Change file creation time on a FAT filesystem

The simplest : you could use :

touch -r Referencefile  THEFILE  

to give THEFILE the same time as Referencefile so:

rm -f Referencefile
echo > Referencefile  #to set the creation time
#...do your captures here, then concatenate into THEFILE ....
echo >> Referencefile #to set the modification time
touch -r Referencefile THEFILE

But if you prefer to have a more flexible way:

To have a "reliable" way to get the time of a file in a "portable" format, not depending on if the file last changed within 6 months, etc:

tar cf - file  | tar tvf -

so you could do this to get a time suitable for touch:

gettouchdate () { 
 tar cf - "$1" | tar tvf - | tr ':' ' ' \
  | awk '{mm=sprintf("%02d",(match("JanFebMarAprMayJunJulAugSepOctNovDec",$5)+2)/3) ;
          print $10 mm $6 $7 $8 $9 ;}'
}
#firstdate=$(gettouchdate "/path/to/FIRSTFILE")
lastdate=$(gettouchdate "/path/to/LASTFILE")
touch -r /path/to/FIRSTFILE  THEFILE
touch -m "$lastdate" THEFILE

The behaviour is expected, if rather non-obvious. At least on my system, touch does

utimensat(0, NULL, NULL, 0)

in the first case (touch file), and

utimensat(0, NULL, [UTIME_OMIT, UTIME_NOW], 0)

in the second (touch -m file). The first call is short-hand for "set the access and modification times to the current time"; the second says "leave the access time alone and change the modification time to the current time".

POSIX says

Only a process with the effective user ID equal to the user ID of the file, or with write access to the file, or with appropriate privileges may use futimens() or utimensat() with a null pointer as the times argument or with both tv_nsec fields set to the special value UTIME_NOW. Only a process with the effective user ID equal to the user ID of the file or with appropriate privileges may use futimens() or utimensat() with a non-null times argument that does not have both tv_nsec fields set to UTIME_NOW and does not have both tv_nsec fields set to UTIME_OMIT.

(times is the third argument to utimensat()). touch file falls under the first set of access restrictions: any write access to the file allows the access and modification times to be changed to "now". touch -m file falls under the second set of access restrictions: you need to be root or the owner of the file to be able to change the access and modification times to values which are not either both "now" or both "unchanged".

There are other ways of changing the access and/or modification times to "now" on files you don't own but can read or write:

• reading a file will update the access time only;
• writing a file (without reading it) will update the modification time only.