La aplicación práctica de la doctrina Clinton.

This section describes the use of a terminal multiplexer. Advanced users can find this to be a valuable productivity tool. Beginning users can safely skip this section.

Consider the following scenarios:

• You have an ssh session exactly the way you want it with modules loaded, commands you are using a couple steps back in your history, and in the desired directory. It can be aggravating when the ssh connection is lost due to a network interruption.

• You want to leave an ssh session open for long periods of time so you can check on work in progress. However, the connection gets broken when the laptop is closed.

• You would like a handy key sequence to jump between multiple ssh sessions. All of these are problems that can be solved with a terminal multiplexer. This section will describe the “screen” command which is included with many Linux distributions. There are other, more sophisticated, terminal multiplexers such as tmux that have similar functionality and more, such as split screen capabilities. The older version of screen included with many Linux distributions does not have split screen capability, but newer versions do.

Here is an example of using screen. Login via ssh.

Create a screen session named project1 with the command

screen -S project1

Change directories “cd /opt/asn/doc”, and type “ls”

Create a second shell in this session by pressing “CTRL-A” then “C” In this second session, change to a different directory, and type “ls” Switch between the two shells by pressing “CTRL-A” then “N” for next.

Exit the screen session by pressing “CTRL-A” then “D” for detach. Your shells are still running. You just aren’t attached to them.

HPC User Manual - Edition 10.1 Accessing the Supercomputers

Type “exit” to log out of the system, then log back in via ssh. You must log back in to the same system. Screen sessions started on dmc.asc.edu will not be visible on uv.asc.edu . Screen sessions are lost if the supercomputer system is rebooted, such during a maintenance shutdown.

Find out what screen sessions you have already in process with the command

screen -ls

One of them should be your session with a number assigned, perhaps like this 19520.project1 . Resume this screen session by typing

screen -r 19520.project1

Again, you should be able to jump between your two shells.

Screen saves environment, history, and directory. You can even leave a screen session in the middle of editing a file. However, it does not save a scroll back of information previously displayed (which is handled by the local computer).

If you lose your network connection, the screen session is still there. Sometimes it can take a while to see that you are no longer connected to it.

There are additional options for screen, such as attaching to a session that is in use from another computer. The documentation can be seen with the command

man screen

7. Working with Linux

The Linux operating system is a public domain operating system, which mostly conforms to the POSIX standard (the technical specification for the UNIX operating system). It was developed by large number of volunteer programmers around the world. The original inception and much of the project coordination is attributed to Linus Torvalds, then a student at the University of Helsinki, Finland. Both the DMC and the UV are running versions of Linux. The Linux version on the UV comes with additional tools developed by SGI. The operating system on these machines is very much like Linux operating systems on a wide variety of other computers.

Most of the differences between the operating systems on the SGI Ultraviolet and DMC are items that concern the system administrators, but are not directly visible to the users of these systems. The one exception to this is that users compiling their own MPI parallelized programs will have to follow slightly different procedures in order to use the version of MPI applicable to each system.

Linux provides the standard UNIX commands, libraries, and features, such as user shells, pipes, tees, and filters. Also included are text editors (nano and vi), communications programs (ssh, sftp, and X-Windows), and compilers (C, C++, and FORTRAN90). The job queuing system (Torque/Moab) is a third party add-on to Linux systems. Torque/Moab allows users to create a file of commands for the computer to execute at a later time rather than simply typing in the commands one by one from a terminal. Effective use of the computers, particularly for large jobs, requires use of the Torque/Moab queue system.

Many popular Linux guides and textbooks also provide valuable information and are generally applicable to Linux. Most documentation for Linux can be accessed online via the man command. See the “Online Help” and “Technical Support for Users” sections of this manual for information about getting help.

Using Linux interactively is described in the following pages. The Torque/Moab queue system is designed to accept the same commands as for interactive use. You can prepare a file for submittal to the Torque/Moab system with one of the text editors and then place it into an appropriate batch queue.