Joining splitted files in lunux

1 Hjsplit files:
hjsplit(for windows) software files into small sizes which can be joined later to get original file. we can join these files on linux through command line without any software.

say we have following files in /home/user/Downloads folder

aaa.avi.001
aaa.avi.002
aaa.avi.003

we want to join these files to get aaa.avi file, go to download folder
cd Downloads

use Concatenate FILE(s) command

cat aaa.avi.001 aaa.avi.002 aaa.avi.003 > aaa.avi

it will join all these files and give output aaa.avi file

2 RAR files
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EDIT:
just extract the first file aaa.part1.rar, remaining files would be joined automatically.
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for files splitted with winrar we need software called rar, so install it

sudo apt-get install rar

say we have following files in /home/user/Downloads folder

aaa.part1.rar
aaa.part2.rar
aaa.part3.rar

we want to join these files to get aaa.avi file, go to download folder

cd Downloads

use rar command

rar x -y aaa.part1.rar

it will join all these files and give output aaa.avi file