Pipes support in Unix shell

Shell support for pipes exists on three levels:

1. Internal functions can read from pipe and write to pipe: in Korn shell and derivatives such as bash internal shell functions can serve as pipe input generators as well as pipe output recipients.

2. Loops can get input from pipes and write to pipe. Loops connected to a pipe  are unique to shell. They are very powerful, albeit little known and underappreciated constructs. The key idea is to allow feeding output of the pipe to the loop or feed a sequence generated by a loop to the pipe.

3. Coprocessing mechanism

• Coprocess facility The Korn shell, or POSIX shell, allows you to run one or more commands as background processes. These commands, run from within a shell script, are called coprocesses.
• Redirection of coprocess input and output The standard input and output of a coprocess is reassigned to a numbered file descriptor by using I/O redirection.

4. Process substitution   Process substitution is a  special case of pipe that allows the input or output of a command to appear as a file. Under the hood, process substitution works by creating a named pipe. The command is substituted in-line and can be use as a parameter which normally is a file. This allows programs that normally only accept files to directly read from or write to another program. Classic case is differing the snapshort and existing status of some command

diff  netstat_snapsort121004.txt <(netstat -rn)

Let's assume that we need to find all files that contain string "#!/bin/bash"

cd/ /usr/bin
ls | while read file
do
   echo "Executing grep '#!/bin/bash' $file"
   grep '#!/bin/bash' $file
done

Here we use the ls command to generate the list of the file names and this list it piped into a loop. In a loop we echo command and execute it.

In another example from O'Reilly "Learning Korn Shell" (first edition). Here we will pipe awk output into the loop. This is a  function that, given a pathname as argument, prints its equivalent in tilde notation if possible:

function tildize {
    if [[ $1 = $HOME* ]]; then
        print "\~/${1#$HOME}"
        return 0
    fi
    awk '{FS=":"; print $1, $6}' /etc/passwd | while read user homedir; do
        if [[ $homedir != / && $1 = ${homedir}?(/*) ]]; then
           print "\~$user/${1#$homedir}"
           return 0
        fi
    done
    print "$1"
    return 1
}

Loop can also serve as a source to input for the pipe. For example

{ while read line'?adc> '; do
      print "$(alg2rpn $line)"
  done 
} | dc

As an example; assume that you want to go through all C files of a directory and, if they are readable to you, convert the filenames to contain lowercase letters only (this example may be a little contrived). We can do it it in slightly different ways.

The first script calls tr inside the the for-loop:

#!/bin/sh
for x in *.c
do
  [ -r $x ] && echo $x | tr 'A-Z' 'a-z'
done

#!/bin/sh
for x in *.c
do
  [ -r $x ] && echo $x 
done | tr 'A-Z' 'a-z'

NEWS CONTENTS

• 200305 : Pipe output to function - Bash - The UNIX and Linux Forums ( Pipe output to function - Bash - The UNIX and Linux Forums, )
• 200305 : Process Substitution ( Process Substitution, )

Old News ;-)

Pipe output to function - Bash - The UNIX and Linux Forums

lfile=/var/tmp/$$.tmp

# function read input from stdin and write output to the stdout
# caller must take care about where come stdin and where go stdout
log() 
{
  while read data
  do
      echo "[$(date +"%D %T")] $data" 
  done
}

# how to use this function ?
# - input is pipe and ouput is file
somecmd | log  >> $lfile
# - input is file and output is  stdout
log < somefile
# - input is stdin = if not defined=keyboard
log
# input is pipe and output pipe
somecmd | log | sort

Process Substitution

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
# 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 -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.

Etc

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Last modified: March 12, 2019