use of - character in tar, su commands?

[jwilliams]

What's the difference between:

tar xvf - filename
and
tar xvf filename

and

su - username
and
su username

?

Thanks

[MikeTbob]

What's the difference between:

tar xvf - filename
and
tar xvf filename

and

su - username
and
su username

?

Thanks

For commands like tar, usually - is for options
tar -zxfv

For su it's a bit different
su just logs you in as root
su - logs you in as root with roots path
Try it yourself, login as root with just su
issue this command:
modprobe
Then login as root with su - and modprobe again

[jens]

What's the difference between:

tar xvf - filename
and
tar xvf filename

"-"(stdout/stdin) will tar everyting to stdout, being already the default you shouldn't need this.

su - username
and
su username

same thing as above, "-" is already the default and will not do anything in this case

see:
man stdout
man tar
man su

[devils casper]

same thing as above, "-" is already the default and will not do anything

"-" is not default in RedHat and Fedora.
try this

su 
echo $PATH
exit
su -
echo $PATH

compare output of 'echo $PATH'.







Casper

[jens]

"-" is not default in RedHat and Fedora.
try this

su 
echo $PATH
exit
su -
echo $PATH

compare output of 'echo $PATH'.

Casper

A redhat thingy ...
It doesn't change anything for me:

jens@desktop:~$ su others
Password:
others@desktop:/home/jens$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games
others@desktop:/home/jens$ exit
exit
jens@desktop:~$ su - others
Password:
others@desktop:~$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games
others@desktop:~$

[devils casper]

Normal users have 'common' path and "-" wont do anything but in case of root user, it will make a lot of difference.
as i suggested earlier, execute 'su' and 'su -' only.

[devils casper]

i think i am missing something. i am not in Fedora box right now and can't check myself.
i think "-" changes 'present working directory' too.
will you please execute 'pwd' command in both cases?

[jens]

Normal users have 'common' path and "-" wont do anything

Isn't that all he asked ("su username" vs "su - username"?

but in case of root user, it will make a lot of difference.
as i suggested earlier, execute 'su' and 'su -' only.

"su -" will dump you in / (not really what he asked for in his question though), but their's no standard rule as for why "echo $PATH" would change (as far as I know...).

jens@desktop:~$ su
Password:
desktop:/home/jens# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11
desktop:/home/jens# exit
exit
jens@desktop:~$ su -
Password:
desktop:~# echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11
desktop:~#

Do correct me if I'm missing something...

EDIT: Oh... you just meant the default Redhat profile ...
I was under the impression your meant it was "root" that makes a difference (post6).
I couldn't see your edit and second post before refreshing this page!? (is this forum software supposed to work like this ?).
All I said is that it doesn't change anything in default (meaning you didn't alter your bash_profile).
RH/Fedora uses a modified one (also for normal users and not just root, that's what I meant with the redhat thingy).

[devils casper]

I was under the impression your meant it was "root" that makes a difference (post6).
I couldn't see your edit and second post before refreshing this page!? (is this forum software supposed to work like this ?).

i didn't edit much in second post. there was typo only. i wrote 'hoot' instead of
'root'. .

did you execute pwd command? i can't access Fedora box right now. i will check it in Fedora and post here soon.

[nikunjb]

Plain 'su <user>' spawns a shell thats not a login shell. 'su - <user>' causes a shell to be spawned with first argument as '-' which indicates a login shell.

Most shells take the first arg as '-' to mean that a new login has been initiated i.e. as if the new user (root or otherwise) logged in afresh over a terminal or ssh or telnet link or whatever. Consequently, they run all the usual login scripts (.bashrc and /etc/bash_profile in case of bash) and so you might get a different PATH according to those files as well as get started with <pwd>=<home directory>.

Comparing this to plain 'su <user>' case, just a new shell is spawned and all the environment is inherited from the parent process, hence retaining the old PATH and pwd.

Hope this was not too muddled an explanation.