cat
-concatenates files into one file
-can dump the contents of a file to the screen
-be careful with very large files

cat country1
canada
Brazil
nigeria
Japan

cat country2
nepal
mexico
Chile

cat country1 country2 > allcountries
more allcountries
canada
Brazil
nigeria
Japan
nepal
mexico
Chile

can also do by globbing

cat cou* > mycountrylist
more mycountrylist
canada
Brazil
nigeria
Japan
nepal
mexico
Chile

if you had a directory with 3000 individual fasta files you could combine them into a single fasta file by

cat *fasta > allmyfastafiles
check to see if it worked
grep “>” *fasta |wc -l

grep “>” *fasta -c
gives you the number of fastas in each file