Using sed to replace beginning of line when match found

Question

I have a Java file. I want to comment any line of code that contains the match:

 myvar 

I think sed should help me out here

 sed 's/myVar/not_sure_what_to_put_here/g' MyFile.java 

I don't know what to put in:

not_sure_what_to_put_here 

as in this case, I don't want to replace myVar but the I want to insert

// 

to the beginning of any line myVar occurs on.

Any tips

Answer 1

Capture the whole line that contains myvar:

$ sed 's/\(^.*myvar.*$\)/\/\/\1/' file  $ cat hw.java class hw {     public static void main(String[] args) {         System.out.println("Hello World!");          myvar=1     } }  $ sed 's/\(^.*myvar.*$\)/\/\/\1/' hw.java class hw {     public static void main(String[] args) {         System.out.println("Hello World!");  //        myvar=1     } } 

Use the -i option to save the changes in the file sed -i 's/\(^.*myvar.*$\)/\/\/\1/' file.

Explanation:

(      # Start a capture group ^      # Matches the start of the line  .*     # Matches anything  myvar  # Matches the literal word  .*     # Matches anything $      # Matches the end of the line )      # End capture group  

So this looks at the whole line and if myvar is found the results in stored in the first capture group, referred to a \1. So we replace the whole line \1 with the whole line preceded by 2 forward slashes //\1 of course the forwardslashes need escaping as not to confused sed so \/\/\1 also note that brackets need escaping unless you use the extended regex option of sed.