Edward Bartolo <edbarx@???> writes:
> sed -i "/Comment=/c\ ${comment}" "${launcher}"
>
> Hi Rainer et al,
>
> Using the above command results in replacing lines containing
> "Comment=" by the value of launcher plus one leading space. The weird
> thing is that I found that if I remove the space between
> "/Comment=/c\" and "${comment}", the line will be replaced by the text
> "${Comment}". I am attributing this to a possible shell bug.

Assuming comment is "tnemmoc", running

"/Comment=/c\ ${comment}"

through the shell will result in the string

/Comment=/c tnemmoc

as the shell interprets the \ as "escape the next character" and
escaping a blank is a no-op. If the string's

"/Comment=/c\${comment}"

the $ will be escaped, hence, no interpolation is done and the resulting
string becomes

/Comment=/c${comment}

sed -i "/Comment=/c$comment"

works as intended.

NB: Despite GNU sed has meanwhile gained transparent support for the
usual temporary-file-and-rename hacks, it's still the stream editor and
hence, restricted to one-pass algorithms and the UNIX(*) line-editor is
still called ed.

A one-line removing all lines starting with Comment= followed by adding
the content of the variable comm could be:

printf 'g/^Comment=/d\na\n%s\n.\nwq\n' "$comment" | ed netman.desktop

This uses printf to do the interpolation so that the template string
can be quoted with ' and hence, the shell won't mess with its contents.