Buffering in the UNIX shells.

[Leslie Chaim]

Hello,

In my case its KSH.

Does anyone know if it's possible to turn off buffering in the KSH? I am running a batch process in the background that writes to a file, I would like to monitor this file with tail -f as the process runs.

Thanks

[Antoanetta Naghiu]

I am not sure I properly understand your question, but I guess you can try to use tee (see man tee), or to re-direct the output to a file... and use as you said tail -f.
Depends on type of apps as well.

[Rick Garland]

Doing the tail -f command onto a file (a logfile) as the process runs does not require any tuning or buffer modifications.

If you have a process running in the background that is writing to a logfile, use tail -f on the logfile and the screen will scroll as activity is written to the logfile.

[John Palmer]

How are you writing to the file? Are you doing shell print (or echo) commands?

[James R. Ferguson]

Leslie:

The man pages for tail provide a few interesting comments:

Under a discussion of the -f option..."tail enters an endless loop wherein it sleeps for one second then attempts to read and copy further records from the input file)."

This might suggest that you will not see characters until a newline character is passed (?).

Also: "Tails relative to end-of-file are stored in a 20-Kbyte buffer, and thus are limited in length. Therefore, be wary of the results when piping output from other commands into tail."

Hopefully this provides some insight (?).

...JRF...

[Leslie Chaim]

Well here is some more clarification.

Thanks Antoinette you point out it depends on the apps used. Let me rephrase it:

Is it possible from KSH (or in UNIX) to have the effect of the C function call: setbuf(stdout, NULL)? Within the application itself, buffering is not disabled; I would like to disable it for monitory testing purposes without modifying my application.

Thanks,

Leslie

[Antoanetta Naghiu]

I am not aware about, but does C or other languages external intervention while running?
Did you try to use nobuffering in the application and eventually at compillation time use buffering, debug ...

[Tracey]

tail -f DOES wait until it receives a carriage return before it displays any new characters.

[Leslie Chaim]

Yes I'm aware tail -f waits for the newline. That is OK.
On the other hand, if I do modify my application with a call to setbuf() I get the desired results, even with tail -f.

I would like to avoid the modification of the application by simply doing...
Well that's my question.

Thanks,

Leslie

[James R. Ferguson]

Leslie:

I don't thing you can avoid the modification to your application. Seems like 'tail' is written to wait for the newline (as the man page material I high-lighted above states), and you're "stuck" with that as its implementation.

...JRF...

[John Palmer]

I can't think of a way that the shell can influence your program other than providing something in the environment or a different argument. It would then be up to your program to make the required call to setbuf, there are no other means of overriding buffer behaviour that I am aware of.