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=$( grep word datafile). Process substitution feeds the output of a process to another process (in other words, it sends the results of a command to another command).
Command substitution template
command within parentheses
>(command)
<(command)
These initiate process substitution. This uses /dev/fd/<n> files to send the results of the process within parentheses to another process. [49]
There is no space between the the "<" or ">" and the parentheses. Space there would give an error message.
bash$ 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.)
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
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Advanced Bash−Scripting Guide
#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
#127.0.0.0 0.0.0.0 255.0.0.0 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 |
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done # Same output |
as above. |
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