What's the best way to take a segment out of a text file?

Even simpler:

sed -n '20,45p;45q' < textfile 

The -n flag disables the default output. The "20,45" addresses lines 20 to 45, inclusive. The "p" command prints the current line. And the q quits after printing the line.

This is not an answer but can't post it as a comment.

Another (very fast) way to do it was suggested by mikeserv here:

{ head -n 19 >/dev/null; head -n 26; } <infile 

Using the same test file as here and the same procedure, here are some benchmarks (extracting lines 1000020-1000045):

mikeserv:

{ head -n 1000019 >/dev/null; head -n 26; } <iplist real 0m0.059s 

Stefan:

head iplist -n 1000045 | tail -n 26 real 0m0.054s 

These are by far the fastest solutions and the differences are negligible (for a single pass) (I tried with different ranges: a couple of lines, millions of lines etc).

Doing it without the pipe might offer a significant advantage, however, to an application which needed to seek over multiple ranges of lines in similar fashion, like:

for pass in 0 1 2 3 4 5 6 7 8 9 do printf "pass#$pass:\t" head -n99 >&3; head -n1 done <<1000LINES 3>/dev/null $(seq 1000) 1000LINES 

...which prints...

pass#0: 100 pass#1: 200 pass#2: 300 pass#3: 400 pass#4: 500 pass#5: 600 pass#6: 700 pass#7: 800 pass#8: 900 pass#9: 1000 

...and only reads the file through the one time.

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The other sed/awk/perl solutions read the whole file and since this is about huge files, they're not very efficient. I threw in some alternatives that exit or quit after the last line in the specified range:

Stefan:

awk "1000020 <= NR && NR <= 1000045" iplist real 0m2.448s 

vs.

awk "NR >= 1000020;NR==1000045{exit}" iplist real 0m0.243s 

dkagedal (sed):

sed -n 1000020,1000045p iplist real 0m0.947s 

vs.

sed '1,1000019d;1000045q' iplist real 0m0.143s 

Steven D:

perl -ne 'print if 1000020..1000045' iplist real 0m2.041s 

vs.

perl -ne 'print if $. >= 1000020; exit if $. >= 1000045;' iplist real 0m0.369s 

ruby -ne 'print if 20 .. 45' file 

Since sed and awk were already taken, here is a perl solution:

perl -nle "print if ($. > 19 && $. < 46)" < textfile 

Or, as pointed out in the comments:

perl -ne 'print if 20..45' textfile