Child Process

Subshell () vs Pipeline | vs Script ./script.sh

Subshell () (More at Parenthesis Spawn a Subshell: $() AND ())

• Fork-only: creates a copy of the current shell
• Inherits everything: all variables, functions, aliases (exported or not)
• Example: (echo $myvar; myfunc) works even if nothing is exported

Pipeline |

• Each command in a pipeline runs in a subshell (fork-only)
• Inherits everything like () subshells
• Warning: In some shells, the last command ALSO runs in a subshell
• Example: echo "test" | myfunc | somefunc works even if myfunc isn't exported

Script ./script.sh

• Fork+exec: creates a new, fresh shell process
• Inherits only exported:
  • exported environment variables
  • exported functions (export -f)
    • bash-specific, not POSIX. Many shells don't support exporting functions at all.
• Example: ./script.sh can't see myvar or myfunc unless exported

Key difference: Subshells and pipeline commands are copies of the parent shell with full state. Scripts are new shells that start clean with only exported environment.

Common gotcha: All three create child processes, so variable changes don't affect the parent:

(myvar=1)                    # parent doesn't see this
echo "test" | read myvar     # parent doesn't see this
./script.sh                  # sets myvar=1 inside, parent doesn't see it

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Children

1. Parenthesis Spawn a Subshell: $() AND ()