User:Spoonbender/Temp/Scripting.doc

Writing Basic Shell Programs

• A text file containing UNIX commands
• A text with read and execute permissions
• A program executed/interpreted by the shell
• A tool for automating UNIX tasks
• A program that doesn’t need to be compiled

Step 1. Create a text file using vi

Step 2. Put commands inside the text file

Step 3. Save and exit from vi

Step 4. Add an execute permission to the text file

Step 5. Run the script using either relative or absolute filename

• #!/usr/bin/shInterpret using default shell
• #!/usr/bin/cshInterpret using C shell
• #!/usr/bin/kshInterpret using Korn shell

Variables

Variable assignment:

variable_name=value

or

variable_name=“value string”

Using variable values:

$variable_name

To make a variable an environment variable, we use the command:

export variable_name

Command Line Positional Parameters

The arguments on the command line are referenced within the shell program through special variables that are defined relative to an argument’s position in the command line called positional parameters

$ prog_name arg1 arg2 … argX

Parameter	Description	Variable Substitution
*	All positional parameters	$*
@	All positional parameters	$@
#	Number of positional parameters	$#

Read and Shift Commands

The read command is used to gather information typed at the terminal during program execution

read variable_name

The shift command will reassign the command-line arguments to the positional parameters, allowing you to increment through the command-line arguments

shift [n]

Using Return Codes

All UNIX commands will generate a return code upon completion of the command

echo $?

0 The previous command worked successfully (TRUE)

1 The previous command worked but was not successful (FALSE)

2 The previous command did not work. (FALSE). Probable causes could be invalid command invocation (syntax) or incorrect file/directory name

126 The previous command failed due to inappropriate file/directory permission settings (the message: “permission denied” is usually presented to stderr)

127 The command could not be found and executed due to: incorrect pathname or incorrect command name (the message: “not found” is usually presented to stderr)

129-160 The command exited due to the receipt of a signal

Testing Parameters

The test command is used to evaluate expressions and generate a return code

test expression or [ expression ]

The test command can evaluate the condition of

• Integers
• Strings
• Files

Numeric Tests

[ exp1 –eq exp2 ]

[ exp1 –ne exp2 ]

[ exp1 –lt exp2 ]

[ exp1 –gt exp2 ]

[ exp1 –le exp2 ]

[ exp1 –ge exp2 ]

String Tests

[ string1 = string2 ]

[ string1 != string2 ]

[ string1 < string2 ]

[ string1 > string2 ]

[ -n string ] String is non-zero in length

[ -z string ]String is zero in length

File Tests

-e fileTrue, if file exists

-d dirTrue, if dir is a directory

-f fileTrue, if file exists and is an ordinary file

-s fileTrue, if file exists and has a size greater than zero

-r fileTrue, if file exists and is readable

-x fileTrue, if file exists and is executable

-w fileTrue, if file exists and is writable

Other Operators

[ exp1 –o exp2 ]OR

[ exp1 –a exp2 ]AND

test ! –d fileNOT

[ \(exp1 –a exp2\) –o exp2 ] GROUPING 

The IF and CASE Commands

The if … then … fi Statement

if expression

then

...

commands

...

fi

The else Statement

if expression

then

...

commands executed if condition is True

else

...

commands executed if condition is False

fi

Compound if Statements

if condition_1

then

commands executed if condition_1 is True

elif condition_2

then

commands executed if condition_2 is True

elif condition_3

then

commands executed if condition_3 is True

else

commands executed if condition1,2&3 are False

fi

The case Statement

case value in

pattern_1)commands to execute ;;

pattern_2)commands to execute ;;

pattern_n)commands to execute ;;

esac

Shell Programming Loops

Arithmetic Evaluation

The let command enables the shell scripts to use arithmetic expressions

let expression or (( expression ))

The WHILE Command - Repeat the loop while the condition is true

while condition

do

commands

done

The UNTIL Command - Repeat the loop until the condition is true

until condition

do

commands

done

The FOR Command - For each item in list, repeat the loop, assigning var to the next item in list until the list is exhausted

for var in list

do

commands

done

The break, continue, and exit

break [n] - Terminates the iteration of the loop and skips to the next command after [the nth] done

continue [n] Stops the current iteration of the loop and skips to the beginning of the next iteration [of the nth] enclosing loop

exit [n]Stops the execution of the shell program, and sets the return code to n