Xuan Guo Chapter 5 The Bourne Shell Graham Glass and King Ables, UNIX for Programmers and Users, Third Edition, Pearson Prentice Hall, 2003. Notes by Michael

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Chapter 5The Bourne Shell

Graham Glass and King Ables,

UNIX for Programmers and Users,

Third Edition, Pearson Prentice Hall, 2003.

Notes by Michael Weeks

Updated by Xuan Guo

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Creating/Assigning a Variable

• Name=value– Variable created if it does not exist– Need quotes if value contains spaces– e.g. x=”one two”

• Access variable with $ in front– e.g., echo $x– Also, echo ${x}3

mweeks$ x="one two"mweeks$ echo $xone twomweeks$ echo ${x}3one two3

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Access Variations

• If name not set, replace with word • Variable x is set, so we see it

displayed• Variable x is not set, so we see

“three” ${name-word}

$ echo ${x-three}one two

$ echo ${z-three}three

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Access Variations

• If name is set, replace with word • Variable x is set• Variable z is not set, so nothing is

printed${name+word}

$ echo ${x+three}three

$ echo ${z+three}

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Access Variations

• If name not set, assign with word • Variable x is set, and printed• Variable z is not set, so now it is set

to “three”${name=word}

$ echo ${x=three}one two$ echo ${z=three}three$ echo $zthree

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Access Variations

• If name is set, return it• Variable x is set• If name is not set, print word to

standard error (and quit)• Variable w is not set ${name?word}

$ echo ${x?three}one two

$ echo ${w?three}-bash: w: three

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Reading from Standard Input

• read [-p] {variable}+• Reads 1 line• Examples

$ read v1 v2one two $ echo $v1one$ echo $v2two

$ read v1 v2 one two three four$ echo $v1one$ echo $v2two three four

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Example Reading Multiple Lines

$ cat readme.sh #!/bin/bash# read multiple lines

read v1read v2echo "you said $v1"echo "then you said $v2"

$ ./readme.sh one twothree four fiveyou said one twothen you said three four five

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Exporting Variables

• export [name[=value]]• Makes variables available in

environment• e.g. export x

$ v1="one two"$ export v1$ shsh-3.1$ echo $v1one twosh-3.1$

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Predefined Locals

$ cat predefined.shecho You passed $# parameters.echo These are: "$@"echo process ID of last background process = $!echo process ID of this shell = $$notAcommandecho Last command returned with $? as the status.$ ./predefined.sh one two three fourYou passed 4 parameters.These are: one two three fourprocess ID of last background process =process ID of this shell = 21712./predefined.sh: line 7: notAcommand: command not foundLast command returned with 127 as the status.


Arithmetic

• $ expr expression• Bourne shell does not directly do math• Command expr evaluates expressions

– Supports • Multiplication (\*), Division (/), Remainder

(%)• Add (+), Subtract (-)• Equal (=), Not Equal (!=)• Less (\<), Less/Eq (\<=), Greater (\>),

Greater/Eq (\>=)• And (\&), Or (\|)


expr Command

• expr also evaluates expressions

STRING : REGEXP Anchored pattern match of REGEXP in STRING

match STRING REGEXP Same as STRING : REGEXP

substr STRING POS LENGTH Substring of STRING, POS counted from 1

index STRING CHARS Index in STRING where any CHARS is found, or 0

length STRING Length of STRING


test Command

• $ test expression OR just expression– Returns 0 if true– Returns nonzero if false

• -e file True if file Exists.• -f file True if file is a regular File.• -r file True if file is readable.• -w file True if file is writable.• -x file True if file is executable.• -d file True if file is a Directory.• String1 = String2 True if the strings are equal.

• See page 193 for a more complete list


Case Structure

case word in pattern1) Statement(s) to be executed if pattern1 matches ;; pattern2) Statement(s) to be executed if pattern2 matches ;; pattern3) Statement(s) to be executed if pattern3 matches ;; … *) Statement(s) to be executed if no pattern matches ;;esac


Case Structure Example

$ cat testcase.sh

echo "Type out the word for 1 or 2:"read v1case $v1 in

[Oo]ne)echo "You entered 1" ;;

[Tt]wo)echo "You

entered 2" ;;

*)echo "sorry“exit 1;;

esac

$ ./testcase.sh Type out the word for 1 or 2:twoYou entered 2$ ./testcase.sh Type out the word for 1 or 2:TwoYou entered 2$ ./testcase.sh Type out the word for 1 or 2:threesorry


For Loop

• Loop where name gets each value in word, in turn

• Uses $@ if no word given• End loop: break• Go to next iteration: continue

for name in {word}*do

command listdone


For Loop Example

$ cat testfor.sh

params=$@for value in $paramsdo echo param: $valuedone

$ ./testfor.sh one two treeparam: oneparam: twoparam: tree


If .. Then

if [ expression ]then Statement(s) to be executed if expression is truefi

if [ expression ]then Statement(s) to be executed if expression is trueelse Statement(s) to be executed if expression is not truefi


If .. Then

if [ expression 1 ]then Statement(s) to be executed if expression 1 is trueelif [ expression 2 ]then Statement(s) to be executed if expression 2 is trueelif [ expression 3 ]then Statement(s) to be executed if expression 3 is trueelse Statement(s) to be executed if no expression is truefi


If .. Then Example

$ cat testif.sh echo "enter a word: "read v1if [ -e $v1 ]then echo "A file by that name exists."else echo "No file by that name exists."fi

$ ./testif.sh enter a word: dummyA file by that name exists.$ ./testif.sh enter a word: twoNo file by that name exists.


Responding to a Signal

• $ trap command {signal}+• Executes command when signal is

received• Example: • $ trap 'echo CTRL-C; exit 1' 2

– When user types Control-C (signal 2)– Print “CTRL-C”– Exit with status 1


Until .. do .. done

• End loop: break• Go to next iteration: continue

until commanddo Statement(s) to be executed until command is truedone


Until .. do .. done Example

$ cat testuntil.sh

x=1until [ $x -gt 3 ]do

echo x = $xx=`expr $x + 1`

done

$ ./testuntil.sh x = 1x = 2x = 3


While Loop

• End loop: break• Go to next iteration: continue

while commanddo Statement(s) to be executed if command is truedone


While Loop Example

$ cat testwhile.sh

x=1while [ $x -lt 4 ]do

echo x = ${x}, less than four x=`expr $x + 1`

done

$ ./testwhile.sh x = 1, less than fourx = 2, less than fourx = 3, less than four


Review

• Variable assignment and access• Reading standard input• Arithmetic and pattern matching

(expr)• Control structures (case, for, if,

while, until)

Documents

Xuan Guo Chapter 5 The Bourne Shell Graham Glass and King Ables, UNIX for Programmers and Users, Third Edition, Pearson Prentice Hall, 2003. Notes by Michael