subshell

A subshell is a separate child process created by a shell to execute a group of commands in isolation from the parent shell. Subshells are typically created when a new process is needed to run a sequence of commands, most commonly when command groups are enclosed in parentheses, or through features such as command substitution and pipelines in many shells. In POSIX-compliant shells, a grouped command written as ( commands ) runs in a subshell; the outer shell continues independently. Braces, by contrast, group commands without invoking a subshell; they execute in the current shell process when used with proper syntax.

Because a subshell has its own process space, changes to variables, the current working directory, or other state inside the subshell do not affect the parent shell after the subshell exits. This isolation helps limit side effects and can be used to perform temporary work, computations, or directory changes without altering the parent environment.

Pipelines commonly involve subshells: each component of a pipeline may run in its own subshell, so modifications within one stage do not automatically propagate to other stages. Some shells provide options (for example, lastpipe in certain shells) to alter this behavior and allow the last element of a pipeline to run in the current shell.

Command substitution also relies on a subshell: $( ...) executes the commands in a subshell and substitutes its output into the command line, preserving the caller’s environment.

Subshells are a fundamental concept in shell design, enabling controlled isolation of command execution and state.