DIFF(I)                      5/15/74                      DIFF(I)


     diff - differential file comparator


     diff [ - ] name1 name2


     Diff tells what lines must be changed in two files to  bring

     them  into  agreement.   The normal output contains lines of

     these forms:

          n1 a n3,n4

          n1,n2 d n3

          n1,n2 c n3,n4

     These lines resemble ed commands to convert file name1  into

     file  name2.   The numbers after the letters pertain to file

     name2.  In fact, by  exchanging  `a'  for  `d'  and  reading

     backward one may ascertain equally how to convert file name2

     into name1.  As in ed, identical pairs where n1 = n2 or n3 =

     n4 are abbreviated as a single number.

     Following each of these lines come all the  lines  that  are

     affected  in  the  first  file  flagged by `*', then all the

     lines that are affected in the second file flagged by `.'.

     Under the - option, the output of diff is a script of  a,  c

     and  d  commands  for  the  editor ed, which will change the

     contents of the first file into the contents of the  second.

     In  this  connection,  the  following shell program may help

     maintain multiple versions of a  file.   Only  an  ancestral

     file  ($1)  and  a  chain  of  version-to-version ed scripts

     ($2,$3,...) made  by  diff  need  be  on  hand.   A  `latest

     version' appears on the standard output.

          (cat $2 ... $9; echo "1,$p") | ed - $1

     Except for occasional  `jackpots',  diff  finds  a  smallest

     sufficient set of file differences.


     cmp (I), comm (I), ed (I)


     `jackpot' - To speed things up, the  program  uses  hashing.

     You  have  stumbled  on  a case where there is a chance that

     this has resulted in a difference being  called  where  none

     actually  existed.   Sometimes  reversing the order of files

     will make a jackpot go away.


     Editing scripts produced under the - option are naive  about

     creating lines consisting of a single `.'.