diffutils: Side by Side Format
2.3.1 Controlling Side by Side Format
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The '--side-by-side' ('-y') option selects side by side format. Because
side by side output lines contain two input lines, the output is wider
than usual: normally 130 print columns, which can fit onto a traditional
printer line. You can set the width of the output with the
'--width=COLUMNS' ('-W COLUMNS') option. The output is split into two
halves of equal width, separated by a small gutter to mark differences;
the right half is aligned to a tab stop so that tabs line up. Input
lines that are too long to fit in half of an output line are truncated
for output.
The '--left-column' option prints only the left column of two common
lines. The '--suppress-common-lines' option suppresses common lines
entirely.