09-02-2011
Afaik, in the general case no. But if your new first line has the same number of bytes or shorter and you can pad it with spaces then I believe you can do it quick with low level programming - open for read/write, read some bytes (512 for example) in a buffer, change them, rewind, and write the buffer back.
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VBUF(1) General Commands Manual VBUF(1)
NAME
vbuf - Virtual Ring Buffer shell interface
SYNOPSIS
vbuf [options]
DESCRIPTION
This manual page documents briefly the vbuf command.
vbuf is a program to glue two programs (a producer and a consumer) together, one of which not being able to fulfil the other's constant
data rate requirements.
OPTIONS
-h
Show short help message.
--help
Show long help message.
-p, --progress
Show progress status line.
-q, --quiet
Suppress progress status line. The default is to show the progress display.
-s size, --size=size
Set buffer size in bytes. Suffixes k, m, and g may be used. The actual size used will be rounded up for system mapping require-
ments. The default is 1m, or 1048576 bytes.
-t time, --time=seconds
Set time in seconds between progress display refreshes. Fractions of a second may be used. Default is 1.500000 seconds.
-b, --bits
Set progress rate display units to bits per second.
-B, --bytes
Set progress rate display units to bytes per second. The default is bytes.
-d, --decimal
Set progress rate display base to decimal. The default is decimal.
-x, --hexadecimal
Set progress rate display base to hexadecimal.
--octal
Set progress rate display base to octal.
-i file, --input=file
Specify an input file to be opened and used instead of using stdin.
-o file, --output=file
Specify an output file to be opened and used instead of using stdout.
-M file, --mapfile=file
Specify a file which will be used for backing store for the virtual ring buffer. The allows using filesystem space in case swap
space is full.
-r bytes, --read-min=bytes
Specify the minimum number of bytes to be requested by read. Reading will not happen unless at least this much buffer space is
available. The default is 1.
-R bytes, --read-max=bytes
Specify the maximum number of bytes to be requested by read. Reading will not request more even if more buffer space is available.
The default is the full buffer size.
-w bytes, --write-min=bytes
Specify the minumum number of bytes to be written. Writing will not be done unless at least this much data is available. Once end
of file is reached on input, a smaller amount may be written. The default is 1.
-W bytes, --write-max=bytes
Specify the maximum number of bytes to be written. Writing will not write more each time even if more data is available in the buf-
fer. The default is the full buffer size.
AUTHOR
VRB and vbuf were written by Phil Howard <vrb@ipal.org>.
This manual page was written by Szekelyi Szabolcs <cc@mail.3d.hu>, for the Debian project (but may be used by others).
2006 Sep 5 VBUF(1)