Subshells

- Running a shell script launches a new process, a subshell. "A subshell is a process launched by a shell (or shell script)."

- Each shell script running is, in effect, a subprocess (child process) of the parent shell.

- A shell script can itself launch subprocesses. These subshells let the script do parallel processing, in effect executing multiple subtasks simultaneously.

NOTE : In general, an external command in a script forks off a subprocess, whereas a Bash builtin does not. For this reason, builtins execute more quickly than their external command equivalents.

- Variables in a subshell are not visible outside the block of code in the subshell. They are not accessible to the parent process, to the shell that launched the subshell. 

#!/bin/bash
# subshell-test.sh
(
# Inside parentheses, and therefore a subshell . . .
while [ 1 ] # Endless loop.
do
  echo "Subshell running . . ."
done
)

Now, run the script:
sh subshell-test.sh
And, while the script is running, from a different xterm:
ps -ef | grep subshell-test.sh
UID PID PPID C STIME TTY TIME CMD
500 2698 2502 0   14:26 pts/4 00:00:00    sh subshell-test.sh
500 2699 2698 21 14:26 pts/4 00:00:24    sh subshell-test.sh

- A subshell may be used to set up a "dedicated environment" for a command group.

COMMAND1
COMMAND2
COMMAND3
(
  IFS=:
  PATH=/bin
  unset TERMINFO
  set -C
  shift 5
  COMMAND4
  COMMAND5
  exit 3 # Only exits the subshell!
)
# The parent shell has not been affected, and the environment is preserved.
COMMAND6
COMMAND7
As seen here, the exit command only terminates the subshell in which it is running, not the parent shell or
script.

- Processes may execute in parallel within different subshells. This permits breaking a complex task into subcomponents processed concurrently.

- A command block between curly braces does not launch a subshell.
{ command1; command2; command3; . . . commandN; }