What Does –– (double dash) Mean In SSH Shell Command?

by on December 2, 2013 · 3 comments· LAST UPDATED December 3, 2013

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I see lots of seasoned admins and cloud provider wrapper scripts use ssh client command as follows in shell:

ssh nixcraft@server1.cyberciti.biz --

What the double -- (dash) does here? Why it is used in this shell command and why not just use the following?

ssh nixcraft@server1.cyberciti.biz

Tutorial details
DifficultyEasy (rss)
Root privilegesNo
RequirementsNone
Estimated completion timeN/A
The double dash "--" means "end of command line flags" i.e. it tells ssh command not to try to parse what comes after command line options. You will see something as follows when you use gcutil ssh vmNameHere python wrapper. It will display and execute ssh as follows:

ssh -o UserKnownHostsFile=/dev/null -o CheckHostIP=no -o StrictHostKeyChecking=no -i /Users/vivek/.ssh/google_compute_engine -A -p 22 nixcraft@server1.cyberciti.biz --

This syntax ensures that you can run commands on the remote server without ssh parsing them:
ssh nixcraft@server1.cyberciti.biz -- command1 --arg1 --arg2

The above syntax tell ssh not try to parse --arg1 and --arg2 after -- command line options. This ensures that command1 will accept --arg1 and --arg2 as arguments.

## safe examples ##
ssh nixcraft@server1.cyberciti.biz -- --commandName --arg1 --arg2

This kind of behavior is mostly defined and handled by the ssh command and not by your bash/ksh/csh shell. This is also true for many other commands. For example you can not create or view a file named --file or -f using cat command

## fail ##
cat --file
cat -f

Instead try passing double dash "--" to instruct cat command not to try to parse what comes after command line options:

## works ##
cat -- --file
cat -- -f
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{ 3 comments… read them below or add one }

1 OJay December 3, 2013 at 8:00 am

Interesting stuff! Didn’t know that that option existed. It’s great to see something new here from time to time even after so many years :-)

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2 Bobsleer February 4, 2014 at 11:30 am

Thanx for the info.
Where can one find documentation/manual about this double-dash argument?
(maybe in POSIX?)

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3 Nix Craft February 4, 2014 at 1:03 pm

You are welcome. You can check your current shell man / info page. For starters, try

man bash
man ksh93

For lazy, see bash man page online here and search for ‘--‘.

Hope this help!

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