找到匹配时使用sed替换行的开头

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 .

Answer 2

Try:

sed -n '/myVar/{s|^|//|};p' MyFile.java

which means: when a line contains myVar , replace the beginning of the line with // .

Answer 3

I was researching the same topic and found this solution which is simpler in terms of the regex

sed -e '/myvar/ s/^/\\/\\//' file

This adds // to column 0 of the line with the matching pattern.

However, I was looking for a solution which will allow me to add a character before the first character of the line (not on column 0).