diffutils: Reversed Patches
10.3.2 Applying Reversed Patches
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Sometimes people run 'diff' with the new file first instead of second.
This creates a diff that is "reversed". To apply such patches, give
'patch' the '--reverse' ('-R') option. 'patch' then attempts to swap
each hunk around before applying it. Rejects come out in the swapped
format.
Often 'patch' can guess that the patch is reversed. If the first
hunk of a patch fails, 'patch' reverses the hunk to see if it can apply
it that way. If it can, 'patch' asks you if you want to have the '-R'
option set; if it can't, 'patch' continues to apply the patch normally.
This method cannot detect a reversed patch if it is a normal diff and
the first command is an append (which should have been a delete) since
appends always succeed, because a null context matches anywhere. But
most patches add or change lines rather than delete them, so most
reversed normal diffs begin with a delete, which fails, and 'patch'
notices.
If you apply a patch that you have already applied, 'patch' thinks it
is a reversed patch and offers to un-apply the patch. This could be
construed as a feature. If you did this inadvertently and you don't
want to un-apply the patch, just answer 'n' to this offer and to the
subsequent "apply anyway" question--or type 'C-c' to kill the 'patch'
process.
Info Catalog
diffutils: Changed White Space
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diffutils: Inexact