topic Re: Interesting Scripting Question - Stdout and File Recreation in Operating System - HP-UX

Interesting Scripting Question - Stdout and File Recreation

Ian Dennison_1 — Mon, 11 Feb 2002 10:45:28 GMT

Pop quiz,...

I know we can recreate a Variable and re-assign the value directly,

eg. export PATH=$PATH":/tmp"

BUT! Can I do the same in one command for a log file?

eg. grep -v "Alpha" /tmp/file1 >/tmp/file1

Or am I resigned to 2 commands?
eg. grep -v "Alpha" /tmp/file1 >/tmp/file2
mv /tmp/file2 /tmp/file1

Share and Enjoy! Ian

Re: Interesting Scripting Question - Stdout and File Recreation

Justo Exposito — Mon, 11 Feb 2002 11:06:42 GMT

Hi Ian,

If you do:
grep -v "Alpha" /tmp/file1 |tee /tmp/file1

Works Fine!!

Regards,

Justo.

Re: Interesting Scripting Question - Stdout and File Recreation

Thierry Poels_1 — Mon, 11 Feb 2002 11:08:16 GMT

Hi,

sorry, but you will indeed need an intermediate file if you use redirection.

If you really do not want an extra file, you could use and ed script:

ed yourfile << [EOT]
,g/searchstring/d
w
q
[EOT]

good luck,
Thierry.

Re: Interesting Scripting Question - Stdout and File Recreation

Thierry Poels_1 — Mon, 11 Feb 2002 11:13:25 GMT

We have a winner !! (read as 10-pointer ;-)
The tee trick indeed does the trick perfectly.

Just add "> /dev/null" to get rid of the output on screen (don't know how big the file is).
grep -v "Alpha" /tmp/file1 |tee /tmp/file1 > /dev/null

regards,
Thierry

Re: Interesting Scripting Question - Stdout and File Recreation

Ian Dennison_1 — Mon, 11 Feb 2002 11:21:09 GMT

Magic answers both! Thanks gentlemen, much appreciated!

Ian

Re: Interesting Scripting Question - Stdout and File Recreation

Justo Exposito — Mon, 11 Feb 2002 11:44:21 GMT

Hi Again,

You are welcome Mister.

Best Regards,

Justo.

Re: Interesting Scripting Question - Stdout and File Recreation

John Palmer — Mon, 11 Feb 2002 12:31:06 GMT

Ian,

Be aware that the tee trick only works for small amounts of data.

tee is useful because you specify the output filename as an argument, if you had to use > then the shell truncates it before any processing begins. However tee is actually reading and writing the same file and at some point the write process will effectively truncate the file and you will lose data.

Some tests that I have done indicate that you will get away with writing between 18 and 24Kb but personally I wouldn't trust it unless you're sure that the input file is very small.

Regards,
John

Re: Interesting Scripting Question - Stdout and File Recreation

Ian Dennison_1 — Mon, 11 Feb 2002 12:36:36 GMT

John,

Thanks for the warning, will definitely check sizings before using this methodology.

Ian