How can I get diff to show only added and deleted lines

Question

How can I get the unix command diff show only added and deleted lines? If diff can't do it, what tool can?

Answer 1

I'm not sure this is possible as it will be hard to differentiate between changed, added and deleted lines.

Consider this file:

start
old
old
old
end

We edit it so it looks like this:

start
old
old but now new
new
new
end

If we diff it we get this output:

< old
< old
---
> old but now new
> new
> new

This is straightforward to generate. But if you ask diff to only print added and deleted line I think it becomes a matter of opinion which lines have been added and deleted and which have been changed. For example, did I delete the the last line that said old and replace it with a line that said new or did I edit it?

Answer 2

Does diff -u0 do what you want?

Answer 3

I had the same question. This function was my solution to obtain the maximum change line number (i.e. changes start with the letter '+'). After which I then loop through the diff file again line by line and do not send to the line processor until this triggers the line to process:

#====================================================================
proc_diff_file()    # processes a diff file
#====================================================================
# Arg_1 = Diff File Name
# Arg_2 = New File Name - the one with the new lines
{
  NEW_FILE=$1
  A_FILE=$2
  if [ -f "$A_FILE" ]; then
    echo "processing diff $A_FILE"
    pre_process_diff $A_FILE
    # Set loop separator to end of line
    ndx=0
    BAKIFS=$IFS
    IFS=$(echo -en "\n\b")
    exec 3<&0
    exec 0<$A_FILE
    while read line
    do
      ndx=$(expr $ndx + 1)
      # use $line variable to process line in processLine() function
      if [ $ndx > $max_ndx ]; then
        proc_age $line
      fi
    done
    exec 0<&3
    # restore $IFS which was used to determine what the field separators are
    IFS=$BAKIFS

#    cleanup $NEW_FILE

    echo "processing diff $A_FILE done"
  fi
}

Here is the function:

#====================================================================
pre_process_diff()  # pre-processes a diff file for last changed line
                    # sets a variable named $max_ndx
#====================================================================
# Arg_1 = Diff File Name
{
  A_FILE=$1
  max_ndx=
  # collect last line number of diff + 
  # all lines following are new data
  `grep -n "^[+]" $A_FILE | gawk '-F:' '{ print $1 }' >tmp`
  # Set loop separator to end of line
  # read through to the last line number
  BAKIFS=$IFS
  IFS=$(echo -en "\n\b")
  exec 3<&0
  exec 0<tmp
  while read last_line
  do
    max_ndx=$last_line
  done
  exec 0<&3
  # restore $IFS which was used to determine what the field separators are
  IFS=$BAKIFS
  echo "pre-processing diff $A_FILE done max_ndx=$max_ndx"
}

Steve