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I need to reboot a number of machines remotely. Normally I just issue 

for host in <hostlist>;do ssh ${host} 'sudo shutdown -r now';done

But I want give the users some time before the restart. However they ssh session won’t disconnect even if I use:

ssh -f 'sudo shutdown -r +5 &;disown'

I get an error message:

bash: -c: line 0: syntax error near unexpected token ;' bash: -c: line 0:sudo shutdown -r +5 &;disown'

Any suggestions?

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    I should add that I'm trying to send the shutdown command to background with the "&". If the shell is interpreting the "&" as a command separator then I'm stumped as to how to send it to background so that the "disown" command works. Commented Feb 5, 2015 at 17:34
  • The & character does send the command on the left hand side in the background. That's how it differs from ;. Commented Feb 5, 2015 at 17:39

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As stated by the error message, you have a syntax error in your command. It is due to the slightly surprising fact that & isn't part of a shell command but rather a separator between commands (like ; is). Though both are command separators & has an additional effect on the command on its left hand side. This certainly also confuse me sometimes, and I frequently make the same mistake. Once you know it, it is however easy to fix.

The fix is to not write &; but rather just write one of the two separators depending on your intention. (And in most cases where one has written &; the intention was to only write &).

This should work:

ssh server 'sudo shutdown -r +5 & disown'
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    You win today's prize for teaching me something I didn't know after 25 years of bashing on UNIX. Also +1! Commented Feb 5, 2015 at 8:20
  • What if he intended to run sudo shutdown -r +5 & on background and then run disown? How would bash handle such situation? I would expect this to be perfect syntax for a command, even having &;. If ; is a command separator, then you have 2 commands on your hands. Right? Commented Feb 5, 2015 at 11:10
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    @IsmaelMiguel I don't get your question. Using & will start the first command in the background and once the first command has been started the second command will be run in the foreground. Commented Feb 5, 2015 at 12:22
  • I meant that sudo shutdown -r +5 &; disown doesn't seem like a bad syntax. I mean, my intention could be to run the first part in background and then the 2nd part. Commented Feb 5, 2015 at 12:25
  • @IsmaelMiguel The command parser could probably be extended to consider the empty string to be a valid command which always succeeds. In that case sudo shutdown -r +5 &; disown would be considered to be three valid commands separated by & and ;. It would however mean that one has to be careful not to use an additional separator if the exist status of the previous command is to be used. It does mean that one has to pay attention to tokenizing as well since for example && and & & gets tokenized differently. Commented Feb 5, 2015 at 14:04

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