Delete all lines beginning with a # from a file

This can be done with a sed one-liner:

sed '/^#/d'

This says, "find all lines that start with # and delete them, leaving everything else."

I'm a little surprised nobody has suggested the most obvious solution:

grep -v '^#' filename

This solves the problem as stated.

But note that a common convention is for everything from a # to the end of a line to be treated as a comment:

sed 's/#.*$//' filename

though that treats, for example, a # character within a string literal as the beginning of a comment (which may or may not be relevant for your case) (and it leaves empty lines).

A line starting with arbitrary whitespace followed by # might also be treated as a comment:

grep -v '^ *#' filename

if whitespace is only spaces, or

grep -v '^[  ]#' filename

where the two spaces are actually a space followed by a literal tab character (type "control-v tab").

For all these commands, omit the filename argument to read from standard input (e.g., as part of a pipe).

The opposite of Raymond's solution:

sed -n '/^#/!p'

"don't print anything, except for lines that DON'T start with #"