Re: a sed question

On 2024-12-18 2:46 p.m., Salvador Mirzo wrote:

(*) Summary
 
I wrote a sed script that makes a line replacement after it finds the
right spot.  So far so good.  Then I added quit command after the
change, but the quit does not seem to take effect---violating my
expectation.  I'll appreciate any help on understanding what's going on.

[snip]

So far so good.  I decided to try it on longer files and I wanted to see
the change more quickly (without long files scrolling past my terminal),
so I decided to add a /q/ command right after the c commmand.  I
thought---it will make sed quit right after making the change, so I can
see it works as desired and then I remove the /q/ and release it to
production.  But that did not happen.

[snip]

I failed the exercise I gave myself.  Can you help me to understand why
the q command isn't stopping sed as I thought it would?  I'd like to get
a better intuition.

By default sed prints the "pattern space", i.e. all lines in the file.
You can suppress that with the "-n" option to sed.  (In other words, use
"sed -n" instead of plain "sed" in your script.)

The man page for GNU sed 4.9 says about the "q" command:

"Immediately quit the sed script without processing any more input,
except that if auto-print is not disabled the current pattern space will
be printed."

So the behaviour you're seeing without the "-n" option is to be
expected:  pattern space still gets auto-printed after the "q" command.