use variable expansion failing in sed for inserting string with 4 tabs in a file

Team,

I need to insert a string at a certain line in a file after certain tabs.

bonus: I also need to skip insertion if it already exists.

cat test.txt

1
2
3
4
5

sed -i '3i\ \t\t\t\t<file branch-rate="0.0">' test.txt

cat test.txt < after above insertion.

1
2
                                <file branch-rate="0.0">
3
4
5

using variable assignment

ln=5

sed -i '${ln}i\ \t\t\t\t<file branch-rate="0.0">' test.txt
sed: -e expression #1, char 4: extra characters after command

You need to use double quotes " so your variables can expand. It is also a good idea to create a backup file while using in-place editing.

You can try this sed

$ sed -i.bak "${ln}i\ \t\t\t\t<file branch-rate=\"0.0\">" test.txt