Linux file permissions
7 = 4+2+1 (read/write/execute)
6 = 4+2 (read/write)
5 = 4+1 (read/execute)
4 = 4 (read)
3 = 2+1 (write/execute)
2 = 2 (write)
1 = 1 (execute)
E.g
chmod 666 myfile.txt -> read/write by anybody.
chmod 755 myfile.txt -> rwx for owner, rx for group and rx for the world.
chmod 777 myfile.txt -> read, write, execute for all.