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).

Command substitution template

command within parentheses



These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process within parentheses to another process. [1]


There is no space between the the "<" or ">" and the parentheses. Space there would give an error message.

bash$ echo >(true)

bash$ echo <(true)
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.)

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'.

diff <(command1) <(command2)    # Gives difference in command output.

tar cf >(bzip2 -c > file.tar.bz2) dir
# Calls "tar cf /dev/fd/?? dir", and "bzip2 -c > file.tar.bz2".
# Because of the /dev/fd/<n> system feature,
# the pipe between both commands does not need to be named.
# This can be emulated.
bzip2 -c < pipe > file.tar.bz2&
tar cf pipe dir
rm pipe
#        or
exec 3>&1
tar cf /dev/fd/4 dir 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
exec 3>&-

# Thanks, S.C.

A reader of this document sent in the following interesting example of process substitution.

# Script fragment taken from SuSE distribution:

while read  des what mask iface; do
# Some commands ...
done < <(route -n)  

# To test it, let's make it do something.
while read  des what mask iface; do
  echo $des $what $mask $iface
done < <(route -n)  

# Output:
# Kernel IP routing table
# Destination Gateway Genmask Flags Metric Ref Use Iface
# U 0 0 0 lo

# As S.C. points out, an easier-to-understand equivalent is:
route -n |
  while read des what mask iface; do   # Variables set from output of pipe.
    echo $des $what $mask $iface
  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.