Linux Filesystem Hierarchy

set group ID (SGID)
The SGID permission causes a script to run with its group set to the group of the script, rather than the
group of the user who started it. It is normally considered extremely bad practice to run a program in
this way as it can pose many security problems. Later versions of the Linux kernel will even prohibit
the running of shell scripts that have this attribute set.
set user ID (SUID)
The SUID permission causes a script to run as the user who is the owner of the script, rather than the
user who started it. It is normally considered extremely bad practice to run a program in this way as it
can pose many security problems. Later versions of the Linux kernel will even prohibit the running of
shell scripts that have this attribute set.
signal
Software interrupts sent to a program to indicate that an important event has occurred. The events can
vary from user requests to illegal memory access errors. Some signals, like the interrupt signal,
indicate that a user has asked the program to do something that is not in the usual flow of control.
SSH
The Secure Shell, or SSH, provides a way of running command line and graphical applications, and
transferring files, over an encrypted connection, all that will be seen is junk. It is both a protocol and a
suite of small command line applications, which can be used for various functions. SSH replaces the
old Telnet application, and can be used for secure remote administration of machines across the
Internet. However, it also has other features. SSH increases the ease of running applications remotely
by setting up X permissions automatically. If you can log into a machine, it allows you to run a
graphical application on it, unlike Telnet, which requires users to have an understanding of the X
authentication mechanisms that are manipulated through the xauth and xhost commands. SSH also
has inbuilt compression, which allows your graphic applications to run much faster over the network.
SCP (Secure Copy) and SFTP (Secure FTP) allow transfer of files over the remote link, either via
SSH's own command line utilities or graphical tools like Gnome's GFTP. Like Telnet, SSH is
cross−platform. You can find SSH server and clients for Linux, Unix and all flavours of Windows,
BeOS, PalmOS, Java and embedded Oses used in routers.