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Old 06-08-2015, 12:39 PM   #1
rookee
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Question on shell scripting: grep with backslash


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.
 
Old 06-08-2015, 01:27 PM   #2
rknichols
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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 is
Code:
grep -- -5 phone list
The "--" 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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Old 06-08-2015, 02:17 PM   #3
rookee
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Thank you very much for the explanation rknichols.
 
Old 06-08-2015, 03:04 PM   #4
joec@home
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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

Last edited by joec@home; 06-08-2015 at 03:06 PM.
 
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Old 06-10-2015, 05:44 AM   #5
chrism01
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Use single quotes to protect regex from the shell thus
Code:
grep '\-5' yourfile

Stair, Chris 5-5972
Benson, Sam 4-5587
 
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Old 06-10-2015, 06:46 AM   #6
pan64
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you can also try [], that is sometimes more readable:
grep '[-]5'
 
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Old 06-10-2015, 01:30 PM   #7
joec@home
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Quote:
Originally Posted by chrism01 View Post
Use single quotes to protect regex from the shell thus
Code:
grep '\-5' yourfile

Stair, Chris 5-5972
Benson, Sam 4-5587
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.

Code:
[root@localhost ~]# A="TEST1"
[root@localhost ~]# B="TEST2"
[root@localhost ~]# C="TEST3"
[root@localhost ~]# echo '123 $A$B$C xyz'
123 $A$B$C xyz
[root@localhost ~]# echo "123 $A$B$C xyz"
123 TEST1TEST2TEST3 xyz
 
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