The ed text editor does not allow you to filter lines of the buffer through external commands using the ! command. What you can do is to read the output of a command and append the output to a specific address within the text. The general solution is to use a temporary file to hold the output of the command, and then read that file into the buffer at the desired location.

Here is how you can do it:

1. First, write the lines you want to filter to a temporary file. For example, if you wish to filter lines 1 to 5, you can do this from within ed:

   1,5w /tmp/tempfile.txt

2. Next, delete the lines you want to filter from the buffer:

   1,5d

3. Now apply the external command to the temporary file and redirect and read the output to the location you want to insert the output. Finally, remove the temporary file:

   1r !sort /tmp/tempfile.txt 
   !rm /tmp/sortedfile.txt

Of course, this might be a bit dumbersome, but it should work with any external command. If you find yourself doing this more often, it might be worthwhile to writ a short shell script to automate the process.

Finally, if you use GNU ed, you can use the addressing commands together with the ! command to achieve a similar effect without needing a temporary file. For example, to filter lines 1 to 5 through sort, you can do:

1,5!sort

to achieve the same effect.