How to insert the 1st arg into the middle of the file

Shell Programming and Scripting
1

I wrote a script like
#!/bin/bash
echo $1 > temp
cat $2 >> temp
mv temp $2

now I have problem appending the above script(only using bash shell) so that it now inserts the first argument into the middle of the file.

I have tried using $(('wc -l < file' / 2 ))
but invain so could any one help.
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2

something like this,

to be in the middle

a=$((`wc -l < filename` / 2))
awk -v var=$a '{ if( NR == var) { print "newline to be added"; print } else { print } }' filename > newf
mv newf filename
3

Try this

a=$((`wc -l < filename` / 2))
sed ''$a' a this is new line' filename
4

when I insert this command and run the sript it give a message stating Syntax error.....is there any other command like head or tail command

5 
sed "${a}a\\
this is new line" filename