Remote Access to a Linux Server Postscript and pdf versions are available.

Terry Sergeant

Fall 2009

Contents

The Big Picture

Preliminaries

This document assumes you have completed the steps outlined in the Linux Quickstart document. and the Using Linux in JB 202 document. These documents are available online at:

http://josephus.hsutx.edu/classes/all/.

If that is the case then you have an active account on the Linux server named csci.hsutx.edu and you have successfully made use of your account by connecting from JB 202. For the remainder of this document I will refer to the server as csci.

This document attempts to explain other ways in which you can connect to the server to perform useful work. This will be helpful because the JB 202 lab maintains limited hours. There are other computer labs on campus that have longer hours. One of those labs (JB 104) has software that will make connecting to csci easier.

Connecting to the Server

There are three protocols through which you can establish a connection to csci (from off-campus):
  1. http (view web pages on csci using a browser)
  2. ssh (log in to csci and type commands using an ssh client)
  3. sftp (transfer files to/from csci using an sftp client)

Most people are familiar with making http (web) connections to a server: to view a web page you simply start your favorite browser, enter the URL of the site you want to contact, and voila, you have made a connection to the server and it provides you with information.

In the same way, to make an ssh connection to the server you can simply launch your favorite ssh client and enter the name of the server to which you want to connect! Of course, there are two problems with this advice:

  1. chances are, you do not have a favorite ssh client, and
  2. when you connect using ssh instead of being greeted with friendly, graphical information, you are confronted with a command-line interface that wants to know your username and password.

Allow me to address these issues invidually.

Finding an ssh Client

There are a variety of Windows-based ssh clients. One of the free ones is Putty (available at http://www.chiark.greenend.org.uk/~sgtatham/putty/). The remainder of the document is written assuming that you are using Putty. I recommend that you do a full download and install so that you also get the sftp client. NOTE: There are free ssh clients for the Macintosh platform as well. One is http://www.macssh.com.

Once you download and install the program simply launch it and enter the full server name (csci.hsutx.edu). Upon establishing a connection, you will be prompted for your username and password. After you log in successfully you will be greeted by the Linux command-line interface. At this point it would be helpful if you knew a lot of Linux commands! If that is not the case then you should turn your attention to the next section.

Dealing with the Command-Line Interface

There are two primary methods of dealing with a command-line interface:
  1. learn several useful commands, or
  2. run graphical programs that will perform the useful commands for you

I have used the command-line interface for some time and, in fact, prefer it to a graphical interface for many uses. If you are new to Linux then this interface will be difficult to use at the beginning. One immediate advantage over using a graphical interface is that no additional software is needed.

If you want a graphical user interface to run through your ssh connection on a Linux server then you will have to install and run an X-Server on your Windows computer. Here are some options (in order of preference IMHO):

For Macintosh, (I believe) XDarwin http://www.xdarwin.org is an X-Server.

Transferring Files

To transfer files to/from csci you'll need an sftp client. For Windows I recommend FileZilla which is a free download and can be obtained at http://www.sourceforge.net/projects/filezilla.

Summary

If you read the first part of this section with some level of understanding you know how to connect to a Linux server (using ssh) and you also know that once you've connected you have a choice between using a command-line interface or a graphical interface. Allow me to summarize the advantages and disadvantages of the two approaches:
Command-Line Interface Graphical Interface
no additional software needed requires expensive (either money or time) additional software
very fast (if you know commands and tools well will run slowly, especially when accessing the server from off-campus (because the GUI information has to travel across the network)
very time-consuming to become proficient with the commands and tools needed for assignments issuing commands and running programs will be intuitive using standard GUI widgets

Using the Command-Line Interface

This section describes how to perform various tasks via the command-line interface.

