How do I use floating-point division in bash

Question

I am trying to divide two image widths in a Bash script  but bash gives me 0 as the result   RESULT     IMG WIDTH  IMG2 WIDTH     I did study the Bash guide and I know I should use bc  in all examples in internet they use bc  In echo I tried to put the same thing in my SCALE but it didn t work   Here is the example I found in the tutorials   echo  scale 2    userinput     bc    How can I get Bash to give me a float like 0 5

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There are scenarios in wich you cannot use bc becouse it might simply not be present  like in some cut down versions of busybox or embedded systems  In any case limiting outer dependencies is always a good thing to do so you can always add zeroes to the number being divided by  numerator   that is the same as multiplying by a power of 10  you should choose a power of 10 according to the precision you need   that will make the division output an integer number  Once you have that integer treat it as a string and position the decimal point  moving it from right to left  a number of times equal to the power of ten you multiplied the numerator by  This is a simple way of obtaining float results by using only integer numbers

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It s perfect time to try zsh  an  almost  bash superset  with many additional nice features including floating point math  Here is what your example would be like in zsh     IMG WIDTH 1080   IMG2 WIDTH 640   result    IMG WIDTH 1 0 IMG2 WIDTH     echo  result 1 6875   This post may help you  bash - Worth switching to zsh for casual use

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As others have indicated  bash does not have built-in floating-point operators  You can implement floating-point in bash  even without using calculator programs like bc and awk  or any external programs for that matter  I m doing exactly this in my project  shellmath  in three basic steps   Break the numbers down into their integer and fractional parts Use the built-in integer operators to process the parts separately while being careful about place-value and carrying Recombine the results  As a teaser  I ve added a demo script that calculates e using its Taylor series centered at x 0  Please check it out if you have a moment  I welcome your feedback

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It s not really floating point  but if you want something that sets more than one result in one invocation of bc     source  dev stdin  lt  lt  lt   bc  lt  lt  lt    d   1  3 1415926535897932384626433832795 2 print  d   d   n  a   1    1  3 1415926535897932384626433832795 print  a   a   n      echo bc radius  1 area  a diameter  d   computes the area and diameter of a circle whose radius is given in  1

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here is awk command   -F   field separator       echo  2 1 3 1     awk -F       print   1  2

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How to do floating point calculations in bash  Instead of using  quot here strings quot    lt  lt  lt   with the bc command  like one of the most-upvoted examples does  here s my favorite bc floating point example  right from the EXAMPLES section of the bc man pages  see man bc for the manual pages   Before we begin  know that an equation for pi is  pi   4 atan 1   a   below is the bc math function for atan     This is how to store the result of a floating point calculation into a bash variable--in this case into a variable called pi  Note that scale 10 sets the number of decimal digits of precision to 10 in this case  Any decimal digits after this place are truncated   pi   echo  quot scale 10  4 a 1  quot    bc -l    Now  to have a single line of code that also prints out the value of this variable  simply add the echo command to the end as a follow-up command  as follows  Note the truncation at 10 decimal places  as commanded   pi   echo  quot scale 10  4 a 1  quot    bc -l   echo  pi  3 1415926532   Finally  let s throw in some rounding  Here we will use the printf function to round to 4 decimal places  Note that the 3 14159    rounds now to 3 1416  Since we are rounding  we no longer need to use scale 10 to truncate to 10 decimal places  so we ll just remove that part  Here s the end solution   pi   printf   4f   echo  quot 4 a 1  quot    bc -l    echo  pi  3 1416    Here s another really great application and demo of the above techniques  measuring and printing run-time   See also my other answer here   Note that dt min gets rounded from 0 01666666666    to 0 017  start  SECONDS  sleep 1  end  SECONDS  dt sec     end - start     dt min   printf   3f   echo  quot  dt sec 60 quot    bc -l    echo  quot dt sec    dt sec  dt min    dt min quot  dt sec   1  dt min   0 017  Related    my answer  https   unix stackexchange com questions 52313 how-to-get-execution-time-of-a-script-effectively 547849 547849  my question  What do three left angle brackets    lt  lt  lt    mean in bash  https   unix stackexchange com questions 80362 what-does-mean 80368 80368 https   askubuntu com questions 179898 how-to-round-decimals-using-bc-in-bash 574474 574474

