2>&1

[Shivkumar]

Dear Sirs;

Please see the below command:
/usr/lbin/qs >> /var/adm/qs/qs.log 2>&1

In this what is the meaning of 2>&1 ?
I have seen this in most of the cron jobs and some shell scripts.

What would be repercussion of this if we forget to mentioned 2>&1 ?

Thanks,
Shiv

[Rajeev Shukla]

Hi Shiv,
This type of syntax is used to collect the standard output and standard error in the file preceeding it.
Like in your case when the script /usr/lbin/qs runs the standard output and standard error of the files will be appended to the file /var/adm/qs/qs.log and if you miss the syntax 2>&1 then only the output of the command will be in the file.
Have a look at the man pages of sh-bourne for more details

Cheers
Rajeev

[Patrick Wallek · Accepted solution]

If you want to get really technical you need to know what those numbers mean.

Technically those are file descriptors.

File descriptor 0 = standard input
File descriptor 1 = standard output
File descriptor 2 = standard error

Now normally when you do things via a terminal session file descriptors 1 and 2 both come back to the screen.

But if you do a command like:

# ll > afile

That will just redirect file descriptor 1 to the file afile. If there are any errors, then you would see them echo'd to the screen.

If you did:

# ll > afile 2>&1

Then you are telling the shell to redirect file descriptor 2 (standard error) to the same location as file descriptor 1 (standard out) which is redirected to the file afile.

Now you could get more complicated and redirect standard out and standard error to different locations if you want to.

Something like:

# ll 1>file.log 2>file.error

The above will send file descriptor 1 (standard output) to file.log and file descriptor 2 (standard error) to file.error.

[Rajeev Shukla]

Thats right patrick,
The only difference in using 2>&1 and >afile 2>err.log is that in the first the output and error is sent synchronously to single file whereas in the other case the output is to different files asynchronously.

[Cem Tugrul]

No any additions to Patrick's and Rajeev's
explanations...
very clear
Good Luck,

[Cem Tugrul]

Here are the other useful links below;

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=891230

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=841251

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=126984

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=113110

Good luck,

[Steve McClendon_1]

Shiv,

You asked about repercussions if you don't use it?

Cron jobs should not have any output going to standard output. Using the 2>&1 on the command line lets you log any errors and keep them from going to standard output.

Steve

[Vibhor Kumar Agarwal]

Right,

Otherwise it might mess up your normal work.
Since cron runs in background, you should output its error to a log file.

[Sandman!]

Essentially 2>&1 sends the standard output and standard error stream to the same file. Take a look at the URL below for a detailed explanation of I/O re-direction.

http://wks.uts.ohio-state.edu/unix_course/intro-69.html