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).

Process substitution template

command within parentheses

>(command)

<(command)

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

Note

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
 	      
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.)

Process substitution can compare the output of two different commands, or even the output of different options to the same command.

 bash$ comm <(ls -l) <(ls -al)
 total 12
-rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0
-rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2
-rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh
        total 20
        drwxrwxrwx    2 bozo bozo     4096 Mar 10 18:10 .
        drwx------   72 bozo bozo     4096 Mar 10 17:58 ..
        -rw-rw-r--    1 bozo bozo       78 Mar 10 12:58 File0
        -rw-rw-r--    1 bozo bozo       42 Mar 10 12:58 File2
        -rw-rw-r--    1 bozo bozo      103 Mar 10 12:58 t2.sh

Using process substitution to compare the contents of two directories (to see which filenames are in one, but not the other):
   1 diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

   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, St´phane Chazelas

A reader 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 
  20 # As Stéphane Chazelas points out, an easier-to-understand equivalent is:
  21 route -n |
  22   while read des what mask iface; do   # Variables set from output of pipe.
  23     echo $des $what $mask $iface
  24   done  #  This yields the same output as above.
  25         #  However, as Ulrich Gayer points out . . .
  26         #+ this simplified equivalent uses a subshell for the while loop,
  27         #+ and therefore the variables disappear when the pipe terminates.
  28 	
  29 
  30 	
  31 #  However, Filip Moritz comments that there is a subtle difference
  32 #+ between the above two examples, as the following shows.
  33 
  34 (
  35 route -n | while read x; do ((y++)); done
  36 echo $y # $y is still unset
  37 
  38 while read x; do ((y++)); done < <(route -n)
  39 echo $y # $y has the number of lines of output of route -n
  40 )
  41 
  42 More generally spoken
  43 (
  44 : | x=x
  45 # seems to start a subshell like
  46 : | ( x=x )
  47 # while
  48 x=x < <(:)
  49 # does not
  50 )
  51 
  52 # This is useful, when parsing csv and the like.
  53 # That is, in effect, what the original SuSE code fragment does.

Notes

[1]

This has the same effect as a named pipe (temp file), and, in fact, named pipes were at one time used in process substitution.