Re: [tlug] Adding text to the beginning of a file

["Stephen J. Turnbull" <turnbull@example.com>]

Arwyn Hainsworth writes:
 > On 16/03/07, Dave M G <martin@example.com> wrote:

 > > Can I wipe out everything up to and including the "--" and then past in
 > > new text?

 > Sed is your friend:
 > 
 > sed '1,/^#--/ d' < file_with_data_to_replace > tmp_file
 > cat file_with_new_data tmp_file > end_result_file

Note that (as in all regexps) whitespace is significant.  So the
target regexp needs to be at least '^# --'.  Note that the classic sig
separator uses significant whitespace to help disambiguate: '^-- $'.
I would suggest matching that seperator exactly: '^# --$'.  This will
help to avoid grief someday when (eg) you include a commented-out mail
message (such as this one, except I currently have no sig :-), where
the commented mail sig separator matches '^# -- $' -- and '^# --' as
well.  You'd be amazed how often the naked eye misses such details
when debugging.

Also, something that might be important in writing a "secure" script
that doesn't have write permission on any directory in particular,
just that file, you can cat stdin explicitly by using '-' as the file
name (and if you actually have a file named '-' in the current
directory, you must qualify it explicitly: './-').  Thus:

sed '1,/^#--/ d' < the_file | cat new_head - > the_file

Note that this may not be robust, although it Works For Me (with bash
2.05b.0(1)-release on Mac OS X).  Those familiar with stdio
redirection will know that '> the_file' truncates the_file before
opening it and passing the fd to the process, but apparently sed gets
to open the file before the shell truncates it for cat.

This may be a race condition that happens to work out, or it may be
required by POSIX, any shell-heads out there that know?