How to keep overwriting the contents of a file instead of appending to it

Question

How do I clear the contents of a file before writing to it? For example:

echo one > filename.tmp
# filename.tmp now contains "one"

echo two > filename.tmp
# filename.tmp should now contain "two", not "one" and "two"

My goal is:

1. Start a listener

   $ nc -l 7007 > /var/tmp/test.log
   

2. Send some data

   $ telnet localhost 7007
   hi
   second_word
   

3. Test the file

   $ cat /var/tmp/test.log
   second_word
   

I don't want "hi" to show up in the log; I want "second_word" to have replaced it

Answer 1

You need a separate program to clear and write the new file since nc doesn't offer that option.

nc -l 7007 | while true; do
    while read line; do
      echo "$line" > /tmp/test
    done
done

You can save everything after the pipe in a separate script that accepts a file path.

save-last-line.sh

while true; do
    while read line; do
        echo "$line" > $1
    done
done

Then it's simply:

nc -l 7007 | save-last-line.sh /var/tmp/test.log

You'll want to add checks to make sure $1 is writable and show usage when $1 isn't specified.

Answer 2

command > /path/to/file will clear the file and write the output of command to it.

When you don't want to clear the file, it's command >> /path/to/file.

One thing to be aware of is the noclobber option in your shell. It will prevent you from clearing files with the > operator. You can override the option by using >! instead, or you can unset it with set +o noclobber.

Try this at your command line:

# echo "Hello, " > /tmp/test
# cat /tmp/test
  Hello
# echo "U&L" > /tmp/test
# cat /tmp/test
  U&L
# echo "Hello," > /tmp/test
# echo "U&L" >> /tmp/test
# cat /tmp/test
  Hello,
  U&L
#