Answer by Kamil Maciorowski for How to achieve side by side output in Bash?

A portable tool for this is paste:

(    echo A    echo a    echo B    echo b    echo C    echo c    echo D    echo d) | paste - -

From the specification:

The default operation of paste shall concatenate the corresponding lines of the input files. The <newline> of every line except the line from the last input file shall be replaced with a <tab>.

[…]

If - is specified for one or more of the files, the standard input shall be used; the standard input shall be read one line at a time, circularly, for each instance of -.

Lines of varying length may generate output that is not perfectly aligned. To see an example, run:

(    echo A line longer than others    echo a    echo B    echo b) | paste - -

You can fix this by piping to column:

(    echo A line longer than others    echo a    echo B    echo b) | paste - - | column -t -s $'\t'

Notes:

• $'\t' generates a tab character in some shells, portably you can do it with "$(printf '\t')".
• column itself is not portable though.
• column needs to know its whole input before it prints any output, so it's not a good tool to process something you need to see immediately; and it's the wrong tool to process data that may never end (e.g. from a potentially endless loop).