Managing Files

Here are some file-manipulation commands you will likely want to use.
ls
lists files in a directory. There are many options that can be supplied to modify the behavior of ls. Here are some examples of using ls.

ls lists all files in current directory
ls -F modifies file listing to differentiate between types of files (directories, including permissions
ls ~/programs/c/ lists all files in the subdirectory c in the subdirectory programs in your home directory
ls -a shows all files, including hidden ones; files with names that begin with a period are designated ``hidden'' and are immune from some file operations
ls *cpp shows all files in the current directory whose names end with cpp
ls -al same as doing both -a and -l

mv
moves or renames a file. This command can be used rename a file or to move it from one directory to another. It should be noted that mv can be applied to directories as well as files. Examples:

mv old.txt new.dat renames file from old.txt to new.dat.
mv myprogs/cool.cpp . moves file named cool.cpp in the myprog directory into the current directory
mv myprogs oldprogs if we assume that myprogs is a directory (as in the previous example) then this would rename the directory to be oldprogs

cp
copies a file. This works pretty much the same way as mv with two exceptions. First, the original file is left intact. Secondly, cp doesn't copy entire directories (unless you use the -r option).
rm
removes a file.
chmod
changes mode (access permissions) for a file. Every file and directory has read, write, and executable permissions for three classifications of users: the user (u), the group (g), and others (o).2 For example, a file can be set to be readable and writeable, but not executable for the user/owner and not readable, writeable, or executable for group members or for anyone else. Likewise, you may want to allow anyone to view a particular file, but not write to it, etc. To view the current settings you can use the ls -l command which will produce output similar to this:
drwxrwxr-x   3 sergeant sergeant     1024 Aug 14 10:27 office52
-rw-------   1 sergeant sergeant    26907 Jul 30  1999 personal.xls
drwxrwxr-x   2 sergeant sergeant     1024 Jul 24 07:50 rpm
-rw-rw-r--   1 sergeant webcache    28492 Aug 18 18:29 stats.ps
-rw-rw-rw-   1 sergeant sergeant     5373 Feb  5  2000 questions.tex

Each line contains information for a single file. The first ten characters provide information about permissions. The first character has a d if the file is a directory and a - otherwise (usually). The remaining nine characters specify the read (r), write (w), and executable (x) permissions for user, group, and others. The file named personal.xls is readable and writeable by the user (sergeant) and not available to anyone else. The file stats.ps is available for reading and writing to the user (sergeant) and the group (webcache), but only readable to everyone else. You may notice that both directories (office52 and rpm) have executable permissions turned on ... without those permissions a directory is not listable/viewable.

There are two main ways to invoke the chmod command. Only the mnemonic method is explained here. The command is followed by a request for new permissions and lastly the filename(s) to which the new permissions should be applied.

chmod guo+rw public.txt makes the file public.txt readable and writeable by anyone
chmod go-rwx *cpp remove all types of access from group or others for all files whose names end with cpp

mkdir
makes a new directory. Simply follow the command with the desired directory name.
rmdir
removes a directory. The main thing to be aware of here is that the directory must be empty before it can be removed with rmdir.
cd
changes to a new working directory. Two notes of interest: cd .. moves one level up the directory tree. Also, cd with no directory name changes to the user's home directory (the directory you are in when you first log in).
pwd
displays present working directory. Depending on how your prompt is displayed you may not have a reminder as to what directory your are currently working in. This command will let you know where you are.

Here are some additional commands (that don't necessarily deal directly with managing files) that you may find helpful:

man
displays manual pages for any command. For example, if you wanted to find out all of the options available for the ls command you can type man ls.
apropos
displays list of commands relevant to the specified topic. Example: apropos clock displays information about various commands dealing with a clock.
find
finds files. This command has numerous options that allow you to find a file based on all manner of criteria. Example: find -name henry.txt will search the current directory and all its subdirectories for a file named henry.txt.
grep
searches files for given text. If you want to find which files contain certain text then grep is what you want. For example: grep Diana *.txt shows all occurrences of ``Diana'' in all files in the current directory whose names end with .txt.
passwd
changes your password. NOTE: Use the yppasswd command if using a lab computer.
script
captures output to a file. This is especially useful for capturing the output of an interactive program (as is often requested by an instructor). The command works as follows:
script myprog.out
ls
javac prog1.java
java prog1
exit
In this example all of the output generated by the ls command and by the program compilation and by the program that was executed are all sent to the file named myprog.out (as well as to the screen). The command exit tells the script command to stop logging the output to the specified file.

Editing Text

There are many fine text-based editors available on csci. The two most widely used editors are vi and emacs. I know and use vi (actually, vim: vi Improved) so that is what I document here. Feel free to experiment with emacs (or other editors).

First, the bad news: vi has a steep learning curve. Plan on spending quite a bit of time learning to use the editor before you attempt to do something substantial with it.

