grep as a character pattern to search but not as an option. It is mentioned that we straight away can't use grep \-5 phone.list instead we need to use grep \\-5 phone.list or grep '\-5' phone.list. Why do we have to use the second \ when single \ already takes care of quoting the following character - and removing its special meaning and treats it as a normal character. Please explain. Thanks in advance.

The command line is first processed by the shell. The result of an unquoted \- will just be -, and that is all that will be passed to grep, which will treat it as an option flag. The double \\ or a quoted '\' will result in a single \ being passed to grep.

But, this is really the wrong way to indicate that "-5" is not a flag option. The right way isThe "--" is the standard way to indicate that no more option arguments follow, and that nothing that follows should be treated as an option.

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Thank you very much for the explanation rknichols.

A practical example of this, say you are search for an instance of code in a script:

awk '{print $1}' input

The "'" "{" "}" symbols would try to be interpreted by grep as part of the grep regex functions. So you would use the "" symbol to escape them:

Incorrect
grep "awk '{print $1}' input" myfile

Correct
grep "awk \'\{print $1\}\' input" myfile

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Use single quotes to protect regex from the shell thus

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you can also try [], that is sometimes more readable:
grep '[-]5'

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Just to add to this comment the difference between single and double quote.

'123 $A$B$C xyz' verses "123 $A$B$C xyz"

In the single quotes the variable will get passed literally as $A$B$C, where as in the double quote the variables will get processed showing what information is stored and then passed.

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