/dev/null 2>&1 question Post: 35713

Sponsored Content

Top Forums UNIX for Dummies Questions & Answers /dev/null 2>&1 question Post 35713 by Perderabo on Tuesday 29th of April 2003 12:08:08 PM

04-29-2003

Administrator Emeritus

There is a system file table. It has one entry for every open file. Your process has descriptors that point to file table entries. Let's change the example to a text file.

So the shell syntax starts out " > results.txt". To run that the shell will need to do something like:
close(1);
fd=open("results.txt", O_WRONLY, 0);
We actually had to open a file. This filled in a file table entry. And now fd 0 points to that file table entry.

Now the syntax continues "2>&1" and to run that, the shell does something like:
close(2);
fd=dup(1);
Here we don't open a file. It's just that whatever file table entry fd 1 was pointing to has been copied into fd 2.

There is more to this than just saving an expensive open call. If we write to fd 1, what ever we write goes into the file. If we then write some more, the new data goes at the end of the file. We can write to fd 1 and we can write to fd 2 and they don't step on each other. That's because a write to either fd updates the file pointer in the file table entry.

In contrast, " > results.txt 2 > results.txt" gives us two file table entries and two file pointers.

Perderabo

View Public Profile for Perderabo

Find all posts by Perderabo

10 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

>/dev/null

Maybe it's an stupid question but remeber... I'm Junior.. I use command line to run programs, and some of them gives a lot of information when, for example, you open a window or other actions. That's really bad because my terminal gets full of unwanted messages, so I use "bin file & >/dev/null"...

2. Shell Programming and Scripting

> /dev/null

hello all, In many shell scripts i found '> /dev/null' , i am not able to get this, will any one please explain why we are using this. thanks sudha

3. Solaris

What is /dev/tty /dev/null and /dev/console

Hi, Anyone can help My solaris 8 system has the following /dev/null , /dev/tty and /dev/console All permission are lrwxrwxrwx Can this be change to a non-world write ?? any impact ??

4. Shell Programming and Scripting

Meaning of "> /dev/null 2>&1"

Hi, I am new into UNIX shell scripting and I am wondering what is the meaning of the below text which appears at the end of each line in the ".sh" file: > /dev/null 2>&1 For example, the line below: sh $HOME/stats/Rep777/Act_777.sh omc omc > /dev/null 2>&1 I know for sure what "sh...

5. Programming

Question about NULL Character & fgets()

Assume client send the message " Hello ", i get output such as Sent mesg: hello Bytes Sent to Client: 6 bytes_received = recv(clientSockD, data, MAX_DATA, 0); if(bytes_received) { send(clientSockD, data, bytes_received, 0); data = '\0';...

6. Shell Programming and Scripting

Difference between ">/dev/null 2>&1" and "2>&1 >/dev/null"

Does >/dev/null 2>&1 and 2>&1 >/dev/null mean the same?

7. UNIX for Dummies Questions & Answers

/dev/null 2>&1 Versus /dev/null 2>1

How are these two different? They both prevent output and error from being displayed. I don't see the use of the "&" echo "hello" > /dev/null 2>&1 echo "hello" > /dev/null 2>1

8. Shell Programming and Scripting

Redirecting standard out to /dev/null goes to file "/dev/null" instead

I apologize if this question has been answered else where or is too elementary. I ran across a KSH script (long unimportant story) that does this: if ; then CAS_SRC_LOG="/var/log/cas_src.log 2>&1" else CAS_SRC_LOG="/dev/null 2>&1" fithen does this: /usr/bin/echo "heartbeat:...

9. AIX

Size of file /dev/./null 2>&1

Hello Can I just remove/delete flile ? rw-r--r-- 1 root system 2385088512 Jun 30 21:25 /dev/null 2>&1 size of this flile is 2274.75 m and fill up my filesystem: Filesystem MB blocks Used Free %Used Mounted on /dev/hd4 2560.00 2558.59 1.41 100% /

10. AIX

Housekeeping null 2>&1 in /dev

Hello, Does anyone know how to housekeeping the null 2>&1 file in /dev? its fill up my system, please help. Thanks :b:

LEARN ABOUT LINUX

fdformat

FDFORMAT(8)						     Linux Programmer's Manual						       FDFORMAT(8)

NAME

       fdformat - Low-level formats a floppy disk

SYNOPSIS

       fdformat [-n] device

DESCRIPTION

       fdformat  does  a  low  level  format on a floppy disk.	device is usually one of the following (for floppy devices, the major = 2, and the
       minor is shown for informational purposes only):

	      /dev/fd0d360  (minor = 4)
	      /dev/fd0h1200 (minor = 8)
	      /dev/fd0D360  (minor = 12)
	      /dev/fd0H360  (minor = 12)
	      /dev/fd0D720  (minor = 16)
	      /dev/fd0H720  (minor = 16)
	      /dev/fd0h360  (minor = 20)
	      /dev/fd0h720  (minor = 24)
	      /dev/fd0H1440 (minor = 28)

	      /dev/fd1d360  (minor = 5)
	      /dev/fd1h1200 (minor = 9)
	      /dev/fd1D360  (minor = 13)
	      /dev/fd1H360  (minor = 13)
	      /dev/fd1D720  (minor = 17)
	      /dev/fd1H720  (minor = 17)
	      /dev/fd1h360  (minor = 21)
	      /dev/fd1h720  (minor = 25)
	      /dev/fd1H1440 (minor = 29)

       The generic floppy devices, /dev/fd0 and /dev/fd1, will fail to work with fdformat when a non-standard format is being used, or if the for-
       mat has not been autodetected earlier.  In this case, use setfdprm(8) to load the disk parameters.

OPTIONS

       -n     No verify.  This option will disable the verification that is performed after the format.

SEE ALSO

       fd(4), setfdprm(8), mkfs(8), emkfs(8)

AUTHOR

       Werner Almesberger (almesber@nessie.cs.id.ethz.ch)

AVAILABILITY

       The fdformat command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

Linux 0.99							  1 February 1993						       FDFORMAT(8)

10 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

>/dev/null

Discussion started by: piltrafa

2. Shell Programming and Scripting

> /dev/null

Discussion started by: rrs

3. Solaris

What is /dev/tty /dev/null and /dev/console

Discussion started by: civic2005

4. Shell Programming and Scripting

Meaning of "> /dev/null 2>&1"

Discussion started by: salanalani