sed + remove "#" and empty lines with one sed command

Question

how to remove comment lines (as # bal bla ) and empty lines (lines without charecters) from file with one sed command?

THX lidia

Answer 1

If you're worried about starting two sed processes in a pipeline for performance reasons, you probably shouldn't be, it's still very efficient. But based on your comment that you want to do in-place editing, you can still do that with distinct commands (sed commands rather than invocations of sed itself).

You can either use multiple -e arguments or separate commands with a semicolon, something like (just one of these, not both):

sed -i 's/#.*$//' -e '/^$/d' fileName sed -i 's/#.*$//;/^$/d' fileName 

The following transcript shows this in action:

pax> printf 'Line # with a comment\n\n# Line with only a comment\n' >file  pax> cat file Line # with a comment  # Line with only a comment  pax> cp file filex ; sed -i 's/#.*$//;/^$/d' filex ; cat filex Line  pax> cp file filex ; sed -i -e 's/#.*$//' -e '/^$/d' filex ; cat filex Line 

Note how the file is modified in-place even with two -e options. You can see that both commands are executed on each line. The line with a comment first has the comment removed then all is removed because it's empty.

In addition, the original empty line is also removed.

Answer 2

@paxdiablo has a good answer but it can be improved.

(1) The '/^$/d' clause only matches 100% blank lines.

If you want to also match lines that are entirely whitespace (spaces, tabs etc.) use this instead:

'/^\s*$/d' 

(2) The 's/#.*$//' clause only matches lines that start with the # character in column 0.

If you want to also match lines that have only whitespace before the first # use this instead:

'/^\s*#.*$/d' 

The above criteria may not be universal (e.g. within a HEREDOC block, or in a Python multi-line string the different approaches could be significant), but in many cases the conventional definition of "blank" lines include whitespace-only, and "comment" lines include whitespace-then-#.

(3) Lastly, on OSX at least, the @paxdiablo solution in which the first clause turns comment lines into blank lines, and the second clause strips blank lines (including what were originally comments) doesn't work. It seems to be more portable to make both clauses /d delete actions as I've done.

The revised command incorporating the above is:

sed -e '/^\s*#.*$/d' -e '/^\s*$/d' inputFile