Clearing RAM cache and SWAP memory in Linux

If your server get hanged, RAM cache is one of the major reason,. It happens frequently on web server when we are under attack or web developer dosen't consider about performance of application. Miss configuration of servers like mysql and apache is one of the main reason.

We can see ram cache amount on htop, which is user friendly monitoring tool for Linux servers. Because of it we may loss some connections and it's interaction with server. We can do this by our own risk.

What is RAM cache ?

It is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over.

To clear cache :

free -m && sync && echo 3 > /proc/sys/vm/drop_caches && free -m

To clear pagecache change value of echo to 1 on about commend, to clear dentries and inodes change value of echo to 2 on about commend, echo 3 will clear all of three pagecache, dentries and inodes.

To clear swap:

Up-to I know that there is no straight way to this, But we can do it by disable and enable the swap, We can do this by following command,

To turn off : swapoff -a
To turn on : swapon -a 

Finding malware codes on web pages

It is beginners trick, not advised for administrators.

Lot's of tools are available on Linux to find the malware codes injected on server. But most of them not detect the page with url encoded. Because it's php functionality. Mostly of this happens when developer or administrator not aware problem when we providing 777 permissions to pages on web servers. In these cases simple java-script injection will make server to get down.

We can disable this function, urldecode but unfortunately it's bad idea on shared hosting and most of frameworks using url encode and decode. In this case, we can do some manual work for figure-out codes by following commend.

find . -iname '*php' | xargs grep 'urldecode' -sl

This will list the files that have urldecode. We can check page and remove code.

Query to find size of MySQL DB

When we are using shared hosting server with Windows or Linux, without any control panel software, It's little bit hard to find user's disk usage on mysql.

In this case we can use  following command on mysql to get size of database.

SELECT table_schema "DB Name", Round(Sum(data_length + index_length) / 1024 / 1024, 1) "DB Size in MB" FROM information_schema.tables GROUP  BY table_schema;

Copy files with original timestamp in Linux commandline

If you move files on Linux with command mv, time-stamp won't be change, But if you copy with command cp, it'll take current time for time-stamp. If we wanna get original time-stamp, you can use following command on Linux,

cp -R -u -p /source /destination

What is time-stamp ?

When we create or modify file / directory, it'll store creation and / or modification date and time on file / directory properties. Which is called time-stamp.

Setting up default permissions to Files and Directories

Most of the Linux beginners / Linux - Level 1 administrator do this funny thing. Setting up 777 permission to files and folders.

With, chmod -R 777 folder_name

Some times it'll help to destroy the server easily by hackers.

Let's come to topic, How to setup default / basic level permission to files and folders.

chmod -R 777 folder_name will give you files and folders with read, write and execute permission, Mostly we no need of execute permission for files.

If we try to change it as chmod -R 666 folder_name, we are not able to open directory.

If we have 100s of sub directory with 1000 of files in each directory,... oh my god.... How do we fix it worst thing.... Here is command.

first open your parent directory, with
cd parent_directory_name

And run following commands,

find . -type d -exec chmod 755 {} \;
find . -type f -exec chmod 644 {} \;

Working: On first command It'll check type of the object whether it is file or directory, if it is directory it'll change permission to 755, else leave. On second same condition, if it is file then change permission to 644 else leave with same,

Copy mysql database table by query / command line

We can find lot of solutions to make copy of table.
Here is one of the simple solution which I got while google.

Copy table with in a Database

use databasename;

CREATE TABLE exampletable_copy LIKE exampletable;
INSERT exampletable_copy SELECT * FROM exampletable; 


Copy table from Database-1 to Database-2

use databasename2;

CREATE TABLE exampletable_copy LIKE exampletable.databasename1;
INSERT exampletable_copy SELECT * FROM exampletable.databasename1; 


Hint:
First query used to create table with structure of original table.
Second query used to copy data from table to table-copy.

Create mail account and alias by command line with parallel plesk panel

Following command is used to create mail account by Linux command line for parallel plesk panel

/usr/local/psa/bin/mail --create name@domain.com -mailbox true -passwd Pa55w0rd -antivirus in


Following command is used to create mail account by Linux command line for parallel plesk panel

/usr/local/psa/bin/mail -u name@domain.com -aliases add:alias1,alias2,alias3,alias4,alias5,alias6


Note : Change bold place as your requirement


You can use this line multiple time for create more mail accounts / alias accounts, Need to work some more for get input from text file and create mails and alias. I'll try to update soon with script.