Ubuntu – how to remove two forward slashes in a comment with sed

sed

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

file.conf 

aaa
bbb
ccc
// test
ddd
// test

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

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

Some guidance ?

Best Answer

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
aaa
bbb
ccc
test
ddd
test

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:

    s/regexp/replacement/

    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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