paste

The paste command is similar to the cat command, it merges lines together in a file. Let's create a new file with the following contents:

sample2.txt
The
quick
brown
fox

Let's combine all these lines into one line:

$ paste -s sample2.txt

The default delimiter for paste is TAB, so now there is one line with TABs separating each word.

Let's change this delimiter (-d) to something a little more readable:

$ paste -d ' ' -s sample2.txt

Now everything should be on one line delimited by spaces.