Move top 1000 lines from text file to a new file using Unix shell commands

head -1000 input > output && sed -i '1,+999d' input

For example:

$ cat input 123456$ head -3 input > output && sed -i '1,+2d' input$ cat input 456$ cat output 123

head -1000 file.txt > first100lines.txttail --lines=+1001 file.txt > restoffile.txt

Out of curiosity, I found a box with a GNU version of sed (v4.1.5) and tested the (uncached) performance of two approaches suggested so far, using an 11M line text file:

$ wc -l input11771722 input$ time head -1000 input > output; time tail -n +1000 input > input.tmp; time cp input.tmp input; time rm input.tmpreal    0m1.165suser    0m0.030ssys     0m1.130sreal    0m1.256suser    0m0.062ssys     0m1.162sreal    0m4.433suser    0m0.033ssys     0m1.282sreal    0m6.897suser    0m0.000ssys     0m0.159s$ time head -1000 input > output && time sed -i '1,+999d' inputreal    0m0.121suser    0m0.000ssys     0m0.121sreal    0m26.944suser    0m0.227ssys     0m26.624s

This is the Linux I was working with:

$ uname -aLinux hostname 2.6.18-128.1.1.el5 #1 SMP Mon Jan 26 13:58:24 EST 2009 x86_64 x86_64 x86_64 GNU/Linux

For this test, at least, it looks like sed is slower than the tail approach (27 sec vs ~14 sec).