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$ cat a
1
wm.swappiness=99
3
4

$ cat b
1
2
3
4

$ awk '/^wm.swappiness/ {print "replacement"; found=1} !/^wm.swappiness/ {print $0} END {if (!found) {print "appended" }}' a
1
replacement
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4

$ awk '/^wm.swappiness/ {print "replacement"; found=1} !/^wm.swappiness/ {print $0} END {if (!found) {print "appended" }}' b
1
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4
appended