replace string with sed in bash

[gekkos]

Hi,
I try to replace a string in a file like
a=text
b=4
c=5

I would like to replace with sed
$a:$b with $a:$c

I tried

sed 's/$a\:$b/$a\:$c/g' file.txt > file1.txt && mv file1.txt file.txt

Any ideas how to do it in a proper way.
Many thanks

[scathefire]

you have to escape the $ too, since it has special meaning in a regexp (means the end of a string) so try:

sed 's/\$a\:\$b/\$a\:\$c/g'

[gekkos]

Thanks,
That didn't changed it but I found it now

sed "s/$a:$b/$a:$c/g"

[scathefire]

are you sure you are using sed right? sed stands for Stream Editor, meaning it takes stuff from like stdout and such.

the data has to be incoming, so you will need to cat your files you want editted, and pipe them into set

cat file1 | sed 's/\$a\:\$b/\$a\:\$c/g' > newfile; blah; blah etc

so the output will be written into newfile

[gekkos]

thats true,
but thats how i did it

sed "s/$a:$b/$a:$c/g" file1 > newfile && mv newfile file1

[Irithori]

gnu sed knows -i, that is: inplace editing

So, no need for that tmp file and mv.