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; ${userinput}" | bc
How can I get Bash to give me a float like 0.5?
This question is related to
bash
floating-point
arithmetic-expressions
Use calc. It's the easiest I found example:
calc 1+1
2
calc 1/10
0.1
It's perfect time to try zsh, an (almost) bash superset, with many additional nice features including floating point math. Here is what your example would be like in zsh:
% IMG_WIDTH=1080
% IMG2_WIDTH=640
% result=$((IMG_WIDTH*1.0/IMG2_WIDTH))
% echo $result
1.6875
This post may help you: bash - Worth switching to zsh for casual use?
You could use bc by the -l option (the L letter)
RESULT=$(echo "$IMG_WIDTH/$IMG2_WIDTH" | bc -l)
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:
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!
you can do this:
bc <<< 'scale=2; 100/3'
33.33
UPDATE 20130926 : you can use:
bc -l <<< '100/3' # saves a few hits
33.33333333333333333333
** 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 <<< "scale=1 ; $A + $B * 0.1"
Instead of using "here strings" (<<<) with the bc command, like one of the most-upvoted examples does, here's my favorite bc floating point example, right from the EXAMPLES section of the bc man pages (see man bc for the manual pages).
Before we begin, know that an equation for pi is: pi = 4*atan(1). a() below is the bc math function for atan().
This is how to store the result of a floating point calculation into a bash variable--in this case into a variable called pi. Note that scale=10 sets the number of decimal digits of precision to 10 in this case. Any decimal digits after this place are truncated.
pi=$(echo "scale=10; 4*a(1)" | 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 "scale=10; 4*a(1)" | 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 "4*a(1)" | bc -l)); echo $pi
3.1416
(See also my other answer here).
Note that dt_min gets rounded from 0.01666666666... to 0.017:
start=$SECONDS; sleep 1; end=$SECONDS; dt_sec=$(( end - start )); dt_min=$(printf %.3f $(echo "$dt_sec/60" | bc -l)); echo "dt_sec = $dt_sec; dt_min = $dt_min"
dt_sec = 1; dt_min = 0.017
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.
here is awk command: -F = field separator == +
echo "2.1+3.1" | awk -F "+" '{print ($1+$2)}'
If you found the variant of your preference you can also wrap it into a function.
Here I'm wrapping some bashism into a div function:
One liner:
function div { local _d=${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 $_r
}
Now you have the function
div <dividend> <divisor> [<precision=2>]
and use it like
> div 1 2
.50
> div 273 123 5
2.21951
> x=$(div 22 7)
> echo $x
3.14
UPDATE I added a little script which provides you the basic operations with floating point numbers for bash:
Usage:
> add 1.2 3.45
4.65
> sub 1000 .007
999.993
> mul 1.1 7.07
7.7770
> div 10 3
3.
> div 10 3.000
3.333
And here the script:
#!/bin/bash
__op() {
local z=00000000000000000000000000000000
local a1=${1%.*}
local x1=${1//./}
local n1=$((${#x1}-${#a1}))
local a2=${2%.*}
local x2=${2//./}
local n2=$((${#x2}-${#a2}))
local n=$n1
if (($n1 < $n2)); then
local n=$n2
x1=$x1${z:0:$(($n2-$n1))}
fi
if (($n1 > $n2)); then
x2=$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 $2 / ;}
For those trying to calculate percentages with the accepted answer, but are losing precision:
If you run this:
echo "scale=2; (100/180) * 180" | bc
You get 99.00 only, which is losing precision.
If you run it this way:
echo "result = (100/180) * 180; scale=2; result / 1" | bc -l
Now you get 99.99.
Because you're scaling only at the moment of printing.
Refer to here
Well, before float was a time where fixed decimals logic was used:
IMG_WIDTH=100
IMG2_WIDTH=3
RESULT=$((${IMG_WIDTH}00/$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 <<< $((${IMG_WIDTH}00/$IMG2_WIDTH)))
RESULT=$INTEGER.$DECIMAL
echo $RESULT
33.33
The rationale behind the code is: multiply by 100 before divide to get 2 decimals.
It's not really floating point, but if you want something that sets more than one result in one invocation of bc...
source /dev/stdin <<<$(bc <<< '
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
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:
bonus track: bc version x 1000 took 1,8 seconds on my laptop, while the pure bash one took 0,016 seconds.
Improving a little the answer of marvin:
RESULT=$(awk "BEGIN {printf \"%.2f\",${IMG_WIDTH}/${IMG2_WIDTH}}")
bc doesn't come always as installed package.
As an alternative to bc, you can use awk within your script.
For example:
echo "$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 ...`
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 < 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 < 0 ? -t : t) / 10 % $Pn))
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 $(( 100 * 1 / 3 )) | sed 's/..$/.&/'
Output:
.33
See Nilfred's answer for a similar but more concise approach.
Besides the mentioned bc and awk alternatives there are also the following:
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
echo 2k 1 3 /p | dc
echo 1/3.0 | genius
echo 1 3 div = | gs -dNODISPLAY -dQUIET | sed -n '1s/.*>//p'
echo 'pr 1/3.' | gnuplot
convert xc: -format '%[fx:1/3]' info:
or through stdin:
echo 1/3 | { convert xc: -format "%[fx:$(cat)]" info:; }
echo 1/3 | jq -nf /dev/stdin
Or:
jq -n 1/3
echo 'print $(( 1/3. ))' | ksh
lua -e 'print(1/3)'
or through stdin:
echo 'print(1/3)' | lua
echo '1/3,numer;' | maxima
with cleaned up output:
echo '1/3,numer;' | maxima --quiet | sed -En '2s/[^ ]+ [^ ]+ +//p'
echo 1/3 | node -p
echo 1/3 | octave
echo print 1/3 | perl
echo print 1/3. | python2
echo 'print(1/3)' | python3
echo 1/3 | R --no-save
with cleaned up output:
echo 1/3 | R --vanilla --quiet | sed -n '2s/.* //p'
echo print 1/3.0 | ruby
units 1/3
with compact output:
units --co 1/3
echo 1/3 | wcalc
with cleaned up output:
echo 1/3 | wcalc | tr -d ' ' | cut -d= -f2
print $(( 1/3. ))
or through stdin:
echo 'print $(( 1/3. ))' | zsh
#Other sources
Stéphane Chazelas answered a similar question over on Unix.SX.
Source: Stackoverflow.com