Shell scripts are text files that automate a series of UNIX environment-based commands that otherwise must be performed one at a time. Shell scripts are often used to automate command sequences that repeat, such as services that start or stop on system start up or shut down.

Any command that can be performed from the command line, such as ls, can be included in a shell script. Similarly, any command that can be included in a shell script can be performed on the UNIX environment command line.

Users with little or no programming experience can create and run shell scripts. You initiate the sequence of commands in the shell script by simply entering the name of the shell script on a command line.

Determining the Type of Shell to Run a Shell Script

There are several different shells available in the Solaris OS. Two of the most commonly used shells are the Bourne shell and the Korn shell.

To ensure that the correct shell is used to run a shell script, the first line of the script should always begin with the characters #!, followed immediately by the absolute path name of the shell required to run the script. These must be the only characters on the first line of the file.

#!/full-pathname-of-shell

For example:

#!/bin/sh

or

#!/bin/ksh

Comments
Comments are text entries that often provide information about a shell  script. They are inserted into a shell script but have no effect on the script  itself. Comments are ignored by the shell and are solely for the benefit of the  user.
Comments are preceded by the hash (#) character. Whenever the shell  encounters a word beginning with the # character it ignores all text on  that line.
For example:
# this is a comment
ls -l 		# list the files in a directory

Options are switches that control the behavior of the Korn shell. Options are boolean, meaning that they can be either on or off.

To switch an option on, type:

$ set -o option_name

To switch an option off, type:

$ set +o option_name

To show current option settings, type:

$ set -o

Protecting File Content During I/O Redirection

Redirecting standard output to an existing file overwrites the previous file content, which results in data loss. This process of overwriting existing data is known as clobbering. To prevent an overwrite from occurring, the shell supports a noclobber option.

When the noclobber option is set, the shell refuses to redirect standard output to the existing file and displays an error message to the screen.

The noclobber option is activated in the shell using the set command. For example:

$ set -o noclobber
$ set -o | grep noclobber
noclobber        on
$ ps -ef > file_new
$ cat /etc/passwd > file_new
ksh: file_new: file already exists
$

Deactivating the noclobber Option
To temporarily deactivate the noclobber option, use the  >| deactivation syntax on the command line. The noclobber option is ignored for this command line only, and the contents of the file are  overwritten.

$ ls -l >| file_new

 



Note: There is no space between the  > and | on the command line.



 
To deactivate the noclobber option, perform the following  commands:
$ set +o noclobber $ set -o | grep noclobber
 noclobber        off 
$ ls -l > file_new $
 

Original post blogged on b2evolution.

	

Functions are a powerful feature of shell programming used to construct customized commands. A function is a group of UNIX commands organized as separate routines. Using a function involves two steps:

1. Define the function.
2. Invoke the function.

Defining a Function

A function is defined using the general format:

function_name { command; . . . command; }

Function Examples

The following example creates a function called num to perform the who command, directing the output to the wc command to display the total number of users currently logged on the system:

$ function num { who | wc -l; }
$ num
	 9

The following example creates a function called list to perform the ls command, directing the output to the wc command to display the total number of subdirectories and files in the current directory:

$ function list { ls -al | wc -l; }
$ list
	 34

To display a list of all functions, use the following command:

$ typeset -f
function list
{
ls -al | wc -l; }
function num
{
who | wc -l; }

To display just the function names, use the following command:

$ typeset +f
list
num

Configuring the Shell Environment
The shell secondary prompt string is stored in the PS2 shell  variable, and you can customize it as needed.
$ PS2="somethings missing >"
$ echo $PS2
somethings missing >
$
In this example, the secondary prompt displays the message that the command  line is incomplete.



Note: To have this secondary shell prompt  appear in every shell, it must be included in the user's Korn shell  initialization file (usually named .kshrc).