i have a executable file 'test.out'
i go to the directory containing 'test.out', and type

$ test.out

the program dosen't run, but when i type

$ ./test.out

it works. Why does we need ./ to run the file.
I know './' is current directory. i need to know some other reasons !

It could avoid conflict with other programs on your path. For example, you have a program called potato on your path and if you just type potato, you'd fire up the one on your PATH, when you actually want to fire your own potato... for example

Wait till you write your own app to wipe you HD and call it ls. Now you login as root, go to your directory and run ls expecting to get a directory listing. Bingo.

You will probably not call that app ls, but someone with bad intentions will.

The 'problem' might depend on your distro. For a root user in Slackware the current directory is not in the path (see above security risk), for a normal user it is.

I thought for general security purposes the current directory was never included into the path. It is one of the few ways, as you said, that can create wonderful security loopholes and whatnots. I do know there is a way to add the current directory into the path, but I would avoid doing that, just in case...

Here is how you do it. But do so at your own risk.

export PATH=$PATH:.

This will allow you to run programs or scripts without typing ./

This effect is temporally, once you logout the effect is gone.

For permanent effect, see below

For bash shell global environment, as root edit the /etc/profile or bash_profile script. Look for something like this:

An example

export PATH="/bin:/usr/bin:/sbin:/usr/sbin:/usr/X11R6/bin:/opt/bin:"

Add a period at the end, like so

export PATH="/bin:/usr/bin:/sbin:/usr/sbin:/usr/X11R6/bin:/opt/bin:."

The period means current directory.

Two ways to take effect, type source /etc/profile or
logout and login again.

A safer way is create a directory for your programs or shell scripts, for example bin

Add the directory to your PATH environment like so

export PATH="/bin:/usr/bin:/sbin:/usr/sbin:/usr/X11R6/bin:/opt/bin:$HOME/bin"

Logout and login again.

Now you can run your scripts anywhere from your home environment, no need to be in the current directory.

The dot slash parameter in front of the program and or file name tells the kernel to execute the sequence of commands stored in the script and or file that you amend to the end of the aforementioned command line interface (CLI) argument.

thanks to all for solving my query...