subshells

A subshell is a separate instance of a shell that executes a command sequence as a child process of the current shell. Subshells have their own process ID and memory space but inherit the parent’s environment at the moment of creation. They allow isolated changes to variables and the working environment without affecting the parent shell.

Subshells are commonly created in several ways. In many shells, parentheses group commands and run them in a subshell, as in something like (cd /tmp; ls). Command substitution also uses a subshell in most shells, with constructs such as $(command) or, in older syntax, `command`. In pipelines, each element historically runs in its own subshell, so variable assignments or directory changes in one stage do not affect others; some shells offer options or features that can alter this behavior.

Environment and side effects: because a subshell is a separate process, it receives a copy of the parent’s environment, but any changes made inside the subshell do not propagate back to the parent. This isolation can be used to run tasks without altering the current shell state. Redirections performed inside a subshell apply to that subshell’s process only.

Differences and alternatives: if the goal is to group commands without creating a new subshell, many shells support braces for grouping (for example, { cmd1; cmd2; }). Brace grouping executes in the current shell, avoiding the creation of a subshell, though syntax rules for braces are stricter than those for parentheses.

Examples: (cd /var/log; ls) runs in a subshell, so the parent shell’s current directory remains unchanged. VAR=1; (VAR=2; echo "$VAR") prints 2 inside the subshell but leaves the parent’s VAR at 1.