$ id
uid=54482(emagiro) gid=17465(ath)
$ /tmp/showfile
$0 /tmp/showfile
filename /tmp/showfile
__FILE__ /tmp/showfile
$ chmod u+s /tmp/showfile
$ ll /tmp/showfile
-rwsr-xr-x   1 emagiro  ath          208 Sep 15 16:37 /tmp/showfile

~> id
uid=33629(eeivob05) gid=17465(ath)
~> /tmp/showfile
$0 /dev/fd/4
filename /dev/fd/4
__FILE__ /dev/fd/4