Dao's BLOG

If ForwadX11 and ForwardX11Trusted are both enabled in /etc/ssh/ssh_config, when you do “ssh host” it will invoke X11 forwarding by default. This is convenient when you frequently need to run programs with GUI on remote host. However, with X11 forwarding, if you are simply running a text mode command/script via ssh, like “ssh node ls” to list files on a remote host, it may not exit properly with X11 forwarding.

To avoid this, the simplest way is to disable X11 forwarding in the ssh command using -x option: using “ssh -x node command” instead of “ssh node command”.

echo <your password> | ssh <node name> “sudo -S <command>”

For a RAID NAS box, when the NAS box dies, the data on the drives are still intact. They can be retrieved in the following steps:

1. dd disks containing LVM to hard drive; # create images of hard drives
2. losetup /dev/loopx disks.img; # make images appear as loop devices on OS
3. partx -v -add /dev/loopx; # make partitions on the loopback devices available to OS
4. vgscan, vgimport -a; # to import Virtual Groups
5. vgdisplay; # showing imported VG names
6. vgchange -a y vgname; # to activate VG
7. fsck /dev/mapper/vgname; # to check file system of VG
8. fuseext2 -o ro -o sync_read /dev/mapper/vgname /mounting_point; # to mount the VG
9. rsync -av –progress /mounting_point/ /destination/

Then you will get everything at your destination, and then you can safely remove the images.

Sometimes a bash script does not run the commands as cron or start up scripts that usually run when you debugging it. This may be caused by that the embedded running environment does not provide proper search path. To avoid this, always use full path to the executibles.

update.packages(checkBuilt=TRUE, ask=FALSE)

The convenient select copy, mouse middle click paste way does not work on some Fedora desktop environment. Even the Ctrl-C/Ctrl-V/Ctrl-X copy past and menu copy-paste does not work, forcing users to hand type in things that usually just a few mouse clicks away.

This is identified as a buggy package called “clipit”

To identify the problem, please do

$ ps ux | grep clipit

If it shows up like

username       1704  0.1  0.1 290772  5980 ?        Sl   13:04   0:51 clipit

You suffer this problem. To make it instantly working, please kill it:

$ kill <PID>

Then, to permanently remove it, you may simply uninstall it:

$ dnf erase clipit

and allow all packages that depends on it to be removed too.

Linux search /etc/profile.d and ~/.bashrc, ~/.bash_profile for system and user defined environmental variables. One of the most manipulated variable is $PATH. You can see this by running

$ echo $PATH

In most cases, when a new 3rd party software is installed, and you want it to be in your default executable search path, you insert a line like

export PATH=$PATH:/where/your/new_executables

in your ~/.bash_profile or in a file in /etc/profile.d if you are a system administrator. This appends a new search path onto the existing one.

However, if you launch another terminal, a shell, etc., all these scripts are loaded again and you got duplicated entries in $PATH.

This looks harmless and does not affect system performance in most cases since bash looks for the first available executable in the search path and ignore all later appended paths.

However, if you are running some scripts that loop into multiple layers of shells, this could cause the $PATH string overflow and disrupt your script. Also for cosmetic reason, a very long $PATH filled with duplicates are ugly.

To not generate duplicates, we can do:

NEWPATH=/where/your/new_executables 
if [ -d "$NEWPATH" ] && [[ ":$PATH:" != *":$NEWPATH:"* ]]; then
    PATH="${PATH:+"$PATH:"}$NEWPATH"
fi
export PATH;

for appending or

NEWPATH=/where/your/new_executables 
if [ -d "$NEWPATH" ] && [[ ":$PATH:" != *":$NEWPATH:"* ]]; then
    PATH="$NEWPATH${PATH:+":$PATH"}"
fi
export PATH;

for prepending, instead of the

export PATH=$PATH:/where/your/new_executables

way. This, will prevent generating duplicated entries in $PATH since it will only add in a new entry when the new entry is not found in the existing environmental variable.

For CentOS7,

ccmake -DCMAKE_INSTALL_PREFIX=/usr/local/lapack -DCMAKE_BUILD_TYPE=RELEASE -DBUILD_SHARED_LIBS=ON -DLAPACKE=ON ..

Then make, make install will get you a working version of liblapacke.so

The current version 3.8.0 source has bugs that prevent the .so to work properly. Version 3.7.1 is the best to do.

For CentOS8, the Fedora 29 version of Lapack works by dnf install.

This is related with a version compatibility issue.

nano +26 /usr/share/ganglia/cluster_view.php

change

$context_metrics = “”;

to

$context_metrics = array();

will do the job.

A zombie is already dead, so you cannot kill it. To clean up a zombie, it must be waited on by its parent, so killing the parent should work to eliminate the zombie.

An example of how you might send a signal to every process that is the parent of a zombie (note that this is extremely crude and might kill processes that you do not intend. I do not recommend using this sort of sledge hammer):

kill $(ps -A -ostat,ppid | awk ‘/[zZ]/ && !a[$2]++ {print $2}’)