28.2 Comparing Three Different Versions with diff3

diff3	You can use the diff3 command to look at differences between three files. (There's also a GNU version on the CD-ROM.) Here are three sample files, repeated from article 28.1 :

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 (28.1 ) , 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 creates a script for building the first file from the second and third files.

$ diff3 -e test1 test2 test3


3c
walnuts
chestnuts
.
1d
.
w
q

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

$ diff3 -e test1 test3 test2


3c
grapes
.
w
q

As you might guess, this is basically the same output as doing a diff on the first and third files. (The only difference in the output is the result of a rather errant inconsistency between diff and diff3 . The System V version of diff3 produces an ed script that ends with the commands that save the edited version of the file. The Berkeley diff3 , and both versions of diff , require that you supply the w and q . We show the System V version of diff3 here.)