What are all the ways to create a subshell in bash?

Question

There are 2 main ways that I know of so far:

• Explicitly: wrapping parentheses around a list of commands

• Implicitly: every command in a pipeline

Are there more ways, either explicitly or implicitly, in which one creates subshells in bash?

Answer 1

From man bash:

• If a command is terminated by the control operator &, the shell executes the command in the background in a subshell. The shell does not wait for the command to finish, and the return status is 0.
• A coprocess is a shell command preceded by the coproc reserved word.
  A coprocess is executed asynchronously in a subshell, as if the command had been terminated with the & control operator
• Shell builtin complete command: when called with the -C command option, command is executed in a subshell environment, and its output is used as the possible completions.
• Command substitution, commands grouped with parentheses, and asynchronous commands are invoked in a subshell environment that is a duplicate of the shell environment

Answer 2

It depends on what you mean by “subshell”.  And you may be missing the point in your “every command in a pipeline” bullet. 

Any time you run any (external) program (i.e., a script or binary executable, in contrast to a shell builtin), unless you use exec program, you run it in a sub-process (or processes).  The shell forks and executes the program.  ls | wc does not create a subshell any more than ls and wc do alone.

The interesting thing is that including it in a pipeline can cause a shell builtin to run in a subshell.  Consider this example:

$ read v
cat                                     # This is input typed by the user.

$ echo "$v"
cat

$ echo cougar | read v

$ echo "$v"
cat

The second read v command is run in a subshell because it is part of the echo cougar | read v pipeline.  Therefore, the value cougar is lost, and $v retains its first value.

Likewise, commands like

$ echo foo | cd /

$ cd / | cat

$ echo foo | exit

$ exit | cat

do not affect the main shell.