extract a line from a file by line number

Shell Programming and Scripting
1

Hi guys,
does anyone know how to extract(grep) a line from the file, if I know the line number?
Thanks a lot.

2 
ln=14
sed '${ln}!d' file
3

Thanks a lot

4

Possibly :

ln=14
sed -n "${ln}p" file

is more efficient?

5

Not really. If you were dealing with a very large file and there were many lines remaining after printing the target, both approaches could continue reading the entire file. I say "could" because some sed implementations, when using -n, will abort once they know that there's no possibility of an address matching further input (I believe I saw such an optimization in gnu sed a while back).

To ensure that sed doesn't continue reading needlessly, use q:

sed -n "$ln{p; q;}" file

Regards,
Alister

6 
awk -vln=12 'NF==ln{print; exit}'
7

This is what I was referring to http://git.savannah.gnu.org/cgit/sed.git/tree/sed/execute.c\#n1774

#ifdef EXPERIMENTAL_DASH_N_OPTIMIZATION
      /* If our top-level program consists solely of commands with
         ADDR_IS_NUM addresses then once we past the last mentioned
         line we should be able to quit if no_default_output is true,
         or otherwise quickly copy input to output.  Now whether this
         optimization is a win or not depends on how cheaply we can
         implement this for the cases where it doesn't help, as
         compared against how much time is saved.  One semantic
         difference (which I think is an improvement) is that *this*
         version will terminate after printing line two in the script
         "yes | sed -n 2p".
        
     Don't use this when in-place editing is active, because line
     numbers restart each time then. */

Although it seems that this code has been bug-ridden since its inception and is currently #undef'd, http://git.savannah.gnu.org/cgit/sed.git/tree/sed/execute.c\#n19

#undef EXPERIMENTAL_DASH_N_OPTIMIZATION    /*don't use -- is very buggy*/

Tomorrow will be 13 years to the day since it first appeared, only to be undefined a month later. I wonder why they didn't just yank it completely.

Tue Apr 14 17:34:54 PDT 1998  Ken Pizzini <ken@gnu.org>

    * execute.c (execute_program, process_files, count_branches,
    shrink_program): Added a first attempt at program optimization.
    ... <snip> ...
    The code is all conditionally compiled based on the
    EXPERIMENTAL_DASH_N_OPTIMIZATION symbol being #defined,
    so it can be easily omitted if it causes problems.

Sat May 23 16:07:09 HST 1998  Ken Pizzini <ken@gnu.org>
    
    * sed/execute.c: #undef'd EXPERIMENTAL_DASH_N_OPTIMIZATION
    because its code is buggy.


Sat May 30 12:23:16 PDT 1998  Ken Pizzini <ken@gnu.org>

* sed/execute.c[EXPERIMENTAL_DASH_N_OPTIMIZATION conditional
    code]: various modifications intended to keep this
    code in sync with the new changes, but the code still
    retains its previous bugs.

Well, at least now I know that I did not imagine it. I did see this optimization somewhere. So, long long story short, use 'q' when you know you no longer need to continue reading the input. :wink:

Regards,
Alister

8

I think you meant to use NR :cool:

awk -vln=12 'NR==ln{print;exit}'
9 
line=3
ruby -ne 'print; exit if $.=='"$line"' ' file
10

:DYes, it is NR, I didn't test the code.
Thanks