Space formatting issue in sed

How to remove any space around a specific charachter from a string using sed.

for exmple :
the string is like following

str1='"name", "roll", "addr","job", "pay",'

I need to remove all the spaces aronnd the commas.

simplest usage

# sed 's/ "/"/g' infile

echo "$str1" | sed 's/ *, */,/g'

its giving the message :

try.sh[7]: str4=${echo $str1 | sed "s/ *, */,/g"}: bad substitution

Hi, use $( ) instead of ${ } (and use double quotes around $str1)

with tr ..

echo "$str1"|tr -d " "

Try with ( )

str4=$(echo $str1 | sed "s/ *, */,/g")

Thanks a lot guys..it worked....
but, caqn you please explain why we are using () instead of {}?

This is called command substitution.