UNIX for Dummies Questions & Answers

It is not clear what you want.

What do you want to insert?

Where do you want to insert it?

What is $1? Is it text? Is it a filename?

(Note that cat is for concatenating files, not inserting things into them.)

I assume that you do not want it in the middle of the second argument, but in the middle of the file pointed to by the second argument.

If that is the case, you need to find out how many lines are in the file, use head to read that number of lines; insert the text; use tail to get the rest of the file, or use awk.

Code:

lines=$( wc -l < "$2" )
middle=$(( $lines / 2 ))

awk -v mid=$middle -v text="$1" '
   NR == mid { print text }
             { print }' "$2" > tempfile

Using head and tail:

Code:

lines=$( wc -l < "$2" )
middle=$(( $lines / 2 ))

n=0
{
   head -n "$middle" "$2"
   printf "%s\n" "$1"
   tail -n "$(( $middle + 1 ))" "$2"
} > tempfile

Without any external commands, it will be very slow on a long file:

Code:

lines=$( wc -l < "$2" )
middle=$(( $lines / 2 ))

n=0
while IFS= read -r line
do
   n=$(( $n + 1 ))
   printf "%s\n" "$line"
   [ $n -eq $middle ] && printf "%s\n" "$1"
done < "$2" > tempfile

Somewhat faster (it uses cat for the second half of the file):

Code:

while IFS= read -r line
do
   n=$(( $n + 1 ))
   printf "%s\n" "$line"
   [ $n -eq $middle ] && { printf "%s\n" "$1"; cat; exit; }
done < "$2"