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The ed output format consists of one or more hunks of differences. The changes closest to the ends of the files come first so that commands that change the number of lines do not affect how ed interprets line numbers in succeeding commands. ed format hunks look like this:


Because ed uses a single period on a line to indicate the end of input, GNU diff protects lines of changes that contain a single period on a line by writing two periods instead, then writing a subsequent ed command to change the two periods into one. The ed format cannot represent an incomplete line, so if the second file ends in a changed incomplete line, diff reports an error and then pretends that a newline was appended.

There are three types of change commands. Each consists of a line number or comma-separated range of lines in the first file and a single character indicating the kind of change to make. All line numbers are the original line numbers in the file. The types of change commands are:

Add text from the second file after line l in the first file. For example, ‘8a’ means to add the following lines after line 8 of file 1.
Replace the lines in range r in the first file with the following lines. Like a combined add and delete, but more compact. For example, ‘5,7c’ means change lines 5–7 of file 1 to read as the text file 2.
Delete the lines in range r from the first file. For example, ‘5,7d’ means delete lines 5–7 of file 1.