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For those trying to calculate percentages with the accepted answer  but are losing precision   If you run this   echo  scale 2   100 180    180    bc   You get 99 00 only  which is losing precision   If you run it this way    echo  result    100 180    180  scale 2  result   1    bc -l   Now you get 99 99   Because you re scaling only at the moment of printing    Refer to here

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i know it s old  but too tempting  so  the answer is  you can t    but you kind of can  let s try this    IMG WIDTH 1024  IMG2 WIDTH 2048   RATIO      IMG WIDTH    IMG2 WIDTH         IMG WIDTH   100   IMG2 WIDTH    100      like that you get 2 digits after the point  truncated  call it rounding to the lower  haha  in pure bash  no need to launch other processes   of course  if you only need one digit after the point you multiply by 10 and do modulo 10   what this does    first          does integer division  second          does integer division on something 100 times larger  essentially moving your 2 digits to the left of the point  then     getting you only those 2 digits by doing modulo    bonus track  bc version x 1000 took 1 8 seconds on my laptop  while the pure bash one took 0 016 seconds

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If you found the variant of your preference you can also wrap it into a function   Here I m wrapping some bashism into a div function   One liner   function div   local  d   3 -2   local  n 0000000000   n    n 0   d   local  r     1  n  2     r    r 0 -  d     r  -  d   echo   r     Or multi line   function div     local  d   3 -2    local  n 0000000000    n    n 0   d    local  r     1  n  2      r    r 0 -  d     r  -  d    echo   r     Now you have the function  div  lt dividend gt   lt divisor gt    lt precision 2 gt     and use it like   gt  div 1 2  50   gt  div 273 123 5 2 21951   gt  x   div 22 7   gt  echo  x 3 14   UPDATE I added a little script which provides you the basic operations with floating point numbers for bash   Usage    gt  add 1 2 3 45 4 65  gt  sub 1000  007 999 993  gt  mul 1 1 7 07 7 7770  gt  div 10 3 3   gt  div 10 3 000 3 333   And here the script      bin bash   op             local z 00000000000000000000000000000000         local a1   1             local x1   1              local n1       x1 -   a1            local a2   2             local x2   2              local n2       x2 -   a2            local n  n1         if    n1  lt   n2    then                 local n  n2                 x1  x1  z 0     n2- n1            fi         if    n1  gt   n2    then                 x2  x2  z 0     n1- n2            fi         if     3            then                 x1  x1  z 0  n          fi         local r     x1  3  x2           local l       r - n           if     3            then                 l     l- n           fi         echo   r 0  l    r  l    add       op  1  2      sub       op  1  2 -    mul       op  1  2        div       op  1  2

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You can t  bash only does integers  you must delegate to a tool such as bc

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bash As noted by others  bash does not support floating point arithmetic  although you could fake it with some fixed decimal trickery  e g  with two decimals  echo     100   1   3      sed  s       amp     Output   33  See Nilfred s answer for a similar but more concise approach  Alternatives Besides the mentioned bc and awk alternatives there are also the following  clisp clisp -x     1 0 3    with cleaned up output  clisp --quiet -x     1 0 3    or through stdin  echo     1 0 3     clisp --quiet   tail -n1  dc echo 2k 1 3  p   dc  genius cli calculator echo 1 3 0   genius  ghostscript echo 1 3 div     gs -dNODISPLAY -dQUIET   sed -n  1s    gt   p    gnuplot echo  pr 1 3     gnuplot  Imagemagick convert xc  -format    fx 1 3   info   or through stdin  echo 1 3     convert xc  -format  quot   fx   cat   quot  info      jq echo 1 3   jq -nf  dev stdin  Or  jq -n 1 3  ksh echo  print     1 3        ksh  lua lua -e  print 1 3    or through stdin  echo  print 1 3     lua  maxima echo  1 3 numer     maxima  with cleaned up output  echo  1 3 numer     maxima --quiet   sed -En  2s                p   node echo 1 3   node -p  octave echo 1 3   octave  perl echo print 1 3   perl  python2 echo print 1 3    python2  python3 echo  print 1 3     python3  R echo 1 3   R --no-save  with cleaned up output  echo 1 3   R --vanilla --quiet   sed -n  2s      p   ruby echo print 1 3 0   ruby  units units 1 3  with compact output  units --co 1 3  wcalc echo 1 3   wcalc  with cleaned up output  echo 1 3   wcalc   tr -d       cut -d  -f2  zsh print     1 3      or through stdin  echo  print     1 3        zsh   Other sources St  phane Chazelas answered a similar question over on Unix SX

