Advanced Bash−Scripting Guide

This utility records (saves to a file) all the user keystrokes at the command line in a console or an xterm window. This, in effect, create a record of a session.

12.8. Math Commands

Command Listing

factor

Decompose an integer into prime factors.

bash$ factor 27417

27417: 3 13 19 37

bc, dc

These are flexible, arbitrary precision calculation utilities.

bc has a syntax vaguely resembling C.

dc uses RPN ("Reverse Polish Notation").

Of the two, bc seems more useful in scripting. It is a fairly well−behaved UNIX utility, and may therefore be used in a pipe.

Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, bc comes to the rescue.

Here is a simple template for using bc to calculate a script variable. This uses command substitution.

variable=$(echo "OPTIONS; OPERATIONS" | bc)

Example 12−28. Monthly Payment on a Mortgage

#!/bin/bash

#monthlypmt.sh: Calculates monthly payment on a mortgage.

#This is a modification of code in the "mcalc" (mortgage calculator) package,

#by Jeff Schmidt and Mendel Cooper (yours truly, the author of this document).

#http://www.ibiblio.org/pub/Linux/apps/financial/mcalc−1.6.tar.gz [15k]

echo

echo "Given the principal, interest rate, and term of a mortgage," echo "calculate the monthly payment."

bottom=1.0

echo

echo −n "Enter principal (no commas) " read principal

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echo −n "Enter interest rate (percent) " # If 12%, enter "12", not ".12". read interest_r

echo −n "Enter term (months) " read term

interest_r=$(echo "scale=9; $interest_r/100.0" | bc) # Convert to decimal.

# "scale" determines how many decimal places.

interest_rate=$(echo "scale=9; $interest_r/12 + 1.0" | bc)

top=$(echo "scale=9; $principal*$interest_rate^$term" | bc)

echo; echo "Please be patient. This may take a while."

let "months = $term − 1"

for ((x=$months; x > 0; x−−)) do

bot=$(echo "scale=9; $interest_rate^$x" | bc) bottom=$(echo "scale=9; $bottom+$bot" | bc)

# bottom = $(($bottom + $bot")) done

# let "payment = $top/$bottom" payment=$(echo "scale=2; $top/$bottom" | bc)

# Use two decimal places for dollars and cents.

echo

echo "monthly payment = \$$payment" # Echo a dollar sign in front of amount. echo

exit 0

#Exercises:

#1) Filter input to permit commas in principal amount.

#2) Filter input to permit interest to be entered as percent or decimal.

#3) If you are really ambitious,

#expand this script to print complete amortization tables.

Example 12−29. Base Conversion

:

##########################################################################

# Shellscript:			base.sh − print number to different bases (Bourne Shell)
# Author		:	Heiner Steven ([e-mail])
#	Date	:	07−03−95
#	Category	:	Desktop

#$Id: base.sh,v 1.2 2000/02/06 19:55:35 heiner Exp $

##########################################################################

#Description

#

#Changes

#21−03−95 stv fixed error occuring with 0xb as input (0.2)

##########################################################################

#==> Used in this document with the script author's permission.

#==> Comments added by document author.

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NOARGS=65
PN=`basename "$0"`		# Program name
VER=`echo '$Revision: 1.2	$' | cut −d' ' −f2`	# ==> VER=1.2

Usage () {

echo "$PN − print number to different bases, $VER (stv '95) usage: $PN [number ...]

If no number is given, the numbers are read from standard input.

A number may be
binary (base 2)	starting with 0b	(i.e. 0b1100)
octal (base 8)	starting with 0	(i.e. 014)
hexadecimal (base 16)	starting with 0x	(i.e. 0xc)
decimal	otherwise (i.e. 12)" >&2
exit $NOARGS

}# ==> Function to print usage message.

Msg () {
for i	# ==> in [list] missing.
do echo	"$PN: $i" >&2
done
}
Fatal () { Msg "$@"; exit 66; }
PrintBases () {
# Determine base of the number
for i	# ==> in [list] missing...
do	# ==> so operates on command line arg(s).
case "$i" in
	0b*)	ibase=2;;	# binary
	0x*|[a−f]*|[A−F]*)	ibase=16;;	# hexadecimal
	0*)	ibase=8;;	# octal
	[1−9]*)	ibase=10;;	# decimal
	*)

Msg "illegal number $i − ignored" continue;;

esac

#Remove prefix, convert hex digits to uppercase (bc needs this) number=`echo "$i" | sed −e 's:^0[bBxX]::' | tr '[a−f]' '[A−F]'`

#==> Uses ":" as sed separator, rather than "/".

#Convert number to decimal

dec=`echo "ibase=$ibase; $number" | bc`		# ==> 'bc' is calculator utility.
case "$dec" in
[0−9]*)	;;	# number ok
*)	continue;;	# error: ignore
esac

#Print all conversions in one line.

#==> 'here document' feeds command list to 'bc'. echo `bc <<!

obase=16; "hex="; $dec obase=10; "dec="; $dec obase=8; "oct="; $dec obase=2; "bin="; $dec

!

` | sed −e 's: : :g'

done

}

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Advanced Bash−Scripting Guide

while [ $# −gt 0 ]
do
case "$1" in
−−)	shift; break;;
−h)	Usage;;	# ==> Help message.
−*)	Usage;;
*)	break;;	# first number
esac	# ==> More error checking for illegal input would be useful.
shift
done
if [ $# −gt 0 ]
then
PrintBases "$@"
else		# read from stdin
while read line
do

PrintBases $line

done

fi

An alternate method of invoking bc involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to bc.

variable=`bc << LIMIT_STRING options

statements operations LIMIT_STRING

`

...or...

variable=$(bc << LIMIT_STRING options

statements operations LIMIT_STRING

)

Example 12−30. Another way to invoke bc

#!/bin/bash

#Invoking 'bc' using command substitution

#in combination with a 'here document'.

var1=`bc << EOF
18.33 * 19.78
EOF
`
echo $var1	# 362.56

# $( ... ) notation also works. v1=23.53

v2=17.881

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Advanced Bash−Scripting Guide

v3=83.501

v4=171.63

var2=$(bc << EOF
scale = 4
a = ( $v1 + $v2 )
b = ( $v3 * $v4 )
a * b + 15.35
EOF
)
echo $var2	# 593487.8452

var3=$(bc −l << EOF scale = 9

s ( 1.7 ) EOF

)

#Returns the sine of 1.7 radians.

#The "−l" option calls the 'bc' math library.

echo $var3	#	.991664810
# Now, try it in	a	function...
hyp=	#	Declare global variable.
hypotenuse ()	#	Calculate hypotenuse of a right triangle.
{
hyp=$(bc −l << EOF
scale = 9
sqrt ( $1 * $1 +	$2 * $2 )
EOF
)
# Unfortunately,	can't return floating point values from a Bash function.
}
hypotenuse 3.68 7.31
echo "hypotenuse	=	$hyp"	# 8.184039344

exit 0

awk

Yet another way of doing floating point math in a script is using awk's built−in math functions in a shell wrapper.

Example 12−31. Calculating the hypotenuse of a triangle

#!/bin/bash

# hypotenuse.sh: Returns the "hypotenuse" of a right triangle.

#	( square root of sum		of squares of the "legs")
ARGS=2	#	Script needs	sides of triangle passed.
E_BADARGS=65	#	Wrong number	of arguments.

if [ $# −ne "$ARGS" ] # Test number of arguments to script. then

echo "Usage: `basename $0` side_1 side_2" exit $E_BADARGS

fi

12.8. Math Commands

182