$ /usr/bin/chmod 751 data/subdir
$ /usr/bin/chmod 640 data/empty.txt
$ /usr/bin/chmod 640 data/FIRST.txt
$ /usr/bin/chmod 400 data/.hidden.txt
$ /usr/bin/chmod 640 data/pwd.txt
$ /usr/bin/chmod 640 data/yat.txt
$ ./bin/ls -a data
empty.txt
FIRST.txt
.hidden.txt
pwd.txt
subdir
yat.txt
$ ./bin/ls -s data
0 empty.txt
6 FIRST.txt
9 pwd.txt
22 yat.txt
$ ./bin/ls -sa data
0 empty.txt
6 FIRST.txt
5 .hidden.txt
9 pwd.txt
22 yat.txt
$ ./bin/ls -ap data
-rw-r----- empty.txt
-rw-r----- FIRST.txt
-r-------- .hidden.txt
-rw-r----- pwd.txt
drwxr-x--x subdir
-rw-r----- yat.txt
$ ./bin/head -n 1 Makefile
#
$ quit