How do I find line starts with and replace complete line?

File output:

```
 xyz
 abc
 /dev/linux-test1/

```

Code:

```
output=/dev/sda/windows
sed 's/^/dev/linux*/$output/g' file.txt

```

I am getting below Error:

```
 sed: -e expression #1, char 9: unknown option to `s'

```

File Output expected after replacement:

```
 xyz
 abc
 /dev/sda/windows

```