Shell Control Statements

CS465 - UNIX

Shell arithmetic using expr• The standard Bourne shell does not provide built-in

arithmetic operators, so we have to use the expr command.

• Interactive example:$ num=1$ expr $num + 12$

• To assign the result of an expr command to another shell variable, surround it with backquotes:

$ sum=`expr $num + 1`$ echo $sum2$

Why expr is needed

Unless otherwise indicated, all values are assumed to be STRINGS:

$ num=1

$ val=$num+1

$ echo $val

1+1

$

• expr is needed to translate the variables from strings to numbers and performs the indicated math. The result is converted back to a string.

• Example:$ var1=3$ var2=5$ sum=`expr $var1 + $var2`$ echo $sum8$

Shell arithmetic using expr

Spaces around operator required

expr Operators– Math operators:

+, -, *, /, % (Must use backslash with *)

– Comparison of strings (text): < , <=, =, !=, >=, > (Must use backslash with <, <=, >, >=)

– Compounds& (and), | (or)(Must use backslash with both)

• Asterisk (*) is usually a wildcard character

– Must precede the asterisk with a backslash to use it for multiplication within expr.

• Example:

$ num=5$ prod=`expr $num \* 3`$ echo $prod15$

Multiplication using expr

Spaces around operator still required

• expr allows you to group expressions using parentheses, but parentheses are metacharacters used to group commands

– So the “(“ and “)” characters also need to be preceded by backslashes.

• Example:

$ num=2$ echo `expr 5 \* \( $num + 3 \)`25$

Parentheses using expr

$ cat test6#!/bin/sh

echo –n "Enter a number: "read numsqr=`expr $num \* $num`echo The square of $num is $sqr$

Integer restriction on expr

Floats are illegal!

test6Enter a number: 6 The square of 6 is 36$ test6Enter a number: 8.2expr: non-numeric argument$

Student exercise

• Write a script that will read in your age as of Dec 31st (of this year), and then compute and display the year you were born.

Exercise Sample Solution

$ cat birthyr#!/bin/shecho "Enter age on Dec 31st: \c"read agecuryr=2009birthyr=`expr $curyr - $age`echo You were born in $birthyrexit 0$ Enter age on Dec 31st: 47You were born in 1962$

Bourne Shell Control Statements

• if – conditional execution of one block of commands or another

• while – repeatedly execute block of commands until condition is no longer true

• for – iterate over a list of values, executing a block of commands once for every item in the list (not directly analogous to HLL for-loops!)

• case – multi-way conditional execution

The test command

$ cat check echo –n "Yes or no: " read answer test $answer = yes echo $? $ check Yes or no: yes 0 $

Use test command to evaluate logical expressions.

test assigns status 0 if the condition is true or 1 if the condition is false. Variable $? holds the status of the last test.

$ check Yes or no: no 1 $

test equivalence

test condition

is equivalent to:

[ condition ]

Note space between condition & brackets!

Square brackets [ ] are used as a “shortcut” for test command:

test within if statements

test is used by the if statement, to test conditions:

Again, note space betweencondition & brackets!

if test condition OR

then

action

fi

if [ condition ]

then

action

fi

if statement

if [ condition ]then

command(s)

elsecommand(s)

fi

• Must have spaces between condition and square brackets

• There are no curly braces. The "true" command block extends from then to else (or fi). The "false" command block extends from else to fi.

• The else block is optional.

• "fi“, indicating the end, is "if" spelled backwards.

String Conditional Tests

s1 = s2– true if strings s1 and s2 are identical

s1 != s2– true if strings s1 and s2 are not identical

-n s1– true if length of string s1 is nonzero

-z s1– true if length of string s1 is zero

String Condition ExampleTask: Compare two names

$ cat namecmp#!/bin/shecho Enter 2 namesread name1 name2if [ "$name1" = "$name2" ] then

echo Same names!else

echo Different names!fi$

$ namecmpEnter 2 namesPam JoeDifferent names!$$ namecmpjoe JoeDifferent names!$

$ namecmpJoe JoeSame names!$

if string details

• You don't have to use double quotes around a variable expansion, but doing so will prevent syntax errors when the variable is the null string:

• Assume an empty string: string=""

if [ "$string" = foo ] • same as if [ "" = foo ]

if [ $string = foo ]• same as if [ = foo ]This is a syntax error!

Numeric Conditional Testsn1 -eq n2

– true if integers n1 and n2 are equivalentn1 -lt n2

– true if integer n1 is less than n2n1 -gt n2

– true if integer n1 is greater than n2n1 -ge n2

– true if integer n1 is greater than or equal to n2n1 -le n2

– true if integer n1 is less than or equal to n2

Numeric Example #1 – no else

Task: Test to see if any arguments were passed into the script. If there were NOT, display a message.

