| Advanced Bash-Scripting Guide: A complete guide to shell scripting | ||
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| Prev | Chapter 12. External Filters, Programs and Commands | Next |
Decompose an integer into prime factors.
bash$ factor 27417
27417: 3 13 19 37
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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=$(echo "OPTIONS; OPERATIONS" | bc)
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Example 12-28. Monthly Payment on a Mortgage
#!/bin/bash
# monthlypmt.sh: Calculates monthly payment on a mortgage.
# This is a modification of code in the "mcalc" (mortgage calculator) package,
# by Jeff Schmidt and Mendel Cooper (yours truly, the author of this document).
# http://www.ibiblio.org/pub/Linux/apps/financial/mcalc-1.6.tar.gz [15k]
echo
echo "Given the principal, interest rate, and term of a mortgage,"
echo "calculate the monthly payment."
bottom=1.0
echo
echo -n "Enter principal (no commas) "
read principal
echo -n "Enter interest rate (percent) " # If 12%, enter "12", not ".12".
read interest_r
echo -n "Enter term (months) "
read term
interest_r=$(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.
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Example 12-29. Base Conversion
: ########################################################################## # Shellscript: base.sh - print number to different bases (Bourne Shell) # Author : Heiner Steven ([email protected]) # 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. 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 } 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
# $( ... ) notation also works.
v1=23.53
v2=17.881
v3=83.501
v4=171.63
var2=$(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
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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
AWKSCRIPT=' { printf( "%3.7f\n", sqrt($1*$1 + $2*$2) ) } '
# command(s) / parameters passed to awk
echo -n "Hypotenuse of $1 and $2 = "
echo $1 $2 | awk "$AWKSCRIPT"
exit 0
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