A sed command to swap first and last character of each line
You need to escape the +,
sed 's/^\(.\)\(.\+\)\(.\)$/\3\2\1/' input.txt
If you like to try some other, here is a gnu awk version
awk '{a=$1;$1=$NF;$NF=a}1' FS= OFS= input.txtThis sets a to the first character, then sets first to last and last to a
It needs gnu awk, since settings FS to nothing is not in standard awk