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