UNIX for Dummies Questions & Answers

Hi,

I want to write a script that takes a file and a string as params and adds the string to the middle line of the file. Also, I want to output the results back to the original file passed without using temp files.

I am very much new to UNIX so this is all a little like black magic to me at the moment but heres what I was thinking:

#Get the number of lines in the file
MyVar="`wc -w NewFile | awk '{print $1}'`"

#Get The middle of the file
Middle=$ expr $MyVar / 2

#Add Line to middle of file

????

Here my problems, I think i should be using sed but I am not sure about the syntax.

Another way I thought was to use awk to scan through each line of the file and hold it as an array. Then output the first half of the array to the file, then the input string, then the remaining part of the array. Problem here is that I dont know how to pass a string to an awk script. Is this possible?

Thanks, Chris.

Example working code:

Code:

#!/bin/sh
text="Text to add"
file=data.txt

lines=`wc -l $file | awk '{print $1}'`

middle=`expr $lines / 2`

# If the file has an odd number of lines this script adds the text
# after the middle line. Comment this block out to add before
if [ `expr $lines % 2` -eq 1 ]
then
  middle=`expr $middle + 1`
fi

sed -e "${middle}a $text" $file

Hi Unbeliever,

Thanks for your help, and yes your right about correctly determining the middle of the file.

One thing though, could you please explain the syntax of the sed example. I get most of it:

sed -e "${middle}a $text" $file

-e = editing the original file.
${middle} = the line number

But whats the a afterwards all about?
Thanks, Chris.

My example doesn't actually edit the file directly, it simply prints the original text file with the added line. But to answer your question.

sed -e "${middle}a $text" $file

-e: next argument is the sed command to use
${middle} is of course the line number of the original file to work on
a means append the following text

So all in all it mean append the text '$text' at line $middle in the file $file.

So I am assuming:

sed -e "${middle}a $text" $file > $file

will ammend the file, no?

I keep getting this error when i try to run the script:

sed: -e expression #1, char 3: Extra characters after command

Try this:

Code:

sed ${mid}i'\
Your Text' yourfile