Interesting Scripting Question - Stdout and File Recreation

[Ian Dennison_1]

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

[Justo Exposito · Accepted solution]

Hi Ian,

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

Works Fine!!

Regards,

Justo.

[Thierry Poels_1]

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.

[Thierry Poels_1]

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

[Ian Dennison_1]

Magic answers both! Thanks gentlemen, much appreciated!

Ian

[Justo Exposito]

Hi Again,

You are welcome Mister.

Best Regards,

Justo.

[John Palmer]

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

[Ian Dennison_1]

John,

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

Ian