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I have a configuration file and I want to replace a line containing a specific string, lets say test :


// test
// test

My line of code is not working, I'm new to sed.

sed -i `s#^//[ \t]test#test#g'

Some guidance ?

share|improve this question
its an interesting question - any reason why you wish to delete this? –  fossfreedom Jan 30 '13 at 12:40
I'm working on a script that automatically configures and installs lamp so I need to uncomment two lines in phpmyadmin conf file to accept nopasswd login via the web interface. –  neuraload Jan 30 '13 at 12:48
@pyn Consider using tools designed specifically for deploying applications instead: Puppet, Juju, Chef, etc. Writing your own scripts to deploy applications is usually reinventing wheels and/or axles. –  gertvdijk Jan 30 '13 at 12:49
I'm doing this for learning purposes, to get familiar with bash scripting ( CLI ). –  neuraload Jan 30 '13 at 12:53

2 Answers 2

up vote 3 down vote accepted

First of all, I'd recommend to avoid the use of -i as it will replace the files in-place without confirmation.

This is a working version of what you were trying to accomplish and outputs to the standard output (the terminal):

sed 's#^//[ \t]test$#test#g' file.conf

Optionally pipe it through less by appending | less to be able to paginate through large amounts of output.

The above is all very specific to the sequence of test, so to more generalize, you can try this:

sed 's#^//\s\(.*\)$#\1#g' file.conf

It will remove the // part and any whitespace until the actual text, whatever this text is.

If you like the result, you can add the -i flag again to replace the file.

To tell more about your attempt and explain why my example does work:

sed -i `s#^//[ \t]test#test#g'
  • You are using a backtick (`) which is interpreted by your shell rather than Sed. A simple single quote (') should have been used here probably.
  • The hashes (#) are your chosen delimiter, a good alternative to the common forward slash (/) as your match is about this same character and this avoids the need for escaping.
  • The pattern in the command s/regexp/replacement/ is a standard one as mentioned in the manpage, among many others: man 1 sed:


    Attempt to match regexp against the pattern space. If successful, replace that portion matched with replacement. The replacement may contain the special character & to refer to that portion of the pattern space which matched, and the special escapes \1 through \9 to refer to the corresponding matching sub-expressions in the regexp.

  • rexexp in the above is a commonly used abbreviation for regular expressions.
  • The regular expression ^//[ \t]test$ matches lines starting (^) with two forward slashes, followed by a space or a tab character, followed by the exact sequence test and a line ending ($).
  • The g flag is to do this operation in a global way as explained here.
  • An input file is missing and should be given after the Sed-command.
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1. backtick, I miss typed 2. I was thinking to use a different delimiter fo clarity purposes 3. Yes, I know. –  neuraload Jan 30 '13 at 12:41

You do not have to use forward slashes as separator. In fact, I recommend using other separators like comma or hashes for readability.

-i is also your friend, but only use it if you know for sure it is correct. In this case, I would do something like:

sed -r 's,^//[ \t]*(.+),\1,' file.conf | less

I use -r here for extended regexp, so I do not have to escape the parentheses. Furthermore, it allows me to use the plus operator which means "one or more occurrences". The following expression would be equivalent:

sed 's,^//[ \t]*\(..*\),\1,' file.conf | less

(one dot to match at least one character followed by zero or more of any character)

After I am sure that the replacement is correct (type slash followed by your keywords in less for searching), I close less by typing q and navigate to the previous history entry by pressing the Up key. I then delete | less and type -i to proceed with the replacement:

sed -r 's,^//[ \t]*(.+),\1,' file.conf -i

If you want to gain a deeper understanding of sed, I can recommend you reading the info page of sed which is more extended than the manual page:

info sed
share|improve this answer
I didn't know about the use of alternative delimiters. Thanks - I was wrong in my answer apparently - fixed that now. :) –  gertvdijk Jan 30 '13 at 13:53
This^ is what I said above.:) –  neuraload Jan 30 '13 at 14:18

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