The paste command displays the corresponding lines of multiple files side-by-side.

# To join all the lines into a single line, using TAB as delimiter:
paste -s file

# To join all the lines into a single line, using the specified delimiter:
paste -s -d delimiter file

# To merge two files side by side, each in its column, using TAB as delimiter:
paste file1 file2

# To merge two files side by side, each in its column, using the specified delimiter:
paste -d delimiter file1 file2

# To merge two files, with lines added alternatively:
paste -d '\n' file1 file2