Display two files side by side

You can use pr to do this, using the -m flag to merge the files, one per column, and -t to omit headers, eg.

pr -m -t one.txt two.txt

outputs:

apple                               The quick brown fox..
pear                                foo
longer line than the last two       bar
last line                           linux

                                    skipped a line

See Also:

• Print command result side by side
• Combine text files column-wise

To expand a bit on @Hasturkun's answer: by default pr uses only 72 columns for its output, but it's relatively easy to make it use all available columns of your terminal window:

pr -w $COLUMNS -m -t one.txt two.txt

Most shell's will store (and update) your terminal's screenwidth in the $COLUMNS environment variable, so we're just passing that value on to pr to use for its output's width setting.

This also answers @Matt's question:

Is there a way for pr to auto-detect screen width?

So, no: pr itself can't detect the screenwidth, but we're helping out a bit by passing in the terminal's width via the -w option.

If you know the input files have no tabs, then using expand simplifies @oyss's answer:

paste one.txt two.txt | expand --tabs=50

If there could be tabs in the input files, you can always expand first:

paste <(expand one.txt) <(expand two.txt) | expand --tabs=50

paste one.txt two.txt | awk -F'\t' '{
    if (length($1)>max1) {max1=length($1)};
    col1[NR] = $1; col2[NR] = $2 }
    END {for (i = 1; i<=NR; i++) {printf ("%-*s     %s\n", max1, col1[i], col2[i])}
}'

Using * in a format specification allows you to supply the field length dynamically.

There is a sed way:

f1width=$(wc -L <one.txt)
f1blank="$(printf "%${f1width}s" "")"
paste one.txt two.txt |
    sed "
        s/^\(.*\)\t/\1$f1blank\t/;
        s/^\(.\{$f1width\}\) *\t/\1 /;
    "

Under bash, you could use printf -v:

f1width=$(wc -L <one.txt)
printf -v f1blank "%${f1width}s"
paste one.txt two.txt |
    sed "s/^\(.*\)\t/\1$f1blank\t/;
         s/^\(.\{$f1width\}\) *\t/\1 /;"

_{(Of course @Hasturkun 's solution pr is the most accurate!)}:

Advantage of sed over pr

You can finely choose separation width and or separators:

f1width=$(wc -L <one.txt)
(( f1width += 4 ))         # Adding 4 spaces
printf -v f1blank "%${f1width}s"
paste one.txt two.txt |
    sed "s/^\(.*\)\t/\1$f1blank\t/;
         s/^\(.\{$f1width\}\) *\t/\1 /;"

Or, for sample, to mark lines containing line:

f1width=$(wc -L <one.txt)
printf -v f1blank "%${f1width}s"
paste one.txt two.txt |
    sed "s/^\(.*\)\t/\1$f1blank\t/;
  /line/{s/^\(.\{$f1width\}\) *\t/\1 |ln| /;ba};
         s/^\(.\{$f1width\}\) *\t/\1 |  | /;:a"

will render:

apple                         |  | The quick brown fox..
pear                          |  | foo
longer line than the last two |ln| bar 
last line                     |ln| linux
                              |  | 
                              |ln| skipped a line