Terminal/command line skills needed

[I_can_see_the_light]

Hi! I wasn't sure about where to ask this so apologies if it's in the wrong section.

I'm running CrunchBang wich is based on Debian and my screensaver is XScreenSaver. When playing a movie in Gnome Mplayer the screensaver activates unless I edit ~/.mplayer/config and enter (according to XScreenSaver faq)

heartbeat-cmd="xscreensaver-command -deactivate >&- 2>&- &"

My question is, what does the part in bold mean?

[kalfusisagod]

I think the ampersand (the & symbol) puts the task in the background. I'm not sure about this context though. So if you wrote

mv largefile.txt largefile2.txt&

Then this would put it into the background of the terminal so the command prompt would show up right away, and to list background terminal tasks, you can use

ps

for processes or

jobs

Anyone else?

[chipbuster]

http://tldp.org/LDP/abs/html/io-redirection.html

Looks like you're telling it to close both stdout and stderr with the first two. Wonder if that's the same thing as pointing it to /dev/null?

[kalfusisagod]

Oh and I forgot to mention, I think the 2> means that it goes to the error output. Linux has several outputs.
> standard output
1> standard output
2> error output

For example if your typed

ls > files.txt

Then this command would list the current directory and put it into a text file called files. If you typed

ls /fakedirectory 2> files.txt

Then this would put

ls: cannot access /fakedirectory: No such file or directory

into a file called files.txt. This would not happen with a normal > option because error output in not mixed with standard output. Hope this helps.

[Paddy Landau]

The ampersand on its own means to put it into the background as an asynchronous background task. Thus, that's the final ampersand that does it.

The symbol ">" means to redirect output. There are two usual outputs, one being stdout or 1, which is normal output, and the other stderr or 2, which is error messages.

Thus: "1>&-" means to send stderr to something called "&-" (more on that in a moment) instead of its usual terminal; and "2>&-" means to also send stderr to "&-".

">&-" is shorthand for "1>&-".

Incidentally, "&>" is shorthand for "both 1 and 2 >", thus ">&-1 2>&-" could be shortened to "&>&-".

Confused yet?

I do not quite understand what "&-" means, but I think it means to send output to the owning variable; in this case we don't know what it is, because it gets passed to XScreenSaver.

You can read more about redirection by using the command:

man bash

and searching for REDIRECTION. You can also find more about background jobs in the same manual. There is a copy of the bash manual on-line if you find that easier.

[oldos2er]

Moved to Other OS/Distro Talk.

[I_can_see_the_light]

Ok, thanks to all three of you.

This is output from running the command in the terminal:

isen@crunchbang:~$ xscreensaver-command -deactivate >&- 2>&- &
[1] 9007
isen@crunchbang:~$ xscreensaver-command -deactivate >&- 2>&- &
[2] 9008
[1]   Done                    xscreensaver-command -deactivate 1>&- 2>&-
isen@crunchbang:~$

This is what it looks like without redirecting, just putting it in the background.

isen@crunchbang:~$ xscreensaver-command -deactivate &
[1] 9018
isen@crunchbang:~$ xscreensaver-command: not active: idle timer reset.

It looked like I could continue using the terminal but then the idle timer reset message interrupted and stole the prompt. Anyway, I'm marking this thread as solved

[kalfusisagod]

A word of advice, go out and get a good book on the terminal, it's a lifesaver! The terminal will be 90 percent the same across most distros unlike KDE, Gnome, and Unity with their constant changes. A good book I'd recommend is "A Practical Guide to Linux Commands, Editors, And Shell Programming" by Mark Sobell.

[sisco311]

[n]<&word is used to duplicate input file descriptors and [n]>&word is used to duplicate output file descriptors. If word evaluates to -, then file descriptor n is closed.

>&- or 1>&- closes file descriptor 1 (stdout)
and
2>&- closes file descriptor 2 (stderr)

See:

man bash | less +/" +Duplicating File Descriptors"
man bash | less +/"^REDIRECTION"

http://mywiki.wooledge.org/BashGuide/InputAndOutput
http://wiki.bash-hackers.org/syntax/redirection
and the POSIX specification:
http://pubs.opengroup.org/onlinepubs...html#tag_18_07

Another option would be to redirect stdout & stderr to /dev/null.
BASH:

command &> /dev/null

Note: this is not standard, so I'd avoid it in scripts. But it's shorter than the POSIX form, hence it's handy in an interactive BASH shell.

The POSIX form is:

command > /dev/null 2>&1

[I_can_see_the_light]

A word of advice, go out and get a good book on the terminal, it's a lifesaver! The terminal will be 90 percent the same across most distros unlike KDE, Gnome, and Unity with their constant changes. A good book I'd recommend is "A Practical Guide to Linux Commands, Editors, And Shell Programming" by Mark Sobell.

Yeah, that's on my "todo list". I've got one in a Swedish translation/rework from Gareth Anderson's GNU/Linux Command-Line Tools Summary but I'd like to have another one which digs a little deeper. I will have a look at the one you mentioned.