Unix Tip: Using bc for big calculations

For sysadmins who want to perform simple calculations on the command line, there's a considerably more useful tool than expr. Try bc. Like expr, the bc command allows you to do simple math. Here are some examples:

boson> echo 6*6-3 | bc
33
boson> amt=`echo 6*6-3 | bc`
boson> echo $amt
33

You can also go into calculator mode by typing bc on the command line by itself and then feed it as many calculations as you want:

boson% bc
7*7*7
343
256*256
65536
256*256*256
16777216
256*256*256*256
4294967296
256*256*256*256*256*256*256*256
18446744073709551616
quit

Notice that you can calculate some very large values! You can also achieve a high degree of precision by setting the number of places you want to see following the decimal point.

boson% bc
scale=4
177/7
25.2857
quit

The "scale" setting, as you can see from the example above, determines the number of decimal places to save. You can, therefore, get considerably more useful values than the rounded down results of expr. Using bc, you don't have to escape multiplication operations and computing powers of 2 (or any arbitrary number for that matter) is simple, fast and accurate as shown below.

boson% expr 3 \* 3
9
boson% echo 3*3 | bc
9
boson% bc
2^8
256
2^32
4294967296
quit

Look how quickly we can calculate the number of IP addresses that will be available in IPv6 as opposed to IPv4:

boson% bc
2^32
4294967296
2^128
340282366920938463463374607431768211456
quit

That's one staggering large number, but bc can generate even larger numbers with ease and speed:

boson% bc
2^256
11579208923731619542357098500868790785326998466564056403945758400791\
3129639936
2^512
13407807929942597099574024998205846127479365820592393377723561443721\
76403007354697680187429816690342769003185818648605085375388281194656\
9946433649006084096
2^1024
17976931348623159077293051907890247336179769789423065727343008115773\
26758055009631327084773224075360211201138798713933576587897688144166\
22492847430639474124377767893424865485276302219601246094119453082952\
08500576883815068234246288147391311054082723716335051068458629823994\
7245938479716304835356329624224137216
quit

The numbers wrap around in 68-digit segments until the complete number has been printed on the screen. The bc command is also useful for doing base conversions. Say you want to know the hexadecimal equivalent of 255. Just do this:

boson% bc
obase=16
255
FF
quit

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