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Top Forums UNIX for Advanced & Expert Users How to copy a binary file while the file is being written to by another process Post 302603953 by Corona688 on Friday 2nd of March 2012 11:10:07 AM
Old 03-02-2012
Reading from a file doesn't disturb anything, at all. The writer doesn't know or care anything else is reading, and the data being written or position it's being written to won't be upset, reset, or altered.

The real worry would be whether you'd get anything sensible by doing so. You could catch the data file in an in-between or invalid state -- a half-written line or some such -- except you've handily told us your application appends, so that earlier records won't be altered; and does so in individual 512-byte blocks.

If your application writes 512 bytes at a time, i.e. one write of 512 bytes instead of 512 writes of 1 byte, you'll never catch the file in a halfway state -- from a user perspective, the file will grow cleanly in 512-byte jumps.

In short, it ought to be safe to simply copy this file. You don't even need to stop the application.

Last edited by Corona688; 03-02-2012 at 12:25 PM..
 

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DD(1)							      General Commands Manual							     DD(1)

NAME
dd - disk dumper SYNOPSIS
dd [option = value] ... EXAMPLES
dd if=/dev/fd0 of=/dev/fd1 # Copy disk 0 to disk 1 dd if=x of=y bs=1w skip=4 # Copy x to y, skipping 4 words dd if=x of=y count=3 # Copy three 512-byte blocks DESCRIPTION
This command is intended for copying partial files. The block size, skip count, and number of blocks to copy can be specified. The options are: if = file - Input file (default is stdin) of = file - Output file (default is standard output) ibs = n - Input block size (default 512 bytes) obs = n - Output block size (default is 512 bytes) bs = n - Block size; sets ibs and obs (default is 512 bytes) skip = n - Skip n input blocks before reading seek = n - Skip n output blocks before writing count = n - Copy only n input blocks conv = lcase - Convert upper case letters to lower case conv = ucase - Convert lower case letters to upper case conv = swab - Swap every pair of bytes conv = noerror- Ignore errors and just keep going conv = silent- Suppress statistics (Minix specific flag) Where sizes are expected, they are in bytes. However, the letters w, b, or k may be appended to the number to indicate words (2 bytes), blocks (512 bytes), or K (1024 bytes), respectively. When dd is finished, it reports the number of full and partial blocks read and writ- ten. SEE ALSO
vol(1). DD(1)
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