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ssh and redirection

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# 15  
Old 10-07-2011
Quote:
Originally Posted by Corona688
Putting that in quotes instead of quote tags means when I try and quote you, the error message disappears. Use code tags for code.

"no askpass program" means it needs to ask you for a password and can't because it's not in a terminal. Run it in a terminal.

---------- Post updated at 09:12 AM ---------- Previous update was at 09:11 AM ----------

This won't work, again, because the redirection happens before the sudo. The shell doesn't have permissions to overwrite the file, and will fail to redirect into it.

---------- Post updated at 09:14 AM ---------- Previous update was at 09:12 AM ----------

Perhaps:

Code:
ssh -t newuser@server echo "': > /var/spool/mail/user1'" '|' sudo -u user1 /bin/sh

you are right but if newuser hasnot a entry in sudoers then your example also is not solution to me.
in this case sudo is not required and sudo does not get extra rights on files.
maybe `su` is can be usable instead of sudo.
Code:
ssh -tt newuser@server "su -l user1 -c 'echo -n "">/var/spool/mail/user1'"

# 16  
Old 10-07-2011
Thanks for all , it works :

Code:
ssh -t newuser@server echo "': > /var/spool/mail/user1'" '|' sudo -u user1 /bin/sh

regards
Christian
# 17  
Old 10-07-2011
Quote:
Originally Posted by Corona688
That may delete and recreate the file and leave the old file still open hogging disk, unfixable without rebooting the daemon or server! Don't do it!
I had a poke around, but apparently my GoogleFu is weak today. Do you have a link that explains the issues surrounding this?

Also, would
Code:
cat /dev/null > file

have the same potential problem (sudo/redirect issues aside)?

(apologies for drifting off-topic)
# 18  
Old 10-07-2011
Quote:
Originally Posted by CarloM
I had a poke around, but apparently my GoogleFu is weak today. Do you have a link that explains the issues surrounding this?
I can do better than that, I can give you a working example:

Code:
## Demo of deleting a logfile in use
$ echo "the owls are not what they seem" > filename # Create a file
$ exec 5<filename # Open filename for reading, into FD 5
$ rm filename # delete filename.
$ ls -l filename # The directory entry will be gone -
ls: cannot access filename: No such file or directory
cat <&5 # ...but the file still exists, since we have it open.
the owls are not what they seem
$ exec 5<&- # Only on close will it TRULY be deleted from disk.

## Demo of truncating a file in use
echo "the owls are not what they seem" > filename # Create a file
$ exec 5<filename # Open filename for reading, into FD 5
$ : > filename # truncate filename
$ ls -l filename # The same file exists, with zero size
-rw-r--r-- 1 username users 0 Oct  7 11:40 filename
$ cat <&5 # Read from the file.  Nothing there.
$ exec 5<&- # close the file.

cp might overwrite a file, or might delete and recreate it, I wouldn't depend on either -- implementations of things can vary. But a shell redirect > always truncates.

Code:
Also, would 


Code:



cat /dev/null > file






 have the same potential problem (sudo/redirect issues aside)?

Nope. cat /dev/null is effectively the same statement as : -- a statement that prints no output. It's the > that's important.

I used to use echo > filename to truncate things until I realized echo does indeed print slightly more than nothing there -- it prints one blank line.
Quote:
(apologies for drifting off-topic)
This is entirely on-topic.
Last edited by Corona688; 10-07-2011 at 02:48 PM..
This User Gave Thanks to Corona688 For This Post:
CarloM
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