Chapter 22. Process Substitution

Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in.

Process substitution feeds the output of a process (or processes) into the stdin of another process.

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 read -a list < <( od -Ad -w24 -t u2 /dev/urandom ) 2 # Read a list of random numbers from /dev/urandom, 3 #+ process with "od" 4 #+ and feed into stdin of "read" . . . 5  6 # From "insertion-sort.bash" example script. 7 # Courtesy of JuanJo Ciarlante.

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, Stéphane Chazelas

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

1 # Script fragment taken from SuSE distribution: 2  3 # --------------------------------------------------------------# 4 while read des what mask iface; do 5 # Some commands ... 6 done < <(route -n) 7 # First < is redirection, second is process substitution. 8  9 # To test it, let's make it do something. 10 while read des what mask iface; do 11  echo $des $what $mask $iface 12 done < <(route -n) 13  14 # Output: 15 # Kernel IP routing table 16 # Destination Gateway Genmask Flags Metric Ref Use Iface 17 # 127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo 18 # --------------------------------------------------------------# 19  20 # As Stéphane Chazelas points out, 21 #+ an easier-to-understand equivalent is: 22 route -n | 23  while read des what mask iface; do # Variables set from output of pipe. 24  echo $des $what $mask $iface 25  done # This yields the same output as above. 26  # However, as Ulrich Gayer points out . . . 27  #+ this simplified equivalent uses a subshell for the while loop, 28  #+ and therefore the variables disappear when the pipe terminates. 29  30 # --------------------------------------------------------------# 31  32 # However, Filip Moritz comments that there is a subtle difference 33 #+ between the above two examples, as the following shows. 34  35 ( 36 route -n | while read x; do ((y++)); done 37 echo $y # $y is still unset 38  39 while read x; do ((y++)); done < <(route -n) 40 echo $y # $y has the number of lines of output of route -n 41 ) 42  43 More generally spoken 44 ( 45 : | x=x 46 # seems to start a subshell like 47 : | ( x=x ) 48 # while 49 x=x < <(:) 50 # does not 51 ) 52  53 # This is useful, when parsing csv and the like. 54 # That is, in effect, what the original SuSE code fragment does.