You can't. bash only does integers; you must delegate to a tool such as bc.

you can do this:

bc <<< 'scale=2; 100/3'33.33

UPDATE 20130926 : you can use:

bc -l <<< '100/3' # saves a few hits33.33333333333333333333

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 $(( 100 * 1 / 3 )) | sed -e 's/..$/.&/;t' -e 's/.$/.0&/'

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/.*>//p' 

gnuplot

echo 'pr 1/3.' | gnuplot

Imagemagick

convert xc: -format '%[fx:1/3]' info:

or through stdin:

echo 1/3 | { convert xc: -format "%[fx:$(cat)]" info:; }

jq

jq -n 1/3

or through stdin:

echo 1/3 | jq -nf /dev/stdin

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 puts 1/3.0 | ruby

units

units 1/3

with compact output:

units --com 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

Stéphane Chazelas answered a similar question over on UL.