#24 An Interactive Calculator - 脚本编程学习 for unix linux freebsd

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Allowing command-line invocations of bc for floatingpoint calculations was inevitable that I'd write a small wrapper script to create an interactive command-line-based calculator. What's remarkable is that, even with help information, it's very short.

The Code
#!/bin/sh

# calc - A command-line calculator that acts as a front end to bc.

scale=2

show_help()
{
cat << EOF
In addition to standard math functions, calc also supports

a % b       remainder of a/b
a ^ b       exponential: a raised to the b power
s(x)        sine of x, x in radians
c(x)        cosine of x, x in radians
a(x)        arctangent of x, returns radians
l(x)        natural log of x
e(x)        exponential log of raising e to the x
j(n,x)      bessel function of integer order n of x
scale N     show N fractional digits (default = 2)
EOF
}

if [ $# -gt 0 ] ; then
exec scriptbc "$@"
fi

echo "Calc - a simple calculator. Enter 'help' for help, 'quit' to quit."

echo -n "calc> "

while read command args
do
case $command
in
    quit|exit) exit 0                                  ;;
    help|\?)   show_help                               ;;
    scale)     scale=$args                             ;;
    *)         scriptbc -p $scale "$command" "$args"   ;;
esac

echo -n "calc> "
done

echo ""

exit 0

How It Works
There's really remarkably little of a complex nature going on here. Perhaps the most interesting part of the code is the while read statement, which creates an infinite loop that displays the calc> prompt until the user exits, either by typing quit or entering an end-of-file sequence (^D). And, of course, the simplicity of this script is exactly what makes it wonderful: Shell scripts don't need to be extremely complex to be useful!

Running the Script
This script is easily run because by default it's an interactive tool that prompts the user for the desired actions. If it is invoked with arguments, those arguments are passed to the scriptbc command instead.

The Results
$ calc 150 / 3.5
42.85
$ calc
Calc - a simple calculator. Enter 'help' for help, 'quit' to quit.
calc> help
In addition to standard math functions, calc also supports