usando o awk -
echo "{"user":"abcd","Name":"mike","middlename":"B","Lastname":null,"userid":"1234","Birthdate":"01-Jan-1800"}" | awk 'BEGIN {FS=","} {for(i=1;i<=NF;i++) {split($i,a,":"); val=a[2]; gsub("{|}","",val); print val}}'
abcd
mike
B
null
1234