ARSC HPC Users' Newsletter Number 429 2014-02-19

The Arctic Region Supercomputing Center Users' Newsletter provides a
platform for discourse relevant to users of high performance computing
systems.  Topics include: programming, commands, tools, applications,
and more.  (We skipped the January 2014 issue.)

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  - Killing a Hung SSH Session
  - Quick-Tip Q&A

		   -- Killing a Hung SSH Session --
[ By Christopher Howard, ARSC Research Projects Assistant ]

Sometimes we find ourselves in an SSH session where a program or command
becomes unresponsive, but it will not relinquish control with CTRL C. We
can end the SSH session by closing the terminal window, but this can be

Most people are not aware that, even if the login shell becomes
unresponsive, the ssh daemon itself is still listening for commands from
the user. To view these commands, press the ENTER (or RETURN) key, and
then type "~?" (the tilde character followed by a question mark). The
terminal should output text like this:

Supported escape sequences:
   ~.  - terminate session
   ~B  - send a BREAK to the remote system
   ~R  - Request rekey (SSH protocol 2 only)
   ~#  - list forwarded connections
   ~?  - this message
   ~~  - send the escape character by typing it twice
(Note that escapes are only recognized immediately after newline.)

At any time in an SSH session, you can press ENTER and then use the
command "~." (tilde period) to kill the current session. This will take
you back to the terminal prompt from which you launched the SSH command.
Note that you must press ENTER first, because the ssh daemon will only
receive commands after a newline character is received.

                          -- Quick-Tip Q & A --

Last time, we asked:

Q: What are some tips and gotchas for using BSD-style systems versus
SysV-style systems?  (Or, more simply: Mac versus Linux.)  Do you have
favorite aliases, scripts or commands for dealing with the

A reader responded: "ps", which lists processes on a system, can be a hassle.
"ps aux" on Mac, "ps elf" on Linux.  On my Mac, "ps -elf" is different
from "ps elf", and "ps aux" is different from "ps -aux".  On Linux,
"ps -elf" is different from "ps elf," but "ps -aux" is the same as "ps
aux" (though the former yields a warning).  Oh, and on ARSC's pacman
system, there is a cluster-wide process space.  So, "ps -elf" can
generate a whole lot of output!

Here is a question for next time:

Q: Have you tried the Intel compilers or other tools yet, at ARSC
or elsewhere?  What has your experience been?  (ARSC recently
added the Intel compiler collection to available software.)

 [[ Answers, Questions, and Tips Graciously Accepted.  Email them to ]]


About ARSC:

  The Arctic Region Supercomputing Center (ARSC) provides research and
high performance computing, large-scale storage, and related services
to the students, faculty and staff of the University of Alaska.

  Greg Newby, ARSC Director,, 907-450-8663

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