Shell Programming and Scripting

Occasionally I see this in ksh scripts:

somepgm >| someoutputfile

This appears to be redirecting the output of somepgm to the file someoutputfile, and it does do so.

But when you remove the pipe symbol "|" from this command, output simply goes to the screen (stdout). Why do we need the pipe symbol here in order to get output to go to the file?

Is there something special (or wierd) that somepgm must be doing so that one has to use the ">|" to get its output redirected to a file?

ksh man says >| overrides noclobber, so it will not work if the file exists, but I am not sure why you get screen output.

Please show us what you get on screen & the code of somepgm.

Is someoutputfile a normal file ?

Unfortunately, I can't show the source code of somepgm - I do not have it. But one example of somepgm is a data file utility which reformats a binary file into a flat text file. When this utility is run without the ">|" but rather only the ">" redirector, the flat text is output to the screen instead of to the somedata file.

I'm wondering if the source code would reveal that somepgm is looking at the command line parameters, parsing them, and in the absence of ">|" as a "valid" parameter (or is watching for a noclobber exception?), puts the output on stdout?

Can a C/C++ pgm "see" the command line entries, such as redirectors?

No, the > or >| is done by ksh before it starts somepgm, and if it fails, it does not run somepgm, it deals with it as a script failure.

If there is a pipe and a goofy program upstream, it could divert to stdout/fd=1or /dev/tty when the pipe dies unexpectedly.

Does your system have tusc or truss? It provides more information that you can imagine unless you have used it.

Could you write a somepgm shell that simulate the described behaviour ?
so that :

Code:

# somepgm >| someouputfile
# cat someouputfile
Hello World
# somepgm > someouputfile
Hello World
#

I'll try that - should be able to do it tomorrow - have a little bit of a lax schedule then.