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Unix Power Tools
Comparing Three Different Versions with diff3

by Dale Dougherty
02/24/2000

You can use the diff3 command to look at differences between three files. Here are three sample files:

test1 test2 test3
apples apples oranges
oranges oranges walnuts
walnuts grapes chestnuts

For each set of differences, diff3 displays a row of equal signs (====) followed by 1, 2, or 3, indicating which file is different; if no number is specified, then all three files differ. Then, using ed-like notation, the differences are described for each file.

$ diff3 test1 test2 test3
====3
1:1c
2:1c
  apples
3:0a
====
1:3c
  walnuts
2:3c
  grapes
3:2,3c
  walnuts
  chestnuts

With the output of diff3, it is easy to keep track of which file is which; however, the prescription given is a little harder to decipher. To bring these files into agreement, the first range of text (after ====3) shows that you would have to add apples at the beginning of the third file (3:0a). The second range tells you to change line 3 of the second file to line 3 of the first file; and change lines 2 and 3 of the third file, effectively dropping the last line.

The diff3 command also has a -e option for creating an editing script for ed. It doesn't work quite the way you might think. Basically, it identifies the changes made to test2 to produce test3 and writes as a script to apply to test1.

$ diff3 -e test1 test2 test3
3c
walnuts
chestnuts
.
1d

If you reverse the second and third files, a different script is produced:

$ diff3 -e test1 test3 test2
3c
grapes
.

As you might guess, this is basically the same output as doing a diff on the first and second files.


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