Process substitution is the counterpart to command substitution. Command substitution sets a variable to the result of a command, as in dir_contents=`ls -al` or xref=$( grep word datafile). Process substitution feeds the output of a process to another process (in other words, it sends the results of a command to another command).
These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process within parentheses to another process. 
There is no space between the the "<" or ">" and the parentheses. Space there would give an error message.
bash$ echo >(true) /dev/fd/63 bash$ echo <(true) /dev/fd/63
1 cat <(ls -l) 2 # Same as ls -l | cat 3 4 sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin) 5 # Lists all the files in the 3 main 'bin' directories, and sorts by filename. 6 # Note that three (count 'em) distinct commands are fed to 'sort'. 7 8 9 diff <(command1) <(command2) # Gives difference in command output. 10 11 tar cf >(bzip2 -c > file.tar.bz2) $directory_name 12 # Calls "tar cf /dev/fd/?? $directory_name", and "bzip2 -c > file.tar.bz2". 13 # 14 # Because of the /dev/fd/<n> system feature, 15 # the pipe between both commands does not need to be named. 16 # 17 # This can be emulated. 18 # 19 bzip2 -c < pipe > file.tar.bz2& 20 tar cf pipe $directory_name 21 rm pipe 22 # or 23 exec 3>&1 24 tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&- 25 exec 3>&- 26 27 28 # Thanks, S.C.
A reader of this document sent in the following interesting example of process substitution.
1 # Script fragment taken from SuSE distribution: 2 3 while read des what mask iface; do 4 # Some commands ... 5 done < <(route -n) 6 7 8 # To test it, let's make it do something. 9 while read des what mask iface; do 10 echo $des $what $mask $iface 11 done < <(route -n) 12 13 # Output: 14 # Kernel IP routing table 15 # Destination Gateway Genmask Flags Metric Ref Use Iface 16 # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo 17 18 19 # As S.C. points out, an easier-to-understand equivalent is: 20 route -n | 21 while read des what mask iface; do # Variables set from output of pipe. 22 echo $des $what $mask $iface 23 done # Same output as above.
This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in process substitution.