UNIX - Class1 - Basic Shell

UNIX

Basic Shell Scripting

Presentation By

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 Editing
There are two ways of entering either editing mode
Add 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
Command Description
 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. of arguments in command line.
Acquire values from the position

– $1, $2, $3,..$9
– sh file1 10 20 30
– Ex: Suppose the content of the below file test1.sh is

#!/bin/ksh
echo 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 can
use.

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Exit Status

 A shell, like any other process, sets an exit status when it
finishes
executing. 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 exit
status is that set by those commands.
, the exit status is
that of the last command (the one that aborted the script).
 End - If the script runs to completion, the exit status is that of the last
command 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 equals
str1 != str2  True if not equals
-n str1 True if str1 is not a null string
-z str1  True if str1 is a null string Nihar R Paital

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 Operator has to be preceded and followed by a space.

would give an output 10.
 When used with variables, back quotes need to be used.

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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 length
Substr 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

Nihar R Paital

UNIX - Class1 - Basic Shell

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UNIX - Class1 - Basic Shell