How do I open a pipe both to and from a command? For example, open(P, " cmd ") doesn't seem to work.
A1:
This task can actually be quite complicated because reading and writing from the same process can cause deadlock . At this point, your program is expecting cmd to print something and is waiting for data with <P> . Meanwhile, because of some snafu, cmd is actually waiting for your program to print something with print P "..." . In fact, if you have warnings enabled, Perl will inform you with this message: Can't do bidirectional pipe .
If you're prepared for this kind of problem, the IPC::Open2 module will allow you to open a bidirectional pipe. Modules will be discussed in Hour 14, "Using Modules."
Q2:
The code $a=system("cmd") didn't capture the output of cmd in $a as I expected. Why not?
A2:
You're confusing system with backticks ( `` ). The system function doesn't capture cmd 's output. What you probably wanted was $a=`cmd` .
Q3:
When I run external programs under Unix with backticks ( `` ), the error messages are not captured. Why not?
A3:
Because all Unix programs ”including Perl ”have two output file descriptors: STDOUT and STDERR . The STDOUT file descriptor is for normal program output. The STDERR file descriptor is reserved for error messages. Backticks ”and open with pipes ”capture only STDOUT . The short answer is to use the shell to redirect STDOUT into STDERR and then run your command as follows :
$a=`cmd 2>&1`; # run "cmd", capturing output and errors
The Perl FAQ has a lengthy explanation of this and many other techniques for capturing a command's errors. Type perldoc perlfaq8 for the appropriate section of the FAQ.
