These are what I learned in 2017-08-06.

Author: khanhicetea

java/Runing-old-java-applets-on-brower.md

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+- Date : 2017-08-06
+- Tags : #java #security
+
+## Runing old java applets on brower
+
+Mostly morden browser has stop support Java plugins, so you can't run Java applet on browser.
+
+Temporary way :
+- run in IE or Safari
+- run in an old Firefox (version 23)
+
+And what if old java applet can't be runned on Java 8 because of weak signature algorithm. Try this
+
+- Open `java.security` file :
+   - In MacOS, located in `/Library/Java/JavaVirtualMachines/jdk[jdk-version].jdk/Contents/Home/jre/lib/security`
+   - In Windows, located in `C:\Program File x86\Java\jre\lib\security`
+- Comment this line, ```jdk.certpath.disabledAlgorithms=MD2, MD5, RSA keySize < 1024```
+- Rerun applet
+

php/realpath-function.md

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+- Date : 2017-08-06
+- Tags : #php #mistake
+
+## realpath function
+
+If you pass a non-exists path to function `realpath`, it returns empty string. So please don't do something like :
+
+```php
+function storage_path($folder) {
+	return realpath(__DIR__.'/storage/'.$folder);
+}
+```
+
+if you expect it return full path of new folder !

sysadmin/Cleaning-up-old-linux-kernels.md

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+- Date : 2017-08-06
+- Tags : #sysadmin #linux #kernel
+
+## Cleaning up old linux kernels
+
+Last day, I try to reboot a production server which has out-of-space /boot (I upgraded many kernels without rebooting, so system doesn't clean up old ones). And in the end, doom day had come ! It installed new kernel failed and booting to that kernel. My system crashed !
+
+So, I learned from it :
+
+- Never ever upgrade kernel without cleaning up old ones (just reboot)
+- Never ever reboot a production without backup
+- MORE IMPORTANT, NEVER do 2 above things at same time in the weekend !!!
+
+**Solution** :
+
+- Check current kernel : `uname -r`
+- List all kernels : `dpkg --list | grep linux-image `
+- Remove a kernel : `sudo apt-get purge linux-image-x.x.x-x-generic`
+- Finally, update grub after removing all old kernels : `sudo update-grub2`
+
+- YOLO command for DEBIAN distros (to remove all of old kernels in 1 line), from [AskUbuntu](https://askubuntu.com/a/254585)
+
+```bash
+dpkg --list | grep linux-image | awk '{ print $2 }' | sort -V | sed -n '/'`uname -r`'/q;p' | xargs sudo apt-get -y purge
+```
+
+THEN, `sudo reboot`