Replace a string in shell script using a variable

you can use the shell (bash/ksh).

$ var="12345678abc"
$ replace="test"
$ echo ${var//12345678/$replace}
testabc

If you want to interpret $replace, you should not use single quotes since they prevent variable substitution.

Try:

echo $LINE | sed -e "s/12345678/${replace}/g"

Transcript:

pax> export replace=987654321
pax> echo X123456789X | sed "s/123456789/${replace}/"
X987654321X
pax> _

Just be careful to ensure that ${replace} doesn't have any characters of significance to sed (like / for instance) since it will cause confusion unless escaped. But if, as you say, you're replacing one number with another, that shouldn't be a problem.

Not specific to the question, but for folks who need the same kind of functionality expanded for clarity from previous answers:

# create some variables
str="someFileName.foo"
find=".foo"
replace=".bar"
# notice the the str isn't prefixed with $
#    this is just how this feature works :/
result=${str//$find/$replace}
echo $result    
# result is: someFileName.bar

str="someFileName.sally"
find=".foo"
replace=".bar"
result=${str//$find/$replace}
echo $result    
# result is: someFileName.sally because ".foo" was not found