Chapter 22. 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).

bash$ echo >(true) /dev/fd/63 bash$ echo <(true) /dev/fd/63

Bash creates a pipe with two file descriptors, --fIn and fOut--. The stdin of true connects to fOut (dup2(fOut, 0)), then Bash passes a /dev/fd/fIn argument to echo. On systems lacking /dev/fd/<n> files, Bash may use temporary files. (Thanks, S.C.)

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 # This yields the same output as above. 24  # However, as Ulrich Gayer points out . . . 25  #+ this simplified equivalent uses a subshell for the while loop, 26  #+ and therefore the variables disappear when the pipe terminates.