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I edited an answer on Ask Ubuntu that was suggesting the following

nohup gedit >& /dev/null &

When they actually meant 

nohup gedit &> /dev/null &

The latter correctly redirects both stderr and stdout to /dev/null. I was expecting the former to either create a file called & or, more likely, to give an error as it does for other cases:

$ echo "foo" >& 
bash: syntax error near unexpected token `newline'

Instead, it seems to work in exactly the same way as the former, a gedit window appears and no error message is printed.

I should also note that this is shell specific:

  • bash (4.2.45(1)-release), zsh (5.0.2), csh (deb package version: 20110502-2) and tcsh (6.18.01) : works as described above, no error message, no files created.

  • dash (0.5.7-3):

    $ nohup gedit >& /dev/null & 
$ dash: 2: Syntax error: Bad fd number

  • ksh (93u+ 2012-08-01): fails, but a process is apparently started (1223) though no gedit window appears:

    $ nohup gedit >& /dev/null & 
[1] 1223
$ ksh: /dev/null: bad file unit number

  • fish (2.0.0):

    > nohup gedit >& /dev/null & 
fish: Requested redirection to something that is not a file descriptor /dev/null
nohup gedit >& /dev/null & 
               ^

So, why does this command simply run with no errors (and no output file created) in some shells and fail in others? What is the >& doing in the apparently special case of nohup? I am guessing that >& /dev/null is being interpreted as >&/dev/null but why isn't the space causing an error in these shells?

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  • In my machine, Ubuntu 12.04, this command run normally for dash. Commented Mar 5, 2014 at 19:49
  • nohup command, run independent tty your application.According to my memory, dash extended of ash , Debian ash , ash developed byOpenBSD and it's limited shell,even maemo OS(Debian Base on n900 mobile) uses dash,ash family shell have limited usage expect of bash or tcsh. Commented Mar 5, 2014 at 19:50
  • @Gnouc huh, perhaps a different version (I'm on Debian)? I can't figure out how to get my dash to print its version out but the package is 0.5.7-3, what's yours? Also, are you sure you're running dash? That's Ubuntu's default sh isn't it? Commented Mar 5, 2014 at 19:54
  • @MohsenPahlevanzadeh I'm not sure what your point is, I know what nohup does, my question is why the >& seems to work with nohup alone in some shells. Commented Mar 5, 2014 at 19:55
  • You can use the following link for abstarct viewing of shells: unix.stackexchange.com/questions/45684/… Commented Mar 5, 2014 at 19:55

1 Answer 1

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21 
nohup gedit &> /dev/null

is POSIX syntax and is the same as:

nohup gedit &
> /dev/null

That is run nohup gedit in background and then do a > /dev/null redirection without running a command.

nohup gedit >& /dev/null

is not POSIX syntax and is the csh way to redirect both stdout and stderr to /dev/null. csh doesn't have the 2>&1 operator as found in Bourne, so it's the only way csh has to redirect stderr.

zsh (as often) also provides with the csh syntax, but it also supports the x>&y fd duplication operator of the Bourne shell, which means there's a conflict there.

ls >&file

redirects ls's stdout and stderr to file, but if the file is 2, you've got a problem as

ls >&2

means redirect stdout to the resource pointed to by fd 2 (dup(2, 1)). So you need to write it:

ls >& ./2

if you wanted to redirect both the stdout and stderr of ls into a file called 2 in the current directory; or use the standard syntax.

bash initially did no understand >&, but it introduced the &> operator instead for that, breaking POSIX compliance in the process (though it's unlikely a script would use cmd &> xxx).

ksh copied that operator in ksh93t+ in 2009, mksh in R35 in 2008 (disabled in posix mode) but not >&.

bash added support for >& in 2.05.

busybox sh added support for both &> and >& in 1.13 (2008).

Neither >& nor &> as meaning redirect stdout and stderr are POSIX/Bourne.

If you want to redirect both stdout and stderr portably, the syntax is

cmd > file 2>&1
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  • Do you mean Bash in POSIX/Bourne by "Bourne"? Commented Mar 4, 2016 at 11:38

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