I'm looking for a way to remove one specific line from a bunch of files, but only if it occurs more than once in that file. Other lines should be kept, even if they are duplicates.
For example, a file like this where I would like to remove the duplicates of AAA
AAA
BBB
AAA
BBB
CCC
should become
AAA
BBB
BBB
CCC
I guess I should use sed but I have no idea how to write the command.
With GNU sed:
sed '0,/^AAA$/b;//d'
That is, let everything through (b branches off like a continue) up to the first AAA (from the 0th line (that is even before the first line) and the first one matching /^AAA$/ (which could be the first line)), and then for the remaining lines, delete every occurrence of AAA (an empty // pattern reuses the last pattern).
GNU sed is needed for the 0 address (and the ability to have other commands after the b one in the same expression, though that could be easily worked around in other implementations by using two -e expressions)
With awk:
awk '$0 != "AAA" || !n++'
(or for a regexp pattern: awk '!/^AAA$/ || !n++')
a shorthand for:
awk '! (&0 == "AAA" && count > 0) {print; count++}'
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