pass stdout as file name for command line util?

I’m working with a command line utility that requires passing the name of a file to write output to, e.g.

foo -o output.txt

The only thing it writes to stdout is a message that indicates that it ran successfully. I’d like to be able to pipe everything that is written to output.txt to another command line utility. My motivation is that output.txt will end up being a 40 GB file that I don’t need to keep, and I’d rather pipe the streams than work on massive files in a stepwise manner.

Is there any way in this scenario to pipe the real output (i.e. output.txt) to another command? Can I somehow magically pass stdout as the file argument?

Answer

Solution 1: Using process substitution

The most convenient way of doing this is by using process substitution. In bash the syntax looks as follows:

foo -o >(other_command)

(Note that this is a bashism. There’s similar solutions for other shells, but bottom line is that it’s not portable.)

Solution 2: Using named pipes explicitly

You can do the above explicitly / manually as follows:

1. Create a named pipe using the mkfifo command.

   mkfifo my_buf
   

2. Launch your other command with that file as input

   other_command < my_buf
   

3. Execute foo and let it write it’s output to my_buf

   foo -o my_buf
   

Solution 3: Using /dev/stdout

You can also use the device file /dev/stdout as follows

foo -o /dev/stdout | other_command