1 © 2001 John Urrutia. All rights reserved. Chapter 10 using the Bourne Again Shell.

1© 2001 John Urrutia. All rights reserved.

Chapter 10

using the Bourne Again

Shell


TopicsControl Structures

The Here Document

Expanding NULL or unset variables

The Builtins

Functions


Control StructuresSelections

if … then … fi One-Way Selection

if … then … else … fi Two-Way Selection

if … then … elif … fi Multi -Way Selection

The above builtins must be on separate

lines or separated by ;


Control StructuresSyntax:

if test expression or [expression ]

thencommand(s)

fi

…

Must Evaluate True or False

Boolean Expression

Executes if

expression is true

test expression or [expression ]

if

then

Executes if

expression is false

fi


Boolean ExpressionsAlgebra created by George Boole

Always evaluates to a binary stateGenerally:

1 is TRUE0 is FALSE


Boolean or Logical 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 [expression ]

Evaluates the expression and returns a Boolean true or false.

expression can be simple or compound

Criteria:String IntegerFilenameFile descriptor number


Test CriteriaString expressions – applies to string

variables or literalsIs null or

Length is zero (-z) orLength is > 0 (-n)

string1 = or != string2

If you are comparing literals quote them


Testing Strings

…]$ if test "not" != "equal";then echo Not Equal;fiNot Equal…]$ if [ "not" = "equal“ ];then echo Equal;fi…]$


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


Testing Integers

…]$ if test 123 -ne 234;then echo Not Equal;fiNot Equal…]$ if [ 123 -eq 234 ];then echo Equal;fi…]$


Test Criteria (cont.)Filename expression – file exists and has

specific attributesdrewsx

-directory

-readable

-exists

-writable

-size – file has data (length >0)

-x executable


Testing Files

…]$ if [ ! -e “TestFile” ];then echo Not found;fiNot found…]$

…]$ if test -e “TestFile”;then echo Found it;fi…]$


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


if …then … else … fi

Start Test 1 False

True


Bourne - elifelif

“else if” structureLinear in natureSimilar to the case or switch in selection

but completely different in execution.


if …then … elif … else … fi

Start Test 2 False

True

Test 1

True

if

then

elif else


if … then … [ else or elif ] … fi

2 structuresif […] then cmds

if […] then cmds else cmds else cmds fi

if […] then cmds elif […] then cmds else cmds fi


if [ $a –gt 2 ]then


echo value is 4 else

echo value is 3fi

elseif [ $a –gt 1 ]then

echo value is 2else

echo value is 1fi


echo value is 4

elif [ $a –gt 2 ]then

echo value is 3

elif [ $a –gt 1 ] then

echo value is 2

elseecho value is 1

fi


Control StructuresIterations

for … in

for


for… and for …inAssign

element

Docommands

Moreelements

Done


Here we go loop-de-loopfor x in a b c

do cmd

cmd cmd done

Substitutes the x variable with each in variable and executes the commands between do and done


California the land of …echo “California the land of – “

for info in fruits nuts cheese wine

do echo –n “ $info”

done

echo –e “\n And of course Hollywood”


Here we go loop-de-loopfor x

docmdcmdcmd

done

Substitutes the x with command line arguments and executes the commands between do and done


California the land of …echo “California the land of – “

for arg in fruits nuts cheese wine

doecho –n “ $arg”

done

echo “ And of course Hollywood”

This modification causes the scriptto display all command line arguments

again


Control StructuresIterations

while

until


While I’m here – Until it’s gonewhile […] (initial state true - go)

until […] (initial state true – stop)do

cmd cmd cmddone


While you were out… cold

WhileTest

docommands

Done

Pre-Test Loop


Until … The cows come home

untilTest

docommands

Done

Pre-Test Loop


Continue to Breakcontinue and break

Used in for while and until structuresbreak – terminates the structurecontinue – terminates this iteration


continuefor idx in 1 2 3 4 5 6 7 8 9 10do

if [ $idx –le 3 ] ; thenecho “continue”continue

fiecho $idx

done

Continues three times

Echoes 4 – 10


breakfor idx in 1 2 3 4 5 6 7 8 9 10do

if [ $idx –gt 6 ] ; thenecho “break”break

fiecho $idx

done

Stops for loop after

sixth iteration

Echoes 1 – 6


Bourne – caseThe case command tests for multiple

values in a variable

Allows the use of “wild cards”

First match wins


Get on this case

case $var in test-var1 )

cmd1;cmd2cmd3;;

test-var2 )cmds;;

) cmds ;;esac


case “$variable” inA|a )

echo Entered A or a;;

[0-9] )echo Entered number;;

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

esac



The Here Document


The Builtins

Functions


Here boy

<<

The Here DocumentAllows in-stream data to feed a

script.

Must start with << and a data delimiter character

Data delimiter character on line by itself - terminates


Here in the script

#!/bin/bashgrep –i “$1” <<+Alex June 22Babs February 3Leroy January 20+

echo “All Done now”


Bundle script #!/bin/bashecho “# To unbundle sh this filefor ido echo “echo $i 1>&2”

echo “cat >$i <<‘End of $i’”cat $iecho “End of $i”

done


Bundle script -- output # To unbundle sh this fileecho 1st filename 1>&2cat > 1st filename <<‘End of 1st filename’Contents of 1st filename..End of 1st filename



The Here Document


The Builtins

Functions


Expanding Null or Unset Vars. ${name} – expands to the value of

the variable.

If variable is null or not set the expansion produces a null string (\0)

${name:-default} – expands to the value of the variable or the default value if null.


Expanding Null or Unset Vars. ${name:=default} – expands to

the value of the variable or the default value if null and sets the variable to the default value.

: ${name:=default} – if null sets the variable to the default value.

${name:?message} – if null or unset sends message to stdout.



The Here Document


The Builtins

Functions


The execute commandThe exec command

Executes scripts or programs

Runs under the same PID

Provides access to the original environment variables

Terminates current process.


The execute commandThe exec command

Can be used to redirect stdin, stdout and stderr from inside a script.

exec < infile

exec > outfile 2> errfile


I feel like trap – let’s kill a PIDtrap ’action ’ [signal number]

64 signals available 0 – 631 – Phone Hangup2 – Pressing an interrupt key ( ^c)3 – Pressing the quit key (Ctrl+Shft+| or \)9 – Kill command (without mercy ^d)15 – Terminate process (kill command)20 – Job control STOP key (^z)


I feel like trap – let’s kill a PIDkill - [signal number] [pid ]

Sends a signal to one or more PIDs

By default sends signal 15 (SIGTERM)

Using signal 9 is un-trappable

Get-out … NOW!kill –9 0



The Here Document


The Builtins

Functions


Play that Function musicFunctions are like pre-processed

scripts.

Can be stored in .profile or in script files.

Generally used by more than 1 script.


Function declarationFunction-name ()

{commands

}

Functions can be called with arguments and process them the same way a command line would.

1 © 2001 John Urrutia. All rights reserved. Chapter 10 using the Bourne Again Shell.

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Transcript of 1 © 2001 John Urrutia. All rights reserved. Chapter 10 using the Bourne Again Shell.