Very Computer

Simple script myscript.sh:

#!/bin/sh
i=1

date
while [ 1 ]; do
   echo "Count - ${i}"
   cat /etc/system
   i=`expr ${i} + 1`
   sleep 5
done

Run it as:

nohup sh myscript.sh > /tmp/myscript.log &

Let it run for a while.  Then run:

grep Count /tmp/myscript.log | wc -l
ls -l /tmp/myscript.log
cat /dev/null /tmp/myscript.log
ls -l /tmp/myscript.log
ls -l /tmp/myscript.log
ls -l /tmp/myscript.log
grep Count /tmp/myscript.log | wc -l

The file size info on my system (Solaris 8) for the /tmp/myscript.log
is zero at first and then goes right back to where it was but a bit
more when the next iteration of the cat command output /etc/system.

Also, the first grep..wc -l command shows how many times the script
has basically looped.  The second grep will show only a couple.  This
shows that the inode is saying the file contains everything is always
did but the grep command is saying it only contains up to some of
point a buffer - maybe the output file descriptor buffer?  I'm
guessing I'm not a programmer

If you change the ">" to a ">>" with your nohup , it works as
expected. The file is zeroed out and the inode info shows it growing
as you would expect it to.

What's the differences between ">" and ">>" that makes this happen?
Running the script with ksh does the same thing.

Looks like a bug to me.

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Sean O'Neill

'>>' means essentially, "Do a seek to the end of the file before writing
                         anything".

So, if you truncate the file to zero length, then >> will seek to the
new "end" position of 0.
The '>' operator does not seek to the end. So the kernel keeps its prior
last 'write' position.
Note: *the kernel*, not the shell.

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