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Process Substitution in Shell


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It is also possible to pipe the output of other processes into programs which read only from a file, not from the standard (terminal) input. You just have to substitute for the file name the expression "<( command; )". This also allows you to use the output of several commands simultaneously as input for another program. Essentially this provides joining of pipes for Unix. For instance you can compare the contents of two directories by typing:

diff <( ls dir1; ) <( ls dir2; )

We have just learnt how to connect input and output channels of two or more independent programs. However, sometimes a program has to work on the file names output by a different program, ie its command line ought to contain the output of the other program. This is achieved by putting the second program in the first's command line, enclosed in backticks "`". Alternatively, "$(...)" can be used.

To determine the path you can use which command,  which gives the full path of an executable. For instance,

ls -l `which sh`
lists details of the executable of the sh shell. (In Linux it is just a symbolical link to bash.) Similar constructs allow it to apply programs to executables of which you don't know the path. Another application of this feature is to apply an operation to a list of files in a text file. I do this for my backups, like this:
tar cfz backup.tgz `cat backupfiles`
The program tar creates an archive file containing all the files and directories given on its command line after the archive name. cat just writes the contents of a file on the terminal, or in this case into tar's command line. So tar will archive all the files listed in "backupfiles". The file names may even contain wildcard characters since the shell performs file globbing on the output of cat before passing the result on to tar.


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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.

Command list enclosed within parentheses


Process substitution uses /dev/fd/<n> files to send the results of the process(es) 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.)

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
        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

Using process substitution to compare the contents of two directories (to see which filenames are in one, but not the other):

diff <(ls $first_directory) <(ls $second_directory)

Some other usages and uses of process substitution:

read -a list < <( od -Ad -w24 -t u2 /dev/urandom )
#  Read a list of random numbers from /dev/urandom,
#+ process with "od"
#+ and feed into stdin of "read" . . .

#  From "insertion-sort.bash" example script.
#  Courtesy of JuanJo Ciarlante.
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) $directory_name
# Calls "tar cf /dev/fd/?? $directory_name", 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 $directory_name
rm pipe
#        or
exec 3>&1
tar cf /dev/fd/4 $directory_name 4>&1 >&3 3>&- | bzip2 -c > file.tar.bz2 3>&-
exec 3>&-

# Thanks, Stéphane Chazelas

A reader 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)  
#    ^ ^  First < is redirection, second is process substitution.

# 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 Stéphane Chazelas 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  #  This yields the same output as above.
        #  However, as Ulrich Gayer points out . . .
        #+ this simplified equivalent uses a subshell for the while loop,
        #+ and therefore the variables disappear when the pipe terminates.
# --------------------------------------------------------------#
#  However, Filip Moritz comments that there is a subtle difference
#+ between the above two examples, as the following shows.

route -n | while read x; do ((y++)); done
echo $y # $y is still unset

while read x; do ((y++)); done < <(route -n)
echo $y # $y has the number of lines of output of route -n

More generally spoken
: | x=x
# seems to start a subshell like
: | ( x=x )
# while
x=x < <(:)
# does not

# This is useful, when parsing csv and the like.
# That is, in effect, what the original SuSE code fragment does.


[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.


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