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What is file descriptor 10?

Real-world example (where standard output is ignored, standard error redirected to standard output and data from file descriptor 10 is redirected to standard output) that I have lost the documentation/context for:

./cluexec21nodes 'mkdir /home/mortense/sambapub;cd /home/mortense/sambapub;./ >/dev/null 2>&1 10>&1 &'

The standard file descriptors are

0   Standard Input (stdin)
1   Standard Output (stdout)
2   Standard Error (stderr)

but what is file descriptor 10?

This was used on a Linux system (possibly Red Hat).

share|improve this question
Half of the command doesn't make sense. It attempts to run a script from within a directory which it just mkdir'd... – grawity May 10 '12 at 12:47
@grawity: the mkdir, etc. was executed on each of a number nodes in a Linux cluster - the command enclosed in ' s. – Peter Mortensen May 10 '12 at 13:22
I'm aware of that. I was talking specifically about the mkdir /home/mortense/sambapub;cd /home/mortense/sambapub;./ part. – grawity May 10 '12 at 13:27
@grawity: yes, you are right. It was not well thought out (probably a mixup of two different stages). – Peter Mortensen May 10 '12 at 14:00

Beyond the standard file descriptors there are 3-1024. These can be created in scripts with the

exec 10<> afilename

From this point on, anything written to file descriptor 10 gets written to afilename

When you have

./ 10>&1 

You are redirecting anything that would have gone to file descriptor 10 to stdout.

You'll want to review the script to see why it is doing this.

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The syntax is exec 10>afilename in sh. – grawity May 10 '12 at 12:53
@grawity: isn't possible to redirect to another file descriptor in sh? – Peter Mortensen May 10 '12 at 13:31
@Paul: it may have been an arbitrary convention that was used, but given it was used for executing a command on each of a number of nodes in a Linux cluster, couldn't it have something to do with that (network pipes?). Anyway, I will try to find the two scripts. – Peter Mortensen May 10 '12 at 13:38
@Peter: It is, with 10>&1 or similar. – grawity May 10 '12 at 13:56

File descriptors are not fixed; the program may use them in whatever way it wants. (Even fd's 0-2 can be closed, opened, reused.)

In this case, fd 10 seems to be chosen arbitrarily; it could be that ./ expects to be able to write some additional information to it. You'll have to read the script itself to know for sure.

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