I have done some research on pipe and subshell
Each command in a multi-command pipeline, where pipes are created, is executed in its own subshell, which is a separate process (see Command Execution Environment). If the lastpipe option is enabled using the shopt builtin (see The Shopt Builtin), the last element of a pipeline may be run by the shell process when job control is not active.
test code
#!/bin/bash
set +m
shopt -u lastpipe
shopt | grep lastpipe
function test() {
echo "This is first pipe" | echo $BASH_SUBSHELL
}
test
shopt -s lastpipe
shopt | grep lastpipe
function test() {
echo "This is first pipe" | echo $BASH_SUBSHELL
}
test
Output is
lastpipe off
0
lastpipe on
0
Expected result is
lastpipe off
1
lastpipe on
0
Your problem is that variable substitution is performed before the pipeline runs. When you write:
echo "This is first pipe" | echo $BASH_SUBSHELL
The $BASH_SUBSHELL is expanded in the current shell before it runs the pipeline. To properly perform your test, you would need to place your echo statement in a function to defer substitution:
test_subshell() {
echo $BASH_SUBSHELL
}
Then you will see different behavior:
$ test_subshell
0
$ echo "This is first pipe" | test_subshell
1