topic Re: |& - Pipe in Operating System - HP-UX

|& - Pipe

Sundar_7 — Tue, 20 Jul 2004 18:13:24 GMT

I was wondering if anyone could help me understand this |& symbol.

this is from the man page of sh-posix

|& Causes asynchronous execution of the preceding command or
pipeline with a two-way pipe established to the parent
shell. The standard input and output of the spawned command
can be written to and read from by the parent shell using
the -p option of the special commands read and print.

man page doesnt make much sense to me and /sbin/init.d/swconfig seem to use that

# grep "\|\&" /sbin/init.d/swconfig
ls |&
ls |&
cat /var/adm/sw/cleanupfile |&
#

I understand |& will execute the command1 in the background and write to a pipe that can be read using the -p option of read.

I was wondering if you guys are aware of any other use to this syntax or when it can be best used ?

Re: |& - Pipe

Stuart Browne — Wed, 21 Jul 2004 00:08:14 GMT

I can see this pipe being useful for challenge-response type programs where you want to script responses or sequences.

Whilst we do this for a number of bits and pieces using the perl 'open()' pipe, it could just as easily use this.

If you look into where those |& pipes are used in /sbin/init.d/swconfig, you'll see shortly after them what use they are put too:

# If exist, cleanup alternate cmds

if [[ -d /usr/lbin/sw/bin ]]
then
(
typeset first=1
cd /usr/lbin/sw/bin
ls |&
while read -p old_cmd
do
if [[ ${first} = 1 ]]
then
msg star "Removing files from '/usr/lbin/sw/bin'"
first=0
fi
rm -f ${old_cmd}
done
)
fi

It goes through a directory listing of the current directory, and removes them, bypassing the need for shell expansion.

The second entry isn't all that more exciting (identical infact), and the third use makes it just a little testing and stuff, before moving the '/var/adm/sw/cleanupfile' out of the way.

If you had a program of which did question/answer however, the |& starts to be of more use. You send it a question with 'print -p ', then it sends you an answer which you grab using 'read -p VAR'. You can then act on the answer, do actions, then send the next question etc. etc..

Or you could use the '>&p' and '<&p' to output/input from your own custom commands into the backgrounded pipe.

Re: |& - Pipe

Mark Grant — Wed, 21 Jul 2004 02:37:01 GMT

Here's another example that is quite useful.

bc |&
print -p "10 + 10"
read -p answer
echo $answer

Re: |& - Pipe

Laurent Menase — Wed, 21 Jul 2004 03:58:42 GMT

It is a coprocess. You have the possibility to have more than one coprocess, by dupping the pipes to others fds:
example to use a socket in a script:
telnet anothersystem 7 |&

exec 3<&p 4>&p

cat <&3 &
echo hello >&4
exec 4>&-

Re: |& - Pipe

Geoff Wild — Wed, 21 Jul 2004 07:47:13 GMT

You can also use && in a condition statement like this from cron:

0 0 * * * [ -d /oracle/PROD/920_64 ] && /export/APPL/spaceman/cleanarch > /dev/null 2>&1

That checks to see if the directory /oracle/PROD/920_64 exists, and if yes, then executes the cleanarch script...

Rgds...Geoff

Re: |& - Pipe

Sundar_7 — Wed, 21 Jul 2004 17:32:25 GMT

Thanks to all of you.

Sorry Geoff, I think you have misread the question. I was talking about "|&" and not about "&&".