Switch to parent shell without logging out

Question

Let's say I start in my local account:

avindra@host:~>

then I switch to root:

host:~ #

Then I switch to oracle:

[ oracle@host:~]

Is there a way for me to drop back into the root shell (the parent), without logging out of the oracle shell?

This would be convenient, because the oracle account does not have sudo privileges. A typical scenario with oracle is that I end up in /some/really/deeply/nested/directory, and all kinds of special environment variables are set in particular ways.

Here comes the problem: I need to get back into root to touch some system files. Yes, I can log out of oracle to get back to root, but at the cost of losing my current working directory and environment.

Is there a way to "switch" to the parent shell using known conventions?

If you are using a gui desktop, you can just open another terminal window or tab or even switch to another virtual console. If not, use screen as stated in one of the answers. — Joe, Dec 26 '14 at 19:55
I'll look into screen, I haven't used it before. As for tabs, I prefer to use one tab for host. I find a workflow that entails multiple tabs per host to be cumbersome. I do a lot of work in clustered hosts / distributed systems, so even having one tab per node in the cluster can get confusing. In my mind, one tab = one discrete host. — Avindra Goolcharan, Dec 26 '14 at 20:08

Bratchley · Accepted Answer · 2014-12-19T19:49:16.503

41

You can simulate a CTRL-Z (which you normally use to temporarily background a process) using the kill command:

[tsa20@xxx01:/home/tsa20/software]$ kill -19 $$

[1]+  Stopped                 sudo -iu tsa20
[root@xxx01 ~]# fg
sudo -iu tsa20
[tsa20@xxx01:/home/tsa20/software]$

bash just traps the CTRL-Z key combination. kill -19 sends SIGSTP to the process which is effectively the same thing.

edited Dec 19 '14 at 19:49

answered Dec 19 '14 at 19:39

Bratchley

16,824
14
67
103

1

In zsh you can just run suspend. – Mikel Dec 19 '14 at 20:10
3

@Mikel, zsh or csh, tcsh, or bash, or the Bourne shell or ksh... (in ksh, suspend is an alias for 'kill -s STOP $$' (note the bug with the missing quotes around $$)). – Stéphane Chazelas Dec 19 '14 at 23:19
I can't run suspend after doing a sudo -i (Output) in bash. – Bratchley Dec 20 '14 at 03:32
Just reproduced the same error message in zsh. Adding -f makes it work though. – Bratchley Dec 20 '14 at 03:36
Beautiful. This is going to save me so much time. Can't wait to show it to the coworkers. – Avindra Goolcharan Dec 20 '14 at 04:49
1

@StéphaneChazelas Does that really make a difference with a variable that's almost certainly guaranteed to contain a numeric value? – Alexia Luna Dec 20 '14 at 14:18
@nyuszika7h, not-quoting variables is the split+glob operator. It makes no sense at all to invoke it here. The behaviour of the split part depends on the current value of $IFS. See also Security implications of forgetting to quote a variable in bash/POSIX shells. – Stéphane Chazelas Dec 20 '14 at 16:17
@StéphaneChazelas $$ is a read-only variable, though, and you can make assumptions about its value. That said, I do quote those variables when using them. – Alexia Luna Dec 20 '14 at 16:20
2

bash does not trap CTRL-Z, when you press CTRL-Z, your terminal emulator sends a ^Z character (0x1a) to the master side of the pty device. The line discipline of the pty driver then sends SIGTSTP to the foreground process group of the terminal. Until then bash is not involved at all. That SIGTSTP may cause the process group leader to suspend and then the wait() that bash does on it will return. – Stéphane Chazelas Dec 20 '14 at 16:23
5

Note that there's no guarantee that 19 will be the number of SIGTSTP. Use kill -s TSTP or kill -s STOP (TSTP can be trapped or ignored, STOP cannot). – Stéphane Chazelas Dec 20 '14 at 16:24
2

@nyuszika7h, $$ may be readonly, but IFS is not. Try IFS=0123456789; suspend in ksh for instance. – Stéphane Chazelas Dec 20 '14 at 16:26
@StéphaneChazelas I didn't think of that. That's a good point. – Alexia Luna Dec 20 '14 at 16:27

score 3 · Answer 2 · answered Dec 20 '14 at 22:57

Csh, bash, ksh, zsh, have a suspend builtin command (or alias, in ksh) that does exactly that. This command is mostly equivalent to sending a TSTP signal to the shell; bash and zsh do a bit of additional signal handler and juggling, and in these shells the suspend command works even if the shell is currently ignoring TSTP.

You can also send the signal to the shell manually with kill -STOP $$.

score 0 · Answer 3 · edited Dec 24 '14 at 06:45

0

I would also suggest that you could use (install if needed) a program called [screen][1] that let's you have multiple terminal windows open. It was designed for TTYS so it works just fine without needing X. You can use keyboard shortcuts to switch between terminals and disconnect and logout, leaving your terminals still running, then log back in and reconnect to them.

I believe most Linux distributions have packages for this program, and I have used packages on Solaris for it. Worst case of course is you can install from source.

edited Dec 24 '14 at 06:45

qneill

113

answered Dec 23 '14 at 22:09

SiliconValley

1

1

This might be more appropriate as a comment rather than an anaswer. – Barmar Dec 24 '14 at 18:59

Switch to parent shell without logging out

3 Answers3