Hex to Dec conversion with printf in sh fail for more than 16 digits

问题

I only have shell available no bash, Perl, python etc.

Using printf small numbers work:

root@DD-WRT:/jffs# printf "%d\n", 0x15a
346

But large numbers fail.

root@DD-WRT:/jffs# printf "%d\n", 0x15abc12345afda325
sh: invalid number '0x15abc12345afda325'
0

Also is it possible to perform hexadecimal arithmetic for example Module using shell ?

回答1:

What shell is this? On Linux I see:

$ bash -c 'echo $((0x15abc12345afda325))'
6538120775109288741

wrong

$ dash -c 'echo $((0x15abc12345afda325))'
9223372036854775807

wrong

$ ksh -c 'echo $((0x15abc12345afda325))'
2.49848648488188404e+19

right, but wrong output format

$ ksh -c 'printf "%d\n" $((0x15abc12345afda325))'
ksh: printf: warning: 2.49848648488188404e+19: overflow exception
9223372036854775807
$ ksh -c 'printf "%.0f\n" $((0x15abc12345afda325))'
24984864848818840399

GNU awk

$ gawk -v n=0x15abc12345afda325 'BEGIN {print strtonum(n)}'
24984864848818839552
$ gawk --bignum -v n=0x15abc12345afda325 'BEGIN {print strtonum(n)}'
24984864848818840357

Do you have bc available?

$ hex=15abc12345afda325
$ echo "ibase=16; $hex" | bc
(standard_in) 1: syntax error

hex values need to be in upper case?

$ echo "ibase=16; ${hex^^}" | bc
24984864848818840357

Hmm, different from ksh output. WolframAlpha says 24984864848818840357

I see busybox has dc, but sadly, it's crippled:

$ printf "%s\n" 16 i 15ABC12345AFDA325 p | dc
24984864848818840357
$ printf "%s\n" 16 i 15ABC12345AFDA325 p | busybox dc
dc: syntax error at 'i'

回答2:

I've attempted to write an arbitrary-precision converter from hexadecimal to decimal in pure sh (actually ash, since busybox sh runs the built-in ash). It needs a lot more effort than bash due to the limited set of features (no arrays) and "strange" errors without clear documentation (like spaces not allowed in expressions)

#!/bin/ash

obase=1000000000    # 1e9, the largest power of 10 that fits in int32_t
ibase=$((1 << 7*4)) # only 7 hex digits, because 0xFFFFFFFF > 1e9

inp="000000${1#0x}"                 # input value in $1 with optional 0x
inp=${inp:$((${#inp}%7)):${#inp}}   # pad the string length to a multiple of 7

carry=0
# workaround, since sh and ash don't support arrays
result0=0       # output digits will be stored in resultX variables in little endian
MSDindex=0      # index of the most significant digit in the result

print_result()
{
    eval echo -n \$result$MSDindex  # print MSD
    if [ $MSDindex -gt 0 ]; then    # print remaining digits
        for i in $(seq $((MSDindex-1)) -1 0); do eval printf "%09d" \$result$i; done
    fi
    echo
}

# Multiply a digit with the result
# $1 contains the value to multiply with the result array
mul()
{
    carry=0
    for i in $(seq 0 $MSDindex); do
        eval let res="$1\\*result$i+carry"
        eval let result$i=res%obase
        let carry=res/obase
    done

    while [ $carry -ne 0 ]; do
        let MSDindex=MSDindex+1
        eval let result$MSDindex=carry%obase
        let carry=carry/obase
    done
}

# Add a digit with the result
# $1 contains the digit to add with the array
add()
{
    eval let res=$1+result0
    eval let result0=res%obase
    let carry=res/obase

    i=1
    while [ $carry -ne 0 ]
    do
        eval let res=carry+result$i
        eval let result$i=res%obase
        let carry=res/obase
        if [ $i -gt $MSDindex ]; then MSDindex=$i; fi
        let i=i+1
    done
}

# main conversion loop
while [ -n "$inp" ]     # iterate through the hex digits, 7 at a time
do
    hexdigit=${inp:0:7}
    mul $ibase          # result = result*input_base+hexdigit
    add 0x$hexdigit

    if [ ${#inp} -gt 7 ]; then
        inp=${inp: $((7-${#inp}))}
    else
        unset inp
    fi
done

print_result

I checked with busybox in my Ubuntu and saw that it supports 64-bit arithmetic, therefore I need a 32-bit limb to avoid overflow when multiply. I choose the output base as 1 000 000 000 because it's the largest power of 10 that can be represented in a 32-bit int. Then the input base need to be smaller than the base (less carry handling needed), thus I choose 0x10000000, the largest power of 16 that's smaller than 1000000000

Of course if your busybox is so crippled that it doesn't support 64-bit int then you have to use base 0x1000 and process 3 hexadecimal digits at once

Confirmed with bc, the result is the same every time

$ v=15ABC12345AFDA325; busybox sh ./hex2dec.sh $v; echo "ibase=16; $v" | bc
24984864848818840357
24984864848818840357
$ v=2B37340113436BA5C23513A1231111C; busybox sh ./hex2dec.sh $v; echo "ibase=16; $v" | bc
3590214682278754501437472025955340572
3590214682278754501437472025955340572
$ v=60431BCD73610ADF2B37340113436BA5C23513A12311111111111;\
> busybox sh ./hex2dec.sh $v; echo "ibase=16; $v" | bc
2474996796503602902399592755755761709869730986038055786310078737
2474996796503602902399592755755761709869730986038055786310078737

来源：https://stackoverflow.com/questions/52485538/hex-to-dec-conversion-with-printf-in-sh-fail-for-more-than-16-digits

标签

shell

hex

decimal

base-conversion