XXX Chapter 23. Process Substitution¶

   #!/bin/bash
   # wr-ps.bash: while-read loop with process substitution.

   # This example contributed by Tomas Pospisek.
   # (Heavily edited by the ABS Guide author.)

   echo

   echo "random input"while read i
   do
     global=3D": Not available outside the loop."
     # ... because it runs in a subshell.
   done

   echo "\$global (from outside the subprocess) = $global"
   # $global (from outside the subprocess) =

   echo; echo "--"; echo

   while read i
   do
     echo $i
     global=3D": Available outside the loop."
     # ... because it does NOT run in a subshell.
   done < <( echo "random input" )
   #    ^ ^

   echo "\$global (using process substitution) = $global"
   # Random input
   # $global (using process substitution) = 3D: Available outside the loop.


   echo; echo "##########"; echo



   # And likewise . . .

   declare -a inloop
   index=0
   cat $0while read line
   do
     inloop[$index]="$line"
     ((index++))
     # It runs in a subshell, so ...
   done
   echo "OUTPUT = "
   echo ${inloop[*]}           # ... nothing echoes.


   echo; echo "--"; echo


   declare -a outloop
   index=0
   while read line
   do
     outloop[$index]="$line"
     ((index++))
     # It does NOT run in a subshell, so ...
   done < <( cat $0 )
   echo "OUTPUT = "
   echo ${outloop[*]}          # ... the entire script echoes.

   exit $?




This is a similar example.

#!/bin/bash
# psub.bash

# As inspired by Diego Molina (thanks!).

declare -a array0
while read
do
  array0[${#array0[@]}]="$REPLY"
done < <( sed -e 's/bash/CRASH-BANG!/' $0grep bin | awk '{print $1}' )
#  Sets the default 'read' variable, $REPLY, by process substitution,
#+ then copies it into an array.

echo "${array0[@]}"

exit $?

# ====================================== #

bash psub.bash

#!/bin/CRASH-BANG! done #!/bin/CRASH-BANG!

# 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
# 127.0.0.0 0.0.0.0 255.0.0.0 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 -nwhile 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.