Re: Command to terminate current stream processing

[Assaf Gordon]

Hello,

On 2019-04-09 1:00 a.m., Konstantin Andreev wrote:
> sed has convenient commands "q" and "Q" that allow to break stream 
> processing, if you are not interested in the stream any more. E.g., 
> looking for C header inclusion guard may be
[...]
> If I understand correctly, the option "-s" has evolved as a side effect 
> of "-i" option, and, as such, is assumed to work as shortcut that turns 
> iterators like
>
> | $ for j in A B C; do sed SCRIPT $j; done
>
> into the
>
> | $ sed -s SCRIPT A B C

I do not know if "-s/--separate" evolved as a side-effect of "-i" - it
works independently of "-i" (as in your examples above)...
It was added sometime between sed 3.02 and sed 3.95 (circa 1999-2002),
I do no have exact details about it (other are welcomed to comment).

> However, "-s" does not work this way if SCRIPT encounters q/Q command: 
> sed just quits without processing further streams. Formally correct and 
> agreed with documentation, this breaks "-s" option idea.

It is important to realize (and perhaps we should document it better),
that "-s" only affects address matching. It does not affect commands at
all.

I think it was mainly added so that "$" (end of stream / last line)
address condition could match each file instead of just once.

For example:

   $ seq 10 19 > a ; seq 200 209 > b; seq 3000 3009 > c ;

   $ sed -n '1p;$p' a b c
   10
   3009

   $ sed -s -n '1p;$p' a b c
   10
   19
   200
   209
   3000
   3009

> I believe this is because in past times, when sed was able to process 
> only one stream at one run, "terminate stream" and "terminate sed" were 
> the same, and documentation for "q/Q" didn't care about the difference.
>
> Anyway, the action of "q/Q" in multi-stream environment is now beyond 
> changing.
>
> But, ... may we introduce a new command, that behaves identically to "q" 
> in a single-stream environment, but just breaks current stream 
> processing in a multi-stream environment?

"q/Q" is quit (=terminate sed's process) regardless of "-s".

It seems you are looking for a functionality similar to awk's "nextfile" 
command.

While sed has no such built-in command, it is easy to achieve this
using the following sequence, which consumes and discards all input
until the last line (and because of "-s/--separate", the
last line detection is per-file):

  :consume
  $d
  N
  z
  b consume

(a similar sequence is documented in the 'Q' command, here:
https://www.gnu.org/software/sed/manual/sed.html#Extended-Commands ).


For example, if you wanted to 'nextfile' when a line contains
the character '4', you could run:

  $ sed -s '/4/{:x;$d;N;z;bx}' a b c
  10
  11
  12
  13
  200
  201
  202
  203
  3000
  3001
  3002
  3003

If you wanted to also print the matching line, just add 'p':

  $ sed -s '/4/{p;:x;$d;N;z;bx}' a b c
  10
  11
  12
  13
  14
  200
  201
  202
  203
  204
  3000
  3001
  3002
  3003
  3004


Hope this helps,
 - assaf