3.18. Process Substitution

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).

(command)>
<(command)
These initiate process substitution. This uses a named pipe (temp file) to send the results of the process within parentheses to another process.

Note: There are no spaces between the parentheses and the "<" or ">". Space there would simply cause redirection from a subshell, rather than process substitution.

cat <(ls -l) # Same as ls -l | cat sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin) # Lists all the files in the 3 main 'bin' directories, and sorts by filename. # Note that three (count 'em) distinct commands are fed to 'sort'.