UNIX

Basic Shell Scripting

Presentation By

Nihar R Paital

Nihar R Paital

Shell Types in Unix

Bourne Shell. Bourne Again Shell (bash). C Shell (c-shell). Korn Shell (k-shell). TC Shell (tcsh)

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Executing a shell script

There are many ways of executing a shell script:– By passing the shell script name as an argument to the shell. For

example:– sh script1.sh

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Shell Scripts

A script, or file that contains shell commands, is a shell program There are two ways to run a script 1 By using . (dot) command Ex:-

Scriptname – By typing scriptname , if the current directory is part of command

search path . If dot isn’t in your path then type– . /scriptname

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Command-line Editing

Enabling Command-line EditingThere are two ways of entering either editing modeAdd following line in .profile file $ set -o emacs or $ set -o vi

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Simple Control Mode Commands

Basic vi Control Mode Commands CommandDescription h Move left one character l Move right one character w Move right one word b Move left one word e Move to end of current word 0 Move to beginning of line ^ Move to first non-blank character in line $ Move to end of line

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Entering and Changing Text

Commands for Entering vi Input Mode Command Description i Text inserted before current character (insert) a Text inserted after current character (append) I Text inserted at beginning of line A Text inserted at end of line R Text overwrites existing text

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Deletion Commands

Command Description dh Delete one character backwards dl Delete one character forwards db Delete one word backwards dw Delete one word forwards d$ Delete to end of line d0 Delete to beginning of line u undoes the last text modification

command only . redoes the last text modification

command.

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Moving Around in the History File

Command Description k or - Move backward one line j or + Move forward one line G Move to line given by repeat count ?string Search backward for string /string Search forward for string n Repeat search in same direction as

previous N Repeat search in opposite direction of

previous

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The fc Command

fc (fix command) is a shell built-in command It is used to edit one or more commands with editor, and to

run old commands with changes without having to type the entire command in again

The fc -l is to lists previous commands. It takes arguments that refer to commands in the history file.

Arguments can be numbers or alphanumeric strings To see only commands 2 through 4, type fc -l 2 4 To see only one command type fc -l 5 To see commands between ls and cal in history ,type fc -l l c To edit , fc command_no

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The fc Command

With no arguments, fc loads the editor with the most recent command. With a numeric argument, fc loads the editor with the command with

that number. With a string argument, fc loads the most recent command starting

with that string. With two arguments to fc, the arguments specify the beginning and

end of a range of commands,

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Shell Variables.

Positional Parameters. Special Parameters. Named variables

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Positional Parameters. Acquire values from the position of arguments in command line.– $1, $2, $3,..$9– sh file1 10 20 30– Ex: Suppose the content of the below file test1.sh is

#!/bin/kshecho Your arguments are $1 $2 $3

-Run the file test1.sh as

$ test1.sh 10 15 20

Output:

Your arguments are 10 15 20

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Special Parameters.

Shell assigns the value for this parameter.– $$ - PID number.– $# - Number of Command Line Arguments.– $0 – Command Name.– $* - Displays all the command line arguments.– $? – Exit Status.– $- - Shell options– $! - Process number of the last background command– $@ - Same as $*, except when enclosed in double quotes.

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Exit Status

Every UNIX command , at the end of its execution returns a status number to the process that invoked it.

Indicates whether or not the command ran successfully. An exit status of zero is used to indicate successful completion. A nonzero exit

status indicates that the program failed. The shell sets the $? variable to the exit status of the last foreground command that was executed. The constructs if, while, until and the logical AND (&&) and OR (||) operators use exit status to make logical decisions:

0 is a logical "true" (success)1 is a logical "false" (failure)

There are built-in true and false commands which you canuse.

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Exit Status

A shell, like any other process, sets an exit status when it finishesexecuting. Shell scripts will finish in one of the following ways:

Abort - If the script aborts due to an internal errorand exit or return command, the exitstatus is that set by those commands.

, the exit status isthat of the last command (the one that aborted the script).

End - If the script runs to completion, the exit status is that of the lastcommand in the script

Exit - If the script encounters

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Named Variables.

User-defined variable that can be assigned a value. Used extensively in shell-scripts. Used for reading data, storing and displaying it.

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Accepting Data from User.

read.– Accepts input from the user.– Syntax : read variable_name.– Example : $ read sname # This will prompt for user input

Here sname is the user defied variable

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Display Data.

echo– Used to display a message or any data as required by the user.– echo [Message, Variable]– Example: $ echo “IBM.” $ echo $sname # This will display the value of sname

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Comment Line

Normally we use the comment lines for documentation purpose. The comment lines are not compiled by the compiler For make a line a comment line we use # symbol at the beginning of

the line

For Ex:# This is the First Program

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test command.

Used extensively for evaluating shell script conditions. It evaluates the condition on its right and returns a true or false exit

status. The return value is used by the construct for further execution. In place of writing test explicitly, the user could also use [ ].

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test command (Contd).

Operators used with test for evaluating numeral data are: -eq Equal To

-lt Less than -gt Greater than -ge Greater than or equal to -le Less than or equal to -ne not equal to

Operators used with test for evaluating string data are:str1 = str2 True if both equalsstr1 != str2 True if not equals-n str1 True if str1 is not a null string-z str1 True if str1 is a null string

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test command (Contd).

Operators used with test for evaluating file data are:

-f file1 True if file1 exists and is a regular file.-d file1 True if file1 exists and is directory.-s file1 True if file1 exists and has size greater than 0-r file1 True if file1 exists and is readable.-w file1 True if file1 exists and is writable.-x file1 True if file1 exists and is executable.

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Logical Operators.

Logical Operators used with test are:

! Negates the expression. -a Binary ‘and’ operator. -o Binary ‘or’ operator.

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expr command.

Used for evaluating shell expressions. Used for arithmetic and string operations.

– Example : expr 7 + 3 would give an output 10.

When used with variables, back quotes need to be used.

Operator has to be preceded and followed by a space.

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expr command.

String operations

Expr can perform three important string functions: 1) Determine the length of the string 2) Extract a substring 3) Locate the position of a character in a string For manipulating strings ,expr uses two expressions seperated by a

colon.The string to be worked upon is placed on the left of the : and a regular expression is placed on its right.

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1) The length of the string

$ x="shellscripting"$ expr length $x$ expr $x : '.*'$ expr "unix training" : '.*'

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2) Extracting a substring

Syntax: expr substr string position lengthSubstr is a keyword , string is any string

$ x="IBMIndia"$ expr substr $x 2 3

$ y=unix$ expr "$y" : '..\(..\)' O/p :- ix$ expr "$y" : '.\(..\)' O/p: - ni $ expr " abcdef" : '..\(...\)' O/p:- bcd

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3) Locating position of a character

$ expr index $x chars Index is a keyword X is a variable Chars is any character of a string whose position is to be located x=shell $ expr index $x e O/p:- 3

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Conditional Execution.

&&– The second command is executed only when first is successful. – command1 && command2

||– The second command is executed only when the first is

unsuccessful.– command1 || command2

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Program Constructs

if for while until case

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if statement.

Syntax

if control command then

<commands> else

<commands> fi

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Ex:if statement.

1) If [ 10 -gt 5 ] then

echo hi else

echo bye fi

2) If grep "unix" xyz && echo found then

ls -l xyz fi

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