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.)

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, Stepane 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 Stephane 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.