Append or write to file using here-strings

Question

How can I write to a file or append a string to a file using ed only?

I have knowledge of other editors but this particular form of writing in a bash script with ed confuses me a lot:

ed fileName <<< $'a textToWriteInFile\nwq'

The previous line does not work and, although I have read some ed man pages, I am still confused with the here-strings method. I am not interested in the here-document method.

I've tried ed H myFile <<< $'a\nMy line here\n.\nwq' with the H option and I get the error

H: No such file or directory

I have already created a file named myFile and did sudo chmod a+wx myFile in my directory.

Answer 1

tl;dr:

ed myFile <<< $'a\nMy line here\n.\nwq'

---

A sad truth about programming is that you can never automate anything that you don't know how to do manually. If you don't know how to append a line manually with ed, you can't hope to append a line automatically through ed and a here-string.

The first step is therefore to look up how to append lines in ed. Here's info ed:

The sample sessions below illustrate some basic concepts of line editing with 'ed'. We begin by creating a file, 'sonnet', with some help from Shakespeare. As with the shell, all input to 'ed' must be followed by a character. Comments begin with a '#'.

 $ ed
 # The 'a' command is for appending text to the editor buffer.
 a
 No more be grieved at that which thou hast done.
 Roses have thorns, and filvers foutians mud.
 Clouds and eclipses stain both moon and sun,
 And loathsome canker lives in sweetest bud.
 .
 # Entering a single period on a line returns 'ed' to command mode.
 # Now write the buffer to the file 'sonnet' and quit:
 w sonnet
 183
 # 'ed' reports the number of characters written.
 q

Ok, now let's adapt that to append a single line to a file and then quit:

$ touch myFile
$ ed myFile
a
Some text here
.
wq

And let's verify that it worked:

$ cat myFile
Some text here

Yay. Now that we're able to manually append a line, we just have to recreate the same input with a here-string. We can use cat to verify that our input is correct:

$ cat <<< $'a\nMy line here\n.\nwq'
a
My line here
.
wq

Yup, this is exactly the input we used. Now we can plug that into ed:

$ echo "Existing contents" > myFile
$ ed myFile <<< $'a\nMy line here\n.\nwq'
18
31
$ cat myFile
Existing contents
My line here