Here are some resources that should help your transition to vi:

Advice
Don't try to use vi to do anything useful until you've gone through an introductory tutorial and you've taken some time to play around with it. Then, and only then, should you try to use it for an assignment.

Also, in the early stages don't try to use vi when you are in a hurry. The idea is that you want to be proficient before you add the stress of ``hurry'' to the stress of learning something new.

Tutorial
On csci there is a vi tutorial called vimtutor. Spend time to go through that tutorial. You may even want to go through it several times until you become comfortable with the various commands.

Quick Reference
There is a two-page ``vi quick reference card'' written by Don Bindner available at http://limestone.truman.edu/~dbindner/mirror/. I can make copies available on request.

On-Line References
There are many on-line references and all you need to find them is

http://www.google.com.

Compiling Programs

Writing and compiling programs using a text-based interface in Linux involves writing/editing the source code using an editor of your choice following by compiling the program by issuing the appropriate command. Many editors provide commands within them to allow compilation without leaving the editor. I sometimes work with two windows open: one for issuing compilation commands and one for editing the source code.

To compile Java programs use:

javac myprog.java compiles Java program found in myprog.java and, if successful, produces a class file named myprog.class
java myprog runs a compiled Java program

Sending Email

For sending and receiving (text-based) email I use a client called mutt. I like it an use it because it is fast, configurable, and simple to use. But, as with many text-based programs it may take a while to become comfortable using it. To invoke this program simply type mutt at the command-line.

Printing Documents

To print virtually any document I recommend that you use the a2ps print formatter rather than sending your document straight to the printer. To use a2ps to print a document named myprog.cpp issue the following command:
    a2ps --printer=jb105 myprog.java
If you can't figure out that the document will appear on the printer in JB 105 then you'll need me to tell you that you need to change ``jb105'' to ``jb104'' to cause the page to print on the printer in JB 104.

Graphical Interface

You may find that while you prefer to do most tasks via a GUI, that some tasks (such as sending email or printing documents) are quite simple when invoked from the command-line. Feel free to consult various subsections from section 2 as needed.

The irony of having an ssh connection through which you forward GUI information is that the graphical programs have to be invoked from the command line. To run any of the programs discussed in this section simply type the name of the program at the command-line. I recommend that you follow the command with an ampersand (&) which will cause the program to be launched in the background and will leave your command-line open to invoke other programs as well. For example to invoke the gedit editor you could type: gedit&

Managing Files

To load a file managing utility (similar to right-clicking on the Windows Start menu and selecting Explore ...) you can use nautilus.

Editing Text

If you only need an editor for creating source code you will want to read the discussion in the following section before reading this recommendation. For general text editing needs gedit works fine. This editor has a simple interface but performs syntax highlighting for some programming languages (including Java).

Compiling Programs

There are two main options for you to consider for writing programs via a remote connection to a Linux server:

The advantage of the IDE is that you may be generally familiar with that style of program development. On the down side, it can be somewhat of a pain to create a simple stand-alone program. Also, the hefty interface may be exceptionally slow over a network. Lastly, if you have trouble using it or understanding it, I will be of no help because I haven't ever used it!

The advantages of using a graphical editor and invoking the compiler manually are that it will work more quickly over the network and I will be able to be of some assistance if things don't go well. Also, you don't have to create an entire directory (folder) filled with various files just to compile a program. On the down side you will need to learn to use the text-based compiler commands.

A scaled-back IDE falls in the middle of the other two options in terms of required network capacity and ease-of-use.

Sending Email

The thunderbird email client is a good choice for sending or reading email in a graphical interface. Consult the Linux Quickstart document for information about how to set it up. Also, evolution is a good choice.

Printing Documents

Any GUI program will provide a menu option for printing. Rather than printing to the default printer you will want to specify an alternate printer (assuming you want to print in one of the labs). The choices for alternate printers should be obvious.

A Next Step

There are many on-line and in-print resources that provide information on how to use Linux. LINUX: Rute User's Tutorial and Exposition is a book that is available in print and (for free) online (see http://www.kefk.net/RUTE/). It is quite comprehensive and, among other things, provides its own list of helpful resources.

Terry Sergeant tsergeant@hsutx.edu

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