Saying, I have two distinct functions with the same goal (counting lines containing a specific pattern in a file MyFile).
To perform that operation, I used a "while loop" with two different syntax ("grep" command would be much more better in that case but this is not the concern in our case) !:
(1) cat MyFile | while read -r line;do; ... ;done;
(2) while read -r line; do; ...;done < MyFile;
Inside the while loop, I used a variable var to store the result.
My questions:
(1) is that the expected behaviour to not be able to get the value of var outside the while for the first syntax ?
(2) what kind of system's difference between both syntax ?
In the first example, pipe | creates a subshell outside your current shell to execute a loop.
After the while loop is done, var is 0 in your current shell, since the increment was done in subshell.
The second example increments the var in current shell, so everything is known to the program.
Peasant, I do not think a subshell is created when using that syntax: in fact, I show the PID for both functions and I do not notice a specific PID for "while loop" with the first function:
Scrutinizer, I applied your suggestion (illustration below with fnct_dae_V3) but, in that case, it seems that I can not keep the value of the variable with that syntax:
So, I allow myself to ask you one more time: how is it possible to lose the value of the variable if the whole execution use a unique PID (cf. fnct_dae_V1 and I assume fnct_dae_V3) ?
You did not apply Scrutinizer's suggestion. And, a subshell and a separate process are two distinct things. And, again, the response is exactly the same. Using cat and a pipeline to feed input into your while read loop places the while read loop in a subshell (when you are using bash and many other shells). Get rid of the unneeded cat (which creates an extra process, drains system resources, and slows down your script without serving any useful purpose) and the subshell, and it will work just fine. Or, if you use a Korn shell ( ksh ) instead of the shell you're currently using, the last stage of a pipeline will be run in the current shell execution environment instead of in a subshell environment. (But, I haven't checked to see if you are using any other extensions to the POSIX standard shell requirements that might be treated differently by ksh than they are in the shell you're currently using.)
In the cat ... | while read you should see another pid while doing ps -ef | grep <your script pid> .
Another pid will be subshell pid with your script PID as parent.
For instance :
echo $$
var=0
echo "junk" | while read line
do
sleep 100
((var++))
done
echo $var
Yields :
user@machine:~/posao$ ./sshell.sh
2775
user@machine:~/work$ ps -ef | grep 2775
user 2775 2488 0 05:46 pts/0 00:00:00 bash
user 2777 2775 0 05:46 pts/0 00:00:00 bash # var will be incremented in shell with PID 2777, when it completes it will exit, parent (our script with PID 2775) will not be aware of var increment.
user 2782 2528 0 05:46 pts/1 00:00:00 grep 2775
user@machine:~/work$ ps -ef | grep sleep
user 2778 2777 0 05:46 pts/0 00:00:00 sleep 100
user 2784 2528 0 05:46 pts/1 00:00:00 grep sleep
Other example :
echo $$
var=0
while read line
do
sleep 100
((var++))
done < junk.txt
echo $var
Yields :
user@machine:~/work$ ./shell.sh
2755
user@machine:~/work$ ps -ef | grep 2755
user 2755 2488 0 05:40 pts/0 00:00:00 bash
user 2756 2755 0 05:40 pts/0 00:00:00 sleep 100 # after this, var will be incremented in shell with pid 2755 (our script).
user 2759 2528 0 05:41 pts/1 00:00:00 grep 2755
Notice the difference in bash count with parent / children relation in both examples.
