Shell Programming and Scripting

A not-too-ugly solution to this classic problem...

Code:

#!/bin/bash

mknod fifo p
 
(
       echo 'value' > fifo &
)
 
VARIABLE=`cat fifo`
rm fifo

Whatdcha think? Good? No good?

What happens if you have more than 1 instance if this script running?

I would suggest generating a temp filename for the pipe and passing it to the child.

Excellent point. Then, maybe something like:

Code:

#!/bin/bash

TMPFIFO="/tmp/fifo.$$"
 
mknod $TMPFIFO p
 
(
       echo 'value' > $TMPFIFO &
)
 
VARIABLE=`cat $TMPFIFO`
rm $TMPFIFO

Thank you.

Yes or

Code:

#!/bin/bash

PIPE="/tmp/pipe.$$"
mkfifo $PIPE

(
echo 'value' > $PIPE &
)

VARIABLE=`cat $PIPE`
echo $VARIABLE
rm $PIPE

Code:

#!/bin/bash

TMPF="/tmp/tmpf.$$"
mktemp $TMPF >/dev/null

(
echo 'value' > $TMPF
)

VARIABLE=`cat $TMPF`
echo $VARIABLE
rm $TMPF

I would use mkfifo instead of mknod. You do not need the ( ) in this case.. If you have more than one write statement, it is better to use exec, otherwise VARIABLE=`cat $TMPFIFO` goes on after the first command writes an EOF and the sub process will not finish.

Code:

FIFO="/tmp/fifo.$$"
if ! [ -p "$FIFO" ]; then
  mkfifo "$FIFO"
fi
{ exec >"$FIFO"; sleep 2; ps -f ;echo "hello from $FIFO" ;} &
var=$(< "$FIFO")
rm "$FIFO"
echo "$var"

In ksh93 you can use coprocesses:

Code:

#!/bin/ksh
{ sleep 2; ps -f ;echo "hello from coproc" ;} |&
var=$(<&p)
echo "$var"

---------- Post updated at 11:44 ---------- Previous update was at 10:45 ----------

bash 4:

Code:

#!/bin/bash
coproc TEST { sleep 2; ps -f ;echo "hello from coproc" ;}
var=$(cat <&${TEST[0]})
echo "$var"

I agree with your mkfifo sugestion, but in regards to my usage of (), let me explain:

Suppose you want to extract a fixed amount of data, say, 20 bytes, from a device file which spits data out in asynchronous patterns, such as /dev/mouse.

The device file/dev/mouse will sometimes be spitting bytes out, and will sometimes not, depending on how the user moves the cursor around, so you really can't know when exactly you're going to get your 20 bytes from it.

So what do you do? You run dd to get 20 bytes from /dev/mouse, like so:

Code:

dd if=/dev/mouse bs=1 count=20

But this ofcourse freezes the execution of your script until /dev/mouse has produced the 20 required bytes. What do you do then?, well, you run it in the background:

Code:

dd if=/dev/mouse bs=1 count=20 &

But then, this only allows you to output the 20 bytes to stdout and, where's the fun in that? If you wanted to manipulate the 20 bytes in some way or another, you would need to either:
The disadvantage of using binary temp files is that you waste slow HDD cycles and it just makes your code look plain ugly. So we decide to go with variable storage: it uses RAM instead, it's non intrusive with your filesystem and it doesn't carry all the cumbersome complications of checking for occupied filenames and the such. Thus, we unsuspectingly attempt to:

Code:

VAR=`dd if=/dev/mouse bs=1 count=20` &

Which obviously fails, because & runs your background processes in a child shell, which cannot alter it's parent's environment. At this point you can either give up and use the ugly temp.bin alternative:

Code:

dd if=/dev/mouse bs=1 count=20 of=/tmp/temp.bin.$$ &

Or...

3. Communicate with the parent from your background process using a fifo.

Like so:

Code:

TMPFIFO="/tmp/fifo.$$"
 
mkfifo $TMPFIFO
 
dd if=/dev/mouse bs=1 count=20 of=$TMPFIFO &

echo 'I can do something right after I tell dd to start listening'
echo 'to /dev/mouse, like telling my robotic arm to move the'
echo 'mouse around, or whatever :P!'
  
# This part WILL freeze the scrip though, at least
# until /dev/mouse is done with the 20 bytes.
VARIABLE=`cat $TMPFIFO`
rm $TMPFIFO

echo "Use \$VARIABLE for something... etc, etc: $VARIABLE"

This doesn't mess (so much) with your filesystem, uses only RAM, and is faster. I admit it shares with 1. the flaw of having to check for filenames... I only wish it could be perfect...

Anyway, that is why I needed my children talking back at their parents.

Thank you for your criticism, though, I truly appreciate your taking the time to read my post.

---------- Post updated at 01:06 PM ---------- Previous update was at 12:35 PM ----------

mktemp huh... never heard of it, thanks for the tip!

@vomv1988: Thank you for the explanation. You started an interesting thread, I would not say it is criticism, but rather some hopefully helpful comments. Regarding the parentheses in your original script, I was only referring to them because I think they are superfluous...