Sub-shell differences between bash and ksh

Question

I always believed that a sub-shell was not a child process, but another shell environment in the same process.

I use a basic set of built-ins:

(echo          


        

Answer 1

In ksh, a subshell might or might not result in a new process. I don't know what the conditions are, but the shell was optimized for performance on systems where fork() was more expensive than it typically is on Linux, so it avoids creating a new process whenever it can. The specification says a "new environment", but that environmental separation may be done in-process.

Another vaguely-related difference is the use of new processes for pipes. In ksh and zsh, if the last command in a pipeline is a builtin, it runs in the current shell process, so this works:

$ unset x
$ echo foo | read x
$ echo $x
foo
$

In bash, all pipeline commands after the first are run in subshells, so the above doesn't work:

$ unset x
$ echo foo | read x
$ echo $x

$

As @dave-thompson-085 points out, you can get the ksh/zsh behavior in bash versions 4.2 and newer if you turn off job control (set +o monitor) and turn on the lastpipe option (shopt -s lastpipe). But my usual solution is to use process substitution instead:

$ unset x
$ read x < <(echo foo)
$ echo $x
foo