UNIX for Dummies Questions & Answers

First of all, hello there cOmMuNity !

Well, I've got two basic questions:

1) In how many ways it's possible to create a file ?
I know one manner using "touch", other typing something like echo "" > file ...
The fact is that I need to overwrite the file if it exists, and touch isn't capable of doing this (actually, touch isn't that suitable for creating files, is it ? as it's destined to change the access date and time of a file...). So, is the echo "" > file the most elegant way for this purpose ?


2) What is exactly the /dev/null... It isn't the same thing as a NULL value, is it ?
Suppose I've chosen to create a file by the echo "" > file method.
The file starts with a blank line. To avoid this, I've seen
cat /dev/null > file

Is this again the only way ?
I heard also that I could use the -n option of echo, which removes the newline character...
It would be something like echo -n "" > file
It should work out.

Well, that's all for now.
Thank you anyway !!!

435 Gavea - bRaZiL - tHE HEll iS hERe !

1. Depends on your shell. Also a matter of opinion. People can reasonably disagree on "most elegant". With sh, ksh, or bash, my vote goes to:
> file


2. /dev/null is a data trashcan. Use it to discard stuff that you don't want. Suppose that you have a program that updates a data file. But it prints out messages like: "getting starting" "half way done" "all done" "closing file" "exiting". And also suppose that the program does not have an option to sut it up. Just use:
program > /dev/null

Also errors go to file descriptor 2. So you can use 2>/dev/null to discard error messages.

But sending files to /dev/null acts as if I was deleting these files ?



By the way, thanks for the prompt answer !




435 Gavea - bRaZiL - ThE HeLL iS hERe !!!

No you can't send a file to /dev/null. You could display a files contents by doing:
cat filename
by that doesn't use up the file. It's still there. So
cat filename > /dev/null
just wastes time.

To delete a file, do
rm filename

I'm trying to understand what you said about /dev/null...

Looking your example, send this file (that prints irrelevant stuff)to /dev/null would be the same as using "cut" command to cut out this "garbage" ?

no, if your actually talking about file contents then this is different. you can redirect output to /dev/null as an example. your console is actually referenced to as a file in unix, same as the printer and everything else. so if you have a program that outputs "hello world" you could, instead of sending it to the console, redirect it to the printer so it prints, or to /dev/null. the out put sent to /dev/null does not get stored in a file, nor does it get output to anywhere. its gone.

your 'cat /dev/null > file' example would just create a file with no data in it what so ever, its size would be zero.

Maybe a couple more examples will make it clear.

echo hello
echo hello > garbage.txt
echo hello > /dev/tty
echo hello > /dev/null

The first one will display hello for you to see, and the second one will send the word hello into a file. I think that you got that much.

The third one is explicitly doing for the shell did automatically. /dev/tty is not your ordinary file, it is a "special file". Instead of being stored in a file on disk, a special little program called a driver is going to get that "hello" string. The tty driver will figure out how to make the characters appear on your screen. And then it will tell the OS, ok...that "hello" string went to the device.

The fourth one is also going to a special file and again a driver will get the string "hello". But this driver is just going to tell the OS, ok...that "hello" string went to the device. But there is no device and the null driver did not try to do anything. It just tosses the characters away.

You can read from /dev/tty. And the driver will wait for you to type something and read those characters and send them on.

You can read from /dev/null too, but the driver will always claim that there is no data.