Bourne Basic - a BASIC interpreter implemented (painfully) in pure Bourne shell

basic.sh

#!/bin/sh
#
#
# From: allbery@ncoast.UUCP (allbery@ncoast.UUCP)
# Subject: bournebasic
# Newsgroups: comp.sources.misc
# Date: 1987-08-18 18:54:12 PST
# 
# Here's a useful BASIC interpreter written in Bourne shell.
# There's no manual but this demo shows the most salient features:
# 
# 10 print "Enter any number and I will double it for you ";
# 20 input x
# 30 y = 2*x
# 40 print "The answer is ";
# 50 print y
# 60 print "Enter -1 if you want to continue."
# 70 input c
# 80 if c = -1 then 10
# 90 print "Bye"
# 
# Also, the commands 'load', 'save', 'list', 'run', and 'new'
# work as expected. Enjoy!
# 

tfiles=/tmp/bas$$
export tfiles
trap "rm ${tfiles?Bug}*; exit" 2
> ${tfiles}
echo 'Hi there!'
while true
do
    echo -n '>'
    read com com2 || exit
    case $com in
        list)
            cat ${tfiles}
            ;;
        run)
            echo 0 > ${tfiles}.ln
            while true
            do
            while true
            do
                case `cat ${tfiles}.ln` in
                    0)
                        read lineno line || exit
                        ;;
                    *)
                        while true
                        do
                            read lineno line || exit
                            match=`cat ${tfiles}.ln`
                            case $lineno in
                                $match)
                                    break
                                    ;;
                                *)
                                    ;;
                            esac
                        done
                esac
                while true
                do
                    
                    case $line in
                        print*)
                            expr=`echo $line | sed 's/print\ *//'`
                            case $expr in
                                \"*)
                                    echo -n `echo "$expr" | sed 's/^\"//
                                                               s/\".*$//'`
                                    case $line in
                                        *\;)
                                            ;;
                                        *)
                                            echo
                                            ;;
                                    esac
                                    ;;
                                *)
                                    > ${tfiles}.ex
                                    files=`echo ${tfiles}.?`
                                    case $files in
                                        ${tfiles}.\?)
                                            ;;
                                        *)
                                            for i in $files
                                     do
                                         cat $i >> ${tfiles}.ex
                                     done
                                     ;;
                                    esac
                             echo $expr >> ${tfiles}.ex
                                    bc < ${tfiles}.ex > ${tfiles}.res
                                    cat ${tfiles}.res
                                    ;;
                            esac
                            ;;
                        if*)
                            expr=`echo $line | sed '{
                                                    s/if\ */\(/
                                                    s/[#=]/\)-\(/
                                                    s/\ *then.*/\)/
                                                    }'`
                            target=`echo $line | sed 's/.*then\ *//'`
                            > ${tfiles}.ex
                            files=`echo ${tfiles}.?`
                            case $files in
                                ${tfiles}.\?)
                                    ;;
                                *)
                                    for i in $files
                             do
                                 cat $i >> ${tfiles}.ex
                             done
                             ;;
                     esac
                     echo $expr >>${tfiles}.ex
                            res=`bc < ${tfiles}.ex`
                            case $res in
                                0*)
                                    case $line in
                                        *=*)
                                            echo $target > ${tfiles}.ln
                                            exit
                                            ;;
                                        *)
                                            ;;
                                    esac
                                    ;;
                                *)
                                    case $line in
                                        *#*)
                                            echo $target > ${tfiles}.ln
                                            exit
                                            ;;
                                        *)
                                            ;;
                                    esac
                                    ;;
                            esac
                            ;;
                        end*)
                            echo 0 > ${tfiles}.ln
                            exit
                            ;;
                        goto*)
                            expr=`echo $line | sed 's/goto\ *//'`
                            echo $expr > ${tfiles}.ln
                            exit
                            ;;
                        input*)
                            var=`echo $line | sed 's/input\ *//'`
                            echo -n "?"
                            echo -n "$var=" > ${tfiles}.$var
                            while true
                            do
                                read val
                                echo $val
                                break
                            done < /dev/tty >> ${tfiles}.$var
                            ;;
                        *)
                            > ${tfiles}.ex
                            files=`echo ${tfiles}.?`
                            case $files in
                                ${tfiles}.\?)
                                    ;;
                                *)
                                    for i in $files
                             do
                                 cat $i >> ${tfiles}.ex
                             done
                             ;;
                     esac
                     echo $line >>${tfiles}.ex
                            var=`echo $line | sed 's/\ *=.*//'`
                            echo $var >> ${tfiles}.ex
                            bc < ${tfiles}.ex > ${tfiles}.res
                            echo -n $var= > ${tfiles}.$var
                            cat ${tfiles}.res >> ${tfiles}.$var
                            ;;
                    esac
                    read lineno line || exit
                done
            done < ${tfiles}
            case `cat ${tfiles}.ln` in
                0*)
                    break
                    ;;
                *)
                    ;;
            esac
            done
            ;;
        new)
            >${tfiles}
            ;;
        quit)
            echo 'Thank you for your patience.'
            rm ${tfiles?Bug}*
            exit
            ;;
        load)
            cp $com2 ${tfiles}
            ;;
        save)
            cp ${tfiles} $com2
            ;;
        [0-9]*)
            echo $com $com2 > ${tfiles}.nl
            grep -v "^$com" ${tfiles} >> ${tfiles}.nl
            sort -n ${tfiles}.nl > ${tfiles}
            ;;
 "")
     ;;
        *)
            echo 'Illegal command'
    esac
done

I pulled this down off of Usenet back in the day - i.e., long before the Internet was open to commercial riff-raff. Because this script is so painful and slow, I was inspired to design a BASIC interpreter in NAWK (New AWK), which I never implemented. But, I did implement it in Perl - it was my first major Perl program. I'll see if I can find that, too.

10 print "hello"
20 goto 10

does not seem to work, i.e. I think there is some issue with "goto".

Hey @KedalionDaimon - this script is 35 years old. I put it up here to save it because I thought it was an awesome abuse of the shell.

At this point, I'd be surprised if it did work. I'll bet the problem is related to the original Bourne shell vs. modern shells like Bash.