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From: John-Paul Stewart <jpstewart@personalprojects.net>
Newsgroups: comp.unix.shell
Subject: Re: a sed question
Date: Wed, 18 Dec 2024 15:12:11 -0500
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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.