Shell Scripting – Putting it All Together

AgendaEscaping CharactersWildcardsRedirecting OutputChaining and Conditional ChainingUnnamed and Named Replacement ParametersIF statementIF ELSE statementMenusSubroutines and Procedures

Escaping CharactersThe “&” and “|” are special characters to the

command interpreterIf you wish to use these characters in a text

string, they must be “escaped” by using the “^” (caret symbol)

For example you want to echo the phrase “Yes & No”

Caret SymbolInput & Output

By using the ^ symbol the Shell does not interpret the & symbol as a chaining command and thus displays the & as a string

WildcardsMost commands allow you to use wildcard

characters to handle more than one file at a time.

Asterisk (*) – substitutes multiple charactersQuestion Mark (?) – substitutes only 1

character

Wildcards ExampleBUDGET.JAN BUDGET.FEB

BUDGET.MARBANK.DOC REPORT12.DOC

REPORT2.DOC

C:\>dir *.*C:\>dir budget.*C:\>dir b*C:\>dir *.docC:\>dir budget.?a?C:\>dir report?.doc

Redirecting OutputYou can modify where the output of a

command goesBy default the output of a commands is

generally displayed on the screenYou can redirect the output to a file (>, >>)You can redirect the output to be used as

input for another command (|)

Redirecting Output to a File> - when you redirect output to an existing

file, the redirected output replaces the original file

>> - you can add, or append to the end of an existing file

If the file doesn’t exist, it will be created in either case

ExamplesFile1.txt

This is file 1.File2.txt

This is file 2.C:\>type file1.txt >> file2.txtC:\>type file1.txt > file2.txtC:\>type file1.txt > file3.txt

Connecting Commands with a PipeYou can redirect the output of one command

to be the input of another. The two commands become connected using a pipe (|). You pipe the output of one command and use it as the input for a filter command.

C:\>dir \DOS | moreC:\>dir | sort

Chaining and Conditional Chaining& - chains two commands together

ECHO Unsuccessful & GOTO :EOF&& - executes the second command ONLY if

the first command was successfulCOPY *.txt && ECHO Operation Successful

|| - executes the second command ONLY if the first command was unsuccessfulCOPY *.txt || ECHO Operation Unsuccessful

Replaceable ParametersA replaceable parameter is a “placeholder”

for information entered on the command line and inserted by the command interpreter into the script during run time

2 types:Unnamed Replacement Parameters

Use %1, %2, %3, %4, %5, %6, %7, %8, %9Named Replacement Parameters

Use a named variable, either e.g. %path% -- a predefined environment variable,

or SET [variable]=[value] -- user defined variable

Unnamed Replacement Parameters

• Names are “hardwired” in the script – no flexibility

• No user input before script runs – no dynamic changes

• Changes to script must be retyped – thus, error prone

Unnamed Replacement Parameters

• Statements are not “hardwired” – gives flexibility

• User input on the CLI – allows dynamic changes

• No retyping of code – thus, less error prone

Unnamed Replacement Parameters

Using Parameters Requires…

Error EntrapmentGood programming tests for the existence of

parameters and displays a messageDocumentation and messages

Good programming provides the user with correct command syntax

Error Entrapment and Documentation

Named Replacement ParametersSystem-wide variables used to store informationTwo types

Environment variables

Can only be added, deleted or modified by members of the administrators group

user defined

Can be added, deleted and modified by members of the users group

Environment Variables•The environment is an area of memory which the operating system uses as a scratch pad

•Stores “mission critical” information such as:

•COMPUTERNAME

•USERNAME

•USERPROFILE

•OS

•PATH

•TEMP and TMP + MANY MORE SEE SET

Predefined Environment Variables

User Defined Variables

•SET [variable]=[string]

used to create a variable

•SET /P [variable]=prompt[string]

•Used to capture user input

•SET /A [variable]=[value]

•Used to do arithmetic

•All information stored as text strings, even numbers

Named Replacement Parameters

Named Replacement Parameters

GOTO commandUse with caution if the label is before the

script, you can create an endless loop (unless there is a control to bypass the GOTO) for example

GOTO CommandOn the other hand, if the label is after the

GOTO statement, you can skip commands and jump to a new section

Execution of the script will continue to the end and you cannot go back to the unexecuted commands unless you use another GOTO

GOTO Command

Jumps to :Copy and executes the remaining commands

4 Types of Conditional TestsIF [NOT]“string1” == “string2” [command]

compares two strings for same valueIF [NOT] ERRORLEVEL 1 [command]

Commands return all values 1 AND above 0 Success 1 Failure2 execution failure 4 math failure

IF [NOT]“%ERRORLEVEL%” == “2” [command]Tests for a specific value ONLY

IF [NOT] EXIST filename [command]Checks to see if a file is located in that directoryTo check a directory, add “\.” after the directory name

IF [NOT] DEFINED [variable] [command]Tests for the existence of a variable

MenuDisplaying choices to the user and controlling

user selectionBranch using a user defined variable

Using IF or IF /I to compare stringsBranch using ERRORLEVEL

Trap the actual key on the keyboard

Branching – Using variable

Branching – Using Error Level• based on ASCII values

Subroutines and ProceduresNormally the “command interpreter”

executes scripts line by line starting at the beginning of a file

To change the order of execution we use subroutines and procedures

Using Subroutines and Procedures helps to organize your code for easier reading and creates “building blocks” of code which can be easily reused

Subroutines Are created with the GOTO commandThe GOTO command creates an

unconditional branch to a labelA label identifies a location in your script

A label is preceded with a colon, ie. :StartThe Shell then executes the commands after

the label

ProceduresAre created using the CALL command

ExternalInternal

The difference between a subroutine and a procedure is how the Shell behaves

With procedures execution continues at the designated label, proceeds to the end of the file and then returns to the line following the CALL statement

External Procedures

1

2

Internal ProceduresMyScript.cmd

:startcmd1CALL :Copy

:Copycmd1cmd2

:Makecmd1

:EOF

12

GOTO :EOF