How to set password if we are trying to read a file by cat ,it should ask for password after we have given the password it should display the contents of the file.

i need is to prompt for password if we try to use cat over a file.
If it is possible, please tell me what have to be done for the same.

To encrypt the file
$gpg -c /path/of/the/file
It will create a encrypted file as .gpg

to decrypt the file
$gpg /path/of/the/gpg/file.gpg

Brother thank you for your reply. But actually what i need is if i need to display file by cat command it should ask for password prompt. How can set this. If possible please tell me.

Explain why you are trying to do this; cat does not have an option for this http://linux.die.net/man/1/cat

When you use gpg command to encrypt a file, it makes encrypted copy of a file.
Now you try to open original file, it throws garbage character on terminal.
Remove this original file from the directory where encrypted file is placed because when you decrypt it, it overwrites encrypted copy and it can create problem to open it.
If you try to read the .gpg file, it shows garbage character too because it is encrypted.
If you rename it using $ mv and again try to read it, it shows garbage character.
If you want to read the file, you have to decrypt.
Now it makes original copy of your file and you can read this.

It can be protected with a password using mcrypt, as:
For further details, once check here.

There are a gzillion ways to display a file. If you somehow removed all of the tools that can read a file, or modified them to do what you want, it would still be trivial to create a program that would be able to read the file. To make a file unviewable, you will want to use proper file permissions and ownerships.

--- rod.

Agree with Nbomr. Instead of using an encryption or password protection tool without assuring that it will work, you should better go with the concept of file access permissions. Simply create a group and add it as main group of the file and give the group read/write permissions. Then add only those users to this group whom you want to give read permission on that file.

On the other hand, you can go with ACLs, which will be little complicate but will work perfectly.