1 © 2000 John Urrutia. All rights reserved. Session 5 The Bourne Shell.

1© 2000 John Urrutia. All rights reserved.

Session 5

The Bourne Shell


OverviewShell fundamentals

Streams revisited

Processes and Subshells

Shell script basics

Examples


Shell fundamentalsCommand separation

Command grouping

Job control (limited in Bourne)


Command SeparationNewline (nl)X’0D0A’

ends command and initiates execution

Semicolon (;)just separates commands

Backslash (\) X’5C0D0A’at end of line and before you type return

Allows command to be continued


Command Separation (cont.)Ampersand (&)

execute task in the background

Pipe ( | ) pipe


Command GroupingParenthesis used to group commands

causes Shell to create a subshell

additional processes are created as required when the subshell runs the commands within the parenthesis (ls ; date; w) ; more (ls ; date; w) | more


Job ControlAmpersand &

tells the Operating system to run the job in the background

User will still be able to interact with the shell

Pure Bourne shell has limited ability. Can not deal with a specific job it has put into background after initial creation. C shell much better.


Job Control (continued)First two jobs in background, c in

foregrounda & b & c

Entire sequence put into backgrounda | b | c &

All three jobs executed in backgrounda & b & c &


Streams RevisitedThree streams

standard in < or 0<

standard out > or 1>

standard error 2>


Streams standard I/Ocat x y

if x exists and y does not, contents of x and error message due to y are sent to terminal

both standard out and standard error default to the terminal


Streams Continuedcat x y 2>error.log

standard error is sent to a file to separate it from the expected results of the command

cat x y 2>>newfile 1>>newfilestandard out is redirected to newfile


Processes and SubshellsA process is the execution of a command

login to UNIX

execution of a UNIX utility

execution of a shell script creates a new process

script commands each start a new process

Process structure is hierarchical


Process FlowUser logs in: shell process is created

User issues command, enters returnShell creates a subshell

child process is forked or spawnedunless the command is built into the bourne

shell process


Process flow (cont.)Subshell is a clone of the parent shell

Subshell tries to exec the commandIf program, the program runsIf shell script, exec fails and subshell

interprets commands.


Process FlowParent Shell sleeps until child shell

finishes(unless job was executed in background)

Variables that are used in a parent can be sent to a child, but the reverse is not true.


Process FlowShell Scripts need to have execute

permission if you just type the file name as you would a command

Alternative (new subshell): sh file

Alternative (current shell): • file


Shell Script BasicsCreating a Shell Script

Keyword Shell Variables

User Created Variables

Readonly Shell Variables


Creating a Shell ScriptUse a text editor like vi

First line should start with #! Followed by the absolute pathname of the shell that is to interpret the script. (default is C shell)#!/bin/sh

Lines which start with a # are comments(except the special line mentioned above)


Keyword Shell VariablesHOME (contains login directory)

PATH (Used by shell to locate commands you type in)/usr/bin:/usr/sbin:/class/n01/bin:

MAIL (contains name of central post office file for your mail)

PS1, PS2 (primary and secondary prompts)


Keyword Variables (continued)CDPATH

like PATH, except used by cd command

TZtimezone

IFSInternal field separator. Blanks and tabs

are default


User Created VariablesCreate a variable by giving a name of your

choice and an optional valuename=charlie

NO blanks around the equal sign!!

Remove variableunset name

Keep variable but remove valuename=


Readonly Shell VariablesTwo types: user created variable that

has been declared to be readonlyreadonly name

keeps later statements from changing the value

Special Shell variables Positional Variables

Miscellanous variables


Positional Variables$1 through $9

Keep the first nine arguments entered after the name of the shell script

myscript aardvark dog cat$1 will contain the word aardvark

$2 will contain the word dog

$3 will contain the word cat


Miscellaneous Variables$* contains all arguments (not just the

first one)

$@ similar to $*, except that it internally quotes each argument.

$# total number of arguments

$$ process id number (pid) of current process


Shift commandPromotes values of each positional

variable to the left.

Contents of $1 go to ‘bit bucket’

Contents of $2 go to $1

Contents of $3 go to $2

etc (etcetera, not etci)


Boolean ExpressionsAlgebra created by George Boole

Always evaluates to a binary stateGenerally:

1 is TRUE0 is FALSE


Boolean Operators

And-a

Or-o

Not !


and Boolean Truth Tables

Expr 1 Expr 2 -a

True True True

FalseTrue False

False True False

False False False


or Boolean Truth Tables

True True True

FalseTrue True

False True True

False False False

Expr 1 Expr 2 -o


not Boolean Truth Tables

Expr !

FalseTrue

False True


test ing, test ing, test ingtest expression or [ … ]

Evaluates the expression and returns a Boolean true or false.

expression can be simple or compound


Test CriteriaString expresions

Is null orLength is zero (-z) orLength is > 0 (-n)

string1 = or != string2


Test Criteria (cont.)Filename expression

File exists and has specific attributes -block -character -directory -size – file has data (length >0)


Test Criteria (cont.)Integer relationship

-gt greater than

-ge greater than or equal to

-eq equal to

-ne not equal to

-le less than or equal to

-lt less than


Bourne - if, thenif

Establishes a control structureFollowed by a test command

thenCommands executed if test is true


Bourne – else, fielse

Commands executed if test is false

fiTerminates this if statement


Bourne - elifelif

“else if” structureLinear in nature


if $a –gt 2then

if $a –gt 3then

echo value is 4 else

echo value is 3fi

elseif $a –gt 1then

echo value is 2else

echo value is 1fi

if $a –gt 3then

echo value is 4elif $a –gt 2

thenecho value is 3

elif $a –gt 1 then

echo value is 2else

echo value is 1fi


Bourne – caseThe case command tests for

multiple values in a variable

Allows the use of “wild cards”

First match wins


case “$variable” inA|a )

echo Entered A or a;;

[0-9] )echo Entered number;;

?z* )echo Entered z in 2nd position;;

esac


Examples: Myscript#!/bin/sh#-----------------------------------------------------------------## Script Name: myscript# Written by: Charlie Verboom# Date: 9/19/97# Purpose: demonstrate the use of# variables and displays upon terminal# Arguments: 3 arguments used to set# variables to be displayed#-----------------------------------------------------------------#


Myscript (continued)echo the first argument is $1

echo the second argument is $2

echo the third argument is $3

echo $# arguments were typed in

echo the current process number is $$

echo script executed successfully> log.$$


Additions to myscriptecho Please type in your name

read name

echo You typed in $name

echo Please type in your age

age=`head -1`

echo Your age is $age

1 © 2000 John Urrutia. All rights reserved. Session 5 The Bourne Shell.

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Transcript of 1 © 2000 John Urrutia. All rights reserved. Session 5 The Bourne Shell.