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Use calc  It s the easiest I found example      calc 1 1    2      calc 1 10    0 1

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Improving a little the answer of marvin   RESULT   awk  BEGIN  printf     2f     IMG WIDTH    IMG2 WIDTH       bc doesn t come always as installed package

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Well  before float was a time where fixed decimals logic was used   IMG WIDTH 100 IMG2 WIDTH 3 RESULT      IMG WIDTH 00  IMG2 WIDTH   echo    RESULT 0 -2    RESULT  -2   33 33   Last line is a bashim  if not using bash  try this code instead   IMG WIDTH 100 IMG2 WIDTH 3 INTEGER     IMG WIDTH  IMG2 WIDTH   DECIMAL   tail -c 3  lt  lt  lt       IMG WIDTH 00  IMG2 WIDTH    RESULT  INTEGER  DECIMAL echo  RESULT 33 33   The rationale behind the code is  multiply by 100 before divide to get 2 decimals

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Injection-safe floating point math in bash shell     Note  The focus of this answer is provide ideas for injection-safe solution to performing math in bash  or other shells   Of course  same can be used  with minor adjustment to perform advanced string processing  etc   Most of the solution that were by presented  construct small scriptlet on the fly  using external data  variables  files  command line  environment variables   The external input can be used to inject malicious code into the engine  many of them   Below is a comparison on using the various language to perform basic math calculation  where the result in floating point  It calculates A   B   0 1  as floating point    All solution attempt avoid creating dynamic scriptlets  which are extremely hard to maintain  Instead they use static program  and pass parameters into designated variable  They will safely handle parameters with special characters - reducing the possibility of code injection  The exception is  BC  which does not provide input output facility  The exception is  bc   which does not provide any input output  all the data comes via programs in stdin  and all output goes to stdout  All calculation are executing in a sandbox  which does not allow side effect  opening files  etc    In theory  injection safe by design   A 5 2 B 4 3    Awk  Map variable into awk   Exit 0  or just exit  for success  non-zero for error    awk -v A   A  -v B   B   BEGIN   print A   B   0 1   exit 0      Perl perl -e    A  B     ARGV   print  A    B   0 1    A    B     Python 2 python -c  import sys   a   float sys argv 1     b   float sys argv 2     print a b 0 1    A    B     Python 3 python3 -c  import sys   a   float sys argv 1     b   float sys argv 2     print a b 0 1     A    B     BC bc  lt  lt  lt   scale 1    A    B   0 1

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You could use bc by the -l option  the L letter   RESULT   echo   IMG WIDTH  IMG2 WIDTH    bc -l

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While you can t use floating point division in Bash you can use fixed point division   All that you need to do is multiply your integers by a power of 10 and then divide off the integer part and use a modulo operation to get the fractional part   Rounding as needed      bin bash  n  1 d  2    because of rounding this should be 10  i 1    where i is the number of decimal digits wanted i 4 P    10   i 1    Pn     P   10     here we  fix  the decimal place  divide and round tward zero t     n    P    d     n  lt  0   -5   5      then we print the number by dividing off the interger part and   using the modulo operator  after removing the rounding digit  to get the factional part  printf   d  0  i d n      t    P       t  lt  0   -t   t    10    Pn

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you can do this  bc  lt  lt  lt   scale 2  100 3  33 33  UPDATE 20130926   you can use  bc -l  lt  lt  lt   100 3    saves a few hits 33 33333333333333333333

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As an alternative to bc  you can use awk within your script     For example   echo   IMG WIDTH  IMG2 WIDTH    awk   printf    2f  n    1  2     In the above      2f   tells the printf function to return a floating point number with two digits after the decimal place  I used echo to pipe in the variables as fields since awk operates properly on them     1   and    2   refer to the first and second fields input into awk    And you can store the result as some other variable using   RESULT    echo

[bash] How do I use floating-point division in bash?

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