How to append contents of multiple files into one file

You need the cat (short for concatenate) command, with shell redirection (>) into your output file

cat 1.txt 2.txt 3.txt > 0.txt

Another option, for those of you who still stumble upon this post like I did, is to use find -exec:

find . -type f -name '*.txt' -exec cat {} + >> output.file

In my case, I needed a more robust option that would look through multiple subdirectories so I chose to use find. Breaking it down:

find .

Look within the current working directory.

-type f

Only interested in files, not directories, etc.

-name '*.txt'

Whittle down the result set by name

-exec cat {} +

Execute the cat command for each result. "+" means only 1 instance of cat is spawned (thx @gniourf_gniourf)

 >> output.file

As explained in other answers, append the cat-ed contents to the end of an output file.