Be careful, because none of these methods work with a large number of files. Personally, I used this line:
for i in $(ls | grep ".txt");do cat $i >> output.txt;done
EDIT: As someone said in the comments, you can replace $(ls | grep ".txt")
with $(ls *.txt)
EDIT: thanks to @gnourf_gnourf expertise, the use of glob is the correct way to iterate over files in a directory. Consequently, blasphemous expressions like $(ls | grep ".txt")
must be replaced by *.txt
(see the article here).
Good Solution
for i in *.txt;do cat $i >> output.txt;done