UNIX for Dummies Questions & Answers

You forgot the quotes so we can see if there is any whitespace around those. I would suspect a typo somewhere -- I'm not getting any syntax error from a copy+paste of that script.

yeah...my mistake..it worked... i have one question... u suggested to use read.. can u please tell me how to use "read" command.

Is it hard to delete the last 3 three lines in file1.txt using script... i am not sure whether to use script to solve the problem or use c++ program.

This is not particularly elegant, but at least this demonstrates how to use read.

Code:

# Set IFS to just a newline
IFS='
'
reading=true
while $reading; do

  # Copy file1.txt to a temporary file
  cp file1.txt temp

  for lines in zero one two; do
    if read input; then 
      echo "$input" >>temp
    else
      # Short read -- print a diagnostic to standard error
      echo "$0: reading three lines failed -- abandoning after $lines" >&2
      reading=false
      break
    fi
  done

  # Run external program on temporary file
  externalprogram temp

done <file2.txt

If you'd run head -n 3 inside the loop, with standard input for the loop redirected to come from file2.txt, I suppose that would work, too. The trick is to use redirection to open the file once and then keep on reading without closing the file descriptor. Redirection (with <, or with exec) achieves that for you.

Using a temporary file just seems like better hygiene than continuously butchering the input file, and saves you from having to keep track of how many lines exactly to remove again in case of a short read (what with the possibility of missing newlines at the end and other complications).