App-iServ : Understanding Shell Languages - VPS v2 Virtual Server

UNIX is an operating system environment that you can interact with by using a program called a “shell”. The shell interprets your commands to the operating system, and also serves as a programming environment, in which you write programs called scripts.

Shell programming languages are interpretive. A program called the interpreter runs every time the application is to run, and interprets and acts on the application, line by line.

Developing a program in a shell language requires none of the hassle of compilers and loaders - just point the interpreter at the text file containing the program source and it will run.

The shells that come with your VPS v2 Virtual Server include:

bash GNU Bourne-Again shell
csh A shell command interpreter) with C-like syntax
ksh Public domain Korn shell
sh The Bourne shell, developed by Steven Bourne
tcsh C shell with file name completion and command line editing
zsh A powerful shell that uses the best of bash and tcsh.

Note: tcsh is the default shell for your VPS v2 Virtual Server.

Information on each of these shells can be obtained from a man page query:

% man tcsh

You can change a VPS v2 Virtual Server's default login shell by using the chsh command. When you run this command, it starts up whatever you have set as your default editor, and it allows you to change any of the following information:

User database information for VPS v2 Virtual Servers

Shell: /bin/tcsh
Full Name: Enetrics Communications
Location:
Office Phone:
Home Phone:

Changing Your Shell from /bin/tcsh to /bin/bash

To change your default shell:

Type:

% chsh
Change the path for your shell to: /usr/local/bin/bash.
Save the file. The shell takes effect next time you login to the VPS v2 Virtual Server.

Understanding the Tenex C (tcsh) Shell

Since tcsh is the standard shell with the VPS v2 Virtual Server, you should understand how it works with your VPS v2 Virtual Server. Each shell language is also an interpreter. Shells can be used like Perl or other interpreted languages to write scripts, or automate systems administration tasks. For example, a simple tcsh script might look like the following:

#!/bin/tcsh

echo "Content-type: text/plain"

echo ""

printenv

Note: If this script were called from the Web, the user's "environment" would be output to the browser.

Some of C shells features include the ability to:

Pipe output of one program into the input of another program

Use the asterisk ("*") for wildcard filename abbreviations

Use shell variables (such as $HOME) for customizing the environment

Access previous commands (command history)

Create aliases (such as the www alias in the $HOME directory) in a shell program

The C shell configuration files are found in the users $HOME directory:

.cshrc Executes every time a new shell is opened (i.e., every time you make a Telnet connection to your server).
.history Saves a list of commands executed from the command-line.
.login After the .cshrc file is executed, .login is run.
.logout Executed by the shell when the user logs out.

Other important configuration files can be found in your /etc/ directory:

Password file
Sendmail file
Aliases file.

See the csh man page for more information.

UNIX Commands and Descriptions

The following table lists UNIX commands.

Command	Description
bg	Put the current job in the background
break	Resume execution (break out of while or foreach loop)
breaksw	Break out of switch statement
case	Identify a pattern in a switch statement
cd	Change Directory. Default changes user to home directory
chdir	Same as cd
continue	Resumes execution of while or for each loop
default	Labels the default case in a switch statement
dirs	Prints the directory stack
echo	Write supplied string to stdout
end	Ends a foreach or switch statement
endif	Ends an if statement
eval	Eval is usually passed an argument. It resolves the variable then runs the resulting command
exec	Executes a command
exit	Exits a shell script
fg	Brings job to the foreground (see bg)
foreach/end	Runs a foreach loop
glob	Similar to echo, except no \ escapes are recognized. Often used in scripts to force a value to remain the same for the rest of the script
goto	Skips to a line beginning with whatever string you put after the goto command
hashstat	Displays statistics that show the success level of locating commands via the path variable
history	Displays a list of events
if	Begins a conditional statement
Jobs-1	List all running or stopped jobs
kill options id	Terminate the process ID(s) or job ID(s) specified
kill (proc id)	Kill the process id number given, usually found through a ps -auxw command.
limit	Displays limits set on a process or all limits if no arguments are given.
login	Replaces users login shell with /bin/login
logout	Terminates shell login
nice	Changes execution priority for a specified command
rehash	Recomputes the hash table for the PATH variable (when you create a new command, run rehash so the hash table finds the command.
set	Sets a variable to a value.
setenv	Assigns a value to an environment variable name
source	Reads and executes commands in a CSH script. For example, if you add or modify your .cshrc file, type source .cshrc.
stop	Stops a background job from running.
suspend	Suspends the current foreground job.
time	Runs a command to show how much time it uses. Use this in a shell script to tell how long it took to run.
umask	Displays or sets the file creation mask
unalias	Removes an alias from the alias list
unhash	Remove the internal hash table (and instead spends the path in the PATH variable.)
unlimit	Removes allocation limits on resource.
unset	Removes one or more variables set by the set command
unsetenv	Removes an environment variable
wait	Waits until all background jobs are completed.
while/end	While loop

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