Deleting (some) comments from a C program

Question

I need to use a bash script to delete full-line old-style comments from a C program, i.e., comments that begin (/*) and end (*/) on the same line, with no code on the same line.  This is an example of what the C program looks like:

/* Comment 1 */
printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
            /* Comment 4 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7

But I need it to look like this:

printf("It is /* Comment 2 */\n");
 x = 5; /* Comment 3 */
 /* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7

I do know how to delete all comments but just not sure on how to just remove certain ones.

The script should read inputs from a text file, and write outputs into another file, and all the I/O file names must be given in the command-line.

Answer 1

This sed one is portable:

sed '\_^[[:blank:]]*/\*.*\*/[[:blank:]]*$_d' file.c

All lines that begin (^) with zero or more blanks ([[:blank:]]*), start a comment (/\*), have anything else, (.*), close the comment (\*/) and have nothing but blanks in the rest of the line ([[:blank:]]*) will be deleted. Of course, you can also do that with grep -v.

Be aware that this will also delete lines like

/* between two comments */ x = 0; /* could be some code */

Answer 2

This is the same as Philippos’s answer except

• It uses | as the regular expression delimiter (my personal preference).
• It uses [[:space:]] instead of [[:blank:]].  [[:space:]] includes such non-graphic characters as vertical tab, form feed, and carriage return (in addition to space and tab); since C treats all those whitespace characters as blank spaces, [[:space:]] is really the better character class to use for handling C code.  And
• It handles the edge case of multiple comments on the same line.

sed '\|^[[:space:]]*/\*.*\*/[[:space:]]*$| { \|\*/.*[^[:space:]]|!d }'

As in Philippos’s answer, it checks if the first non-blank thing on the line is /* and the last non-blank thing on the line is */.  If that’s true, we have a possible full-line comment; a candidate for removal.  In that case, enter the {…} and look for a */ followed by something non-blank; i.e., a */ that’s not the last non-blank thing on the line.  If we find that, then we know that we have found the end of the first comment, and that there’s something else on the line.  In that case, do nothing.  If we don’t find a */ in the interior of the line, then delete the line.

Answer 3

Tested with sed command and worked fine

command:

  sed -r "s/^\s+//g" filename| sed '/^\/\*.*\*\/$/d'

output:

printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
/* Comment 5 */ y = 0;
/*
* Comment 6
*/
// Comment 7

Answer 4

This should get past the issue of code bracketed between comments

sed  -E '/^\s*\/\*/!bx ; /\*\/\s*$/!bx ; /\*\/\s*\S+.*\/\*/bx ; d;  :x' draft 

If a line doesn't start with a comment marker led only by whitespace then it starts with code so branch past the delete to x

/^\s*\/\*/!bx    

If the line starting as a comment doesn't end as a comment followed only by whitespace then there is code at the end so branch past the delete to x

/\*\/\s*$/!bx

These first two tests can be combined as

/^\s*\/\*.*\*\/\s*$/!bx

If the comment line contains a clsing comment marker followed by at least one non-whitespace character and then another comment start then there is code inside so branch past the delete to x

/\*\/\s*\S+.*\/\*/bx

Since we found no valid code then delete

d

Otherwise finish without doing anything

x

Tested on

/* Comment 1 */
printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
            /* Comment 4 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0;  /*some code */

Output is

printf("It is /* Comment 2 */\n");
x = 5; /* Comment 3 */
/* Comment 5 */ y = 0;
            /*
             * Comment 6
             */
            // Comment 7
/* between two comments */ x = 0; /*some code */