Why piping input to "read" only works when fed into "while read ..." construct?
bash, pipe, piping, while-loop
Solution
How to do a loop against stdin and get result stored in a variable
Under bash (and other shell also), when you pipe something to another command via `|`, you will implicitly create a fork, a subshell that is a child of current session. The subshell can't affect current session's environment.
So this:
TOTAL=0
printf "%s %s\n" 9 4 3 1 77 2 25 12 226 664 |
while read A B;do
((TOTAL+=A-B))
printf "%3d - %3d = %4d -> TOTAL= %4d\n" $A $B $[A-B] $TOTAL
done
echo final total: $TOTAL
won't give expected result! :
9 - 4 = 5 -> TOTAL= 5
3 - 1 = 2 -> TOTAL= 7
77 - 2 = 75 -> TOTAL= 82
25 - 12 = 13 -> TOTAL= 95
226 - 664 = -438 -> TOTAL= -343
echo final total: $TOTAL
final total: 0
Where computed TOTAL could'nt be reused in main script.
Inverting the fork
By using bash Process Substitution, Here Documents or Here Strings, you could inverse the fork:
Here strings
read A B <<<"first second"
echo $A
first
echo $B
second
Here Documents
while read A B;do
echo $A-$B
C=$A-$B
done << eodoc
first second
third fourth
eodoc
first-second
third-fourth
outside of the loop:
echo : $C
: third-fourth
Here Commands
TOTAL=0
while read A B;do
((TOTAL+=A-B))
printf "%3d - %3d = %4d -> TOTAL= %4d\n" $A $B $[A-B] $TOTAL
done < <(
printf "%s %s\n" 9 4 3 1 77 2 25 12 226 664
)
9 - 4 = 5 -> TOTAL= 5
3 - 1 = 2 -> TOTAL= 7
77 - 2 = 75 -> TOTAL= 82
25 - 12 = 13 -> TOTAL= 95
226 - 664 = -438 -> TOTAL= -343
# and finally out of loop:
echo $TOTAL
-343
Now you could use `$TOTAL` in your main script.
Piping to a command list
But for working only against stdin, you may create a kind of script into the fork:
printf "%s %s\n" 9 4 3 1 77 2 25 12 226 664 | {
TOTAL=0
while read A B;do
((TOTAL+=A-B))
printf "%3d - %3d = %4d -> TOTAL= %4d\n" $A $B $[A-B] $TOTAL
done
echo "Out of the loop total:" $TOTAL
}
Will give:
9 - 4 = 5 -> TOTAL= 5
3 - 1 = 2 -> TOTAL= 7
77 - 2 = 75 -> TOTAL= 82
25 - 12 = 13 -> TOTAL= 95
226 - 664 = -438 -> TOTAL= -343
Out of the loop total: -343
Note: `$TOTAL` could not be used in main script (after last right curly bracket `}` ).
Using lastpipe bash option
As @CharlesDuffy correctly pointed out, there is a bash option used to change this behaviour. But for this, we have to first disable job control:
shopt -s lastpipe # Set *lastpipe* option
set +m # Disabling job control
TOTAL=0
printf "%s %s\n" 9 4 3 1 77 2 25 12 226 664 |
while read A B;do
((TOTAL+=A-B))
printf "%3d - %3d = %4d -> TOTAL= %4d\n" $A $B $[A-B] $TOTAL
done
9 - 4 = 5 -> TOTAL= -338
3 - 1 = 2 -> TOTAL= -336
77 - 2 = 75 -> TOTAL= -261
25 - 12 = 13 -> TOTAL= -248
226 - 664 = -438 -> TOTAL= -686
echo final total: $TOTAL
-343
This will work, but I (personally) don't like this because this is not standard and won't help to make script readable. Also disabling job control seem expensive for accessing this behaviour.
Note: Job control is enabled by default only in interactive sessions. So `set +m` is not required in normal scripts.
So forgotten `set +m` in a script would create different behaviours if run in a console or if run in a script. This will not going to make this easy to understand or to debug...
Problem
I've been trying to read input into environment variables from program output like this: ``` echo first second | read A B ; echo $A-$B ``` And the result is: ``` - ``` Both A and B are always empty. I read about bash executing piped commands in sub-shell and that basically preventing one from piping input to read. However, the following: ``` echo first second | while read A B ; do echo $A-$B ; done ``` Seems to work, the result is: ``` first-second ``` Can someone please explain what is the logic here? Is it that the commands inside the `while` ... `done` construct are actually executed in the same shell as `echo` and not in a sub-shell?