Unix, when a program starts another program (more exactly, when a
process starts another process), the new process runs as a subprocess (Section 24.3) or
child process. When a shell
starts another shell, the new shell is called a
So what? There are some important things to know about it: the
child process gets a copy of
its parent's environment, and any changes in the
environment of the child process aren't passed to
its parent. "Still," I hear you
say, "so what??"
are run in a subshell (unless you use the source or . commands (Section 35.29) to start the script). If the script makes
changes to the environment of its (sub)shell, the parent shell
won't see those changes. If the script uses
cd, it doesn't change the current
directory in the parent shell. If the script changes the value of the
TZ (or any) environment variable, that
won't change TZ in the parent
shell. The script can set a
different umask than the parent shell -- no
There are times you might want to start a subshell from your current
shell. Maybe you have a special project where you need to work in a
different current directory, reset environment variables, set a new
home directory, reset some aliases, use a different PATH (Section 35.6), whatever.
When you end the subshell, the parent shell's
environment will be the way it was.
If your parent shell has job control (Section 23.3),
you can stop the subshell and pop back to your parent shell without
losing the changes in the subshell. If the child shell has job
control, too, the command suspend
(or kill -STOP $$ (Section 27.17)) will stop it. Otherwise, just type CTRL-z at
the subshell's prompt. For example:
prompt Section 4.1
myprompt% set prompt="project% "
project% cd project-directory
project% setenv PRINTER plotter
project% set path=($path some-new-directories)
project% setenv EXINIT "se ts=4 sw=4 aw wm=0"
...do some work...
...back to parent shell...
myprompt% fg %csh
...back to subshell...
I use suspend so much that I've
made a CTRL-z-like alias named z:
alias z suspend ...csh
alias z=suspend ...bash, ksh
If you need a different type of shell temporarily, just type that
shell's name at a prompt. When you end the shell by
typing exit or by suspending it (as shown above),
you're back to your usual shell. For example, you
might normally use bash but want to use the
zsh multiline editing for a few loops you need to
run. As another example, I started a lot of different shells while I
was writing this book -- and suspended them, as above, when I
wasn't using them. Very handy.
escape (Section 17.21) starts a subshell.
Do whatever you want to the subshell's environment.
When you end the shell escape, the changes go away.
The su command starts a subshell.
cd anywhere, change environment variables, and so
If you use the
exit command, a subshell (or any shell) will
terminate. In a script, when the shell reads the end of file, that
does an implicit exit. On the command line, an
(usually CTRL-d) will do the same thing. Section 35.16 explains how exit sets a
shell's exit status.
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