With awk
:
$ awk '{NF=NF+1; $NF=$2}1' file
abc 123 pqr 123
lmn 234 rst 234
jkl 100 mon 100
It increments the number of field in one and sets the last one as the 2nd. Then 1
is a true condition, which is evaluated as the default awk behaviour: {print $0}
.
Or also
awk '{print $0, $2}' file
It prints the full line plus the second field.
Or even shorter, thanks Håkon Hægland!:
awk '{$(NF+1)=$2}1' file