how can I "cat" or "grep" a file to ignore lines starting with "#" ???

callagga · 07-09-2008, 03:56 PM

Hi,

How can I "cat" or "grep" a file to ignore lines starting with "#" ???

That is, to list config files that have more comments "# xxxx" than content often. If I just want to see the file without comments and perhaps blank lines.

Cheers

matthewg42 · 07-09-2008, 04:00 PM

Code:

grep -v '^#' filename

callagga · 07-09-2008, 04:04 PM

arrr, excellent thanks - if I wanted to ignore blank lines as well do you know what it would be then?

David the H. · 07-09-2008, 04:09 PM

To provide a little more explanation for the above command, the caret '^' represents the start of a line, so you're testing only for # symbols that appear at the beginning of each line. And the '-v' option in grep inverts the selection, so only lines that don't match get displayed.

I suggest working your way through a simple Regular Expression (regex) tutorial (such as this one) if you really want to know how to harness the power of text matching. It's really worth the effort.

Edit: You can pipe the output through grep again to remove the blank lines. There may be more 'proper' ways to do it, but quick and dirty:

Code:

grep -v '^#' filename | grep -v '^$'

The '$' sign matches the end of a line.

callagga · 07-09-2008, 04:31 PM

thanks - I'd just tried this myself & was pondering a better way to do it

David the H. · 07-09-2008, 04:45 PM

Actually, thinking about it a bit more, a cleaner way would be to use an 'or' operator so you can include both patterns in a single operation. You need to use switch to egrep though.

Code:

egrep -v '(^#|^$)' filename

or alternately

egrep -v '^(#|$)' filename

syg00 · 07-09-2008, 04:58 PM

I always use "egrep -v '(^#|^\s*$)' filename" - especially on Debian derived systems.

kilrogg · 08-16-2013, 06:58 AM

I tend to use egrep -v '(^#|^\s*$|^\s*\t*#)' filename which also excludes comment lines starting with TAB or SPACE and nothing else before the comment (frequent in config files with multi-line comments behind a setting).