Question

I read this example in a book:

 cp /bin/cat proj33
 echo -n x | ./proj33 - pipe33a > pipe33b &
./proj33 <pipe33b >pipe33a &

What do the symbols -, > and < mean?

Answer 1

The > is I/O redirection; it sends the standard output of the command (./proj33) to the file pipe33b.

The < symbol is also I/O redirection; it sends the standard input of the command (./proj33 again) from the file pipe33b.

The - is just an argument. It is often treated by commands as an indication to read standard input instead of a file. In this context, it is likely that proj33 normally requires a file as its first argument, but it reads from the pipe when the argument is -. Sometimes, the - is used to indicate standard output. In extreme cases, you can get one - indicating standard input and one indicating standard output on a single command. With GNU tar, you could have tar -c -f - -T - with the output being written to standard output (-f -) and the list of files to be archived read from standard input (-T -).

If you see --, that is very different; it indicates the ‘end of options’ for the command; anything that follows is not an option, even if it starts with a dash -.