Which of the following codes will run in the same process of the current shell

Question

A. { echo "Hello World"; } >outputfile

B. ( echo "Hello World" ) >outputfile

C. ./anothershell.sh

D. /bin/echo "Hello World"

Which is right? And what kind of command can run in the same process of the current shell?

Answer 1

Only A will run within the process of the current shell.

B will run in a subshell because you asked for a subshell by using paranehteses.

C and D will both run outside of the current shell process because they are invocations of external commands.

Answer 2

You have asked a couple times about how to prove it. You use environment variable setting as a probe:

export testvar=nope
{ echo "Hello World"; export testvar=yep; } >outputfile
printenv testvar

writes yep

export testvar=nope
( echo "Hello World"; export testvar=yep; ) >outputfile
printenv testvar

writes nope

You'll need to edit your script in (C) and compile a modified echo for (D), but then the corresponding constructs for them will also write nope. This shows explicitly that only case (A) and glennjackman's case (E) from the comments run in the same process.

Answer 3

One distinction you may be missing is that B will fork (so it's another process), but it doesn't actually exec (it's a copy of the same shell, it doesn't go and find /bin/sh and re-run initializers etc). And there are special rules ($$ is the pid of the original shell, for example).