Using Awk or Sed to tack on a statement at the end of a specific line

Question

I have a file I named poscar1.cif, and I would like to insert the contents of a variable at a specific line in this file.

For example, line 24

Answer 1

You can use sed to do it, depending on your answer to dawg's question as

sed -i -e '24s/$/5.3827/' poscar1.cif

or if it's the pattern

sed -i -e '/_cell_length_a/s/$/5.3827/' poscar1.cif

The first goes to the line with the given number, the later will apply on any line that matches the pattern in the first set of slashes. In either case it will then "replace" the end of the line with the value between the final two slashes.