$ argscriptNo arguments$ argscript hello$

$ cat argscript#!/bin/shif [ $# -eq 0 ] thenecho No arguments

fiexit 0$

Numeric Example #2 – with elseTask: Test to see if any arguments were passed into the script. Display a message either way.

$ showargsNo arguments$ showargs Ford GM2 arguments: Ford GM$

$ cat showargs#!/bin/shif [ $# -eq 0 ] then echo No arguments

else echo $# arguments: $*

fiexit 0$

Numeric Condition Example #3

$ cat calcmales#!/bin/shecho Enter number of total students:read totalecho Enter percent male \(1-100\):read percentif [ $percent -gt 100 ] then echo Percent entered is too bigelse nummale=`expr $percent \* $total / 100`

echo There are $nummale male studentsfi

$

Example #3 Execution

$ calcmalesEnter number of total students:88Enter percent male (1-100):109Percent entered is too big$$ calcmalesEnter number of total students:88Enter percent male (1-100):50There are 44 male students$

Student Exercise

• Modify previous script to read month and year of birth year and compute your age at the end of the current month..

Exercise Sample Solution$ cat calcage#!/bin/shecho -n "Enter birth year: "read byrecho -n "Enter birth month: "read bmoncuryr=2009curmon=5age=`expr $curyr - $byr`if [ $curmon –lt $bmon ]then

age=`expr $age – 1`fiecho You will be $age at this month\’s endexit 0$ calcageEnter birth year: 1962Enter birth month: 9You will be 46 at this month’s end$

Logical Conditional Operatorsexpr1 –a expr2

– AND operator between two logical conditions

expr1 –o expr2

– OR operator between two logical conditions

! expr1

– unary NOT operator before one logical condition

'(' expr1 -a expr2 ')' -o expr3– Parentheses can be used for grouping– Must use single quotes or backslash

$ cat notify

#!/bin/sh

if [ `whoami` = "smith123" -o \

`whoami` = "jones456" ]

then

echo "See the system administrator! "

fi

exit 0

$ notify

$

Logical Condition Example

File Conditional Tests

-r filename

• true if filename exists and is readable

-w filename

• true if filename exists and is writable

-d filename

• true if filename exists and is a directory

-s filename

• true if filename exists and is nonzero in length

NOTE: See test or sh manpage for more file conditions

File Condition Example #1

Test to see if the file myprogs exists and is a directory. If so, move to the myprogs directory and list the files in it.

$ cat progdir#!/bin/shif [ -d myprogs ]then cd myprogs lsfi$ progdirfile1.c file2.c$

File Condition Example #2

• Script file mvf moves to the user's home directory and prompts for a subdirectory name to move files to. It then checks to see if the named subdirectory exists. If not, the subdirectory is created. Then the files are moved to the subdirectory.

File Condition Script #2

$ cat mvf#! /bin/sh# Moves parameter filenames to subdirectorycdecho Move files to which subdirectory?read subnameif [ ! -d $subname ]then echo Creating subdirectory $subname ...mkdir $subname

fimv $* $subnameecho Move complete.exit 0$

File Condition Script #2 Execution

$ ls -F $HOMEmbox prog1.c prog3.cnotes\ prog2.c$ mvf prog1.c prog2.cMove files to which subdirectory?cprogsCreating subdirectory cprogs ...Move complete.$ ls -F $HOMEcprogs\ mbox notes\ prog3.c$ cd cprogs$ lsprog1.c prog2.c$

Student Exercise

• Write a script which takes ONE argument, a filename. The script should check if the file exists and is readable. If it DOES, display it. If NOT, issue an error message.

$ cat display#!/bin/shfile=$1if [ -r $file ]thencat $file

elseecho File $1 not readable or nonexistent!

fi$ display memoFile memo not readable or nonexistent!$

Exercise Sample Solution

elif syntax (i.e.“else if”)

Can use elif to create a nested set of “if-then-else” structures.

Notice then command appears after “elif” statement.

if [ condition ]then action 1elif [ condition ]then action2else action3fi

elif Example #2$ cat test8#!/bin/shusers=`who | wc -l`if [ $users -ge 10 ]

then echo "Heavy load"elif [ $users -gt 1 ]

then echo "Medium load"else

echo "Just me!"fi$ test8Medium load!$

elif Example # - what type is file?

#!/bin/shif [ -d what ]then echo File is a directory

lselif [ -b what ]then echo File is a block special fileelif [ -c what ]then echo File is a character special fileelif [ -a what ]then echo File exists - unknown file typeelse echo File does not existfi