Linux - Tips

Maintenance

Release/Clear cache memory RAM
Every Linux System has three options to clear cache without interrupting any processes or services. 

 1. Clear PageCache only. 

 # sync; echo 1 > /proc/sys/vm/drop_caches

 

 2. Clear dentries and inodes. 

 # sync; echo 2 > /proc/sys/vm/drop_caches

 

 3.  Clear pagecache, dentries, and inodes. 

 # sync; echo 3 > /proc/sys/vm/drop_caches 

 

 

 Explanation of the above command. 

 sync will flush the file system buffer. Command Separated by “;” run sequentially. The shell waits for each command to terminate before executing the next command in the sequence. As mentioned in the kernel documentation, writing to drop_cache will clean cache without killing any application/service, command echo is doing the job of writing to file. 

 f you have to clear the disk cache, the first command is safest in enterprise and production as  “...echo 1 > ….” will clear the PageCache only. It is not recommended to use the third option above “...echo 3 >” in production until you know what you are doing, as it will clear pagecache , dentries, and inodes . 

 Is it a good idea to free Buffer and Cache in Linux that might be used by Linux Kernel? 

 Free Buffer and Cache in Linux 

 When you are applying various settings and want to check, if it is actually implemented specially on the I/O-extensive benchmark, then you may need to clear the buffer cache. You can drop cache as explained above without rebooting the System i.e., no downtime required. 

 Linux is designed in such a way that it looks into the disk cache before looking onto the disk. If it finds the resource in the cache, then the request doesn’t reach the disk. If we clean the cache, the disk cache will be less useful as the OS will look for the resource on the disk. 

 Moreover, it will also slow the system for a few seconds while the cache is cleaned and every resource required by OS is loaded again in the disk cache. 

 Now we will be creating a shell script to auto clear RAM cache daily at 2 am  via a cron scheduler task. Create a shell script clearcache.sh and add the following lines. 

 #!/bin/bash

# Note, we are using "echo 3", but it is not recommended in production instead use "echo 1"

echo "echo 3 > /proc/sys/vm/drop_caches"

 

 Set execute permission on the clearcache.sh file. 

 # chmod 755 clearcache.sh

 

 Now you may call the script whenever you are required to clear the ram cache. 

 Now set a cron to clear RAM cache every day at 2 am. Open crontab for editing. 

 # crontab -e

 

 Append the below line, save and exit to run it at 2 am  daily. 

 0 2 * * * /path/to/clearcache.sh

 

 For more details on how to cron a job, you may like to check our article on 11 Cron Scheduling Jobs . 

 Is it a good idea to auto clear the RAM cache on the production server? 

 Clear RAM Cache on Linux Production Server? 

 No! it is not. Think of a situation when you have scheduled the script to clear ram cache every day at 2 am . Every day at 2 am the script is executed and it flushes your RAM cache. One day for whatsoever reason may be more than expected users are online on your website and seeking resources from your server. 

 At the same time, the scheduled script runs and clears everything in the cache. Now all the users are fetching data from the disk. It will result in a server crash and corrupt the database. So clear ram-cache only when required, and known your footsteps, else you are a Cargo Cult System Administrator. 

 How to Clear Swap Space in Linux? 

 If you want to clear Swap space, you may like to run the below command. 

 # swapoff -a && swapon -a

 

 Also, you may add the above command to a cron script above, after understanding all the associated risks. 

 

 Now we will be combining both above commands into one single command to make a proper script to clear RAM Cache and Swap Space. 

 # echo 3 > /proc/sys/vm/drop_caches && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'

OR

$ su -c "echo 3 >'/proc/sys/vm/drop_caches' && swapoff -a && swapon -a && printf '\n%s\n' 'Ram-cache and Swap Cleared'" root

 

 After testing both the above commands, we will run the command “ free -h ” before and after running the script and will check the cache.

Large files linux
df -h shows hard drive space 

 

 sudo du -a /dir/ | sort -n -r | head -n 20 

 this will list larger files/folders in /dir 

   

   

 find / -type f -size +20000k -exec ls -lh {} \; 2> /dev/null | awk '{ print $NF ": " $5 }' | sort -nrk 2,2 

   

 this will find files larger than 20000k

Find hidden process running
lsof -Pn|grep txt|grep "/\."

IP Tables
to list all ip tables by chain 

 iptables -L --line-numbers 

 add iptable on line  

 sudo iptables -I INPUT 1 -p tcp -s 10.190.195.123 --dport 22 -j ACCEPT sudo iptables -I INPUT 2 -p tcp -s 10.190.227.123 --dport 22 -j ACCEPT 

 sudo iptables -I INPUT 3 -p tcp -s 10.190.195.123 --dport 8443 -j ACCEPT sudo iptables -I INPUT 4 -p tcp -s 10.190.227.123 --dport 8443 -j ACCEPT 

 sudo iptables -I INPUT 5 -p tcp --dport 22 -j REJECT sudo iptables -I INPUT 6 -p tcp --dport 8443 -j REJECT 

 delete existing chain line iptables -D INPUT 2  this will delete rule 2 

   

   

 to save the iptables service iptables save

Buzz noise audio - KDE
Change your working directory to /etc/modprobe.d: 

 cd /etc/modprobe.d 

 And now, create a new file named audio_disable_powersave.conf and open with the nano text editor using the given command: 

 sudo nano audio_disable_powersave.conf 

 And put the following lines in that file to turn off the power-saving setting in the sound card permanently: 

 options snd_hda_intel power_save=0 

 

 For a USB sound card , you can use snd_usb_audio : 

 options snd_usb_audio power_save=0 

 Now, save changes and exit the Nano text editor by pressing Ctrl+X keys. Reboot your system, and you can enjoy a noise-free workspace.

epomaker keyboard detected as apple
The Problem 

 Recently, my Keychron K8 keyboard has died (PCB 💀), so I started hunting for a new keyboard. Another Keychron was out of the question, so I decided to give Epomaker a try. I stumbled into a B-stock Epomaker Galaxy80 and decided to give it a shot. I am very happy with the feel of the keyboard, typing feels much better than the K8, at least to me. 

 However, I ran into a problem in linux (Fedora Workstation, although this is not distro-specific): I was unable to use the F-row (F1 - F12), so that Alt + F4 would not close the window, F11 would not trigger full screen etc. When I used the online keyboard testers , it would register keys/commands such as “Files”, “Calculator”, “Volume Up/Down” etc.: I searched the keyboard manual, thinking it must be some sort of an Fn lock being activated, but there does not seem to be a feature like that on my keyboard. Even worse, using the Fn key + one of the F-row keys was not triggering the actual F-row key either. I had no (useful) way to use my F-row keys! This issue has been reported with many other Epomaker keyboards, such as EP84, TH80, RT80, RT100 or GMK87, but also with other brands (including some Keychrons). The good news is, the fix should be the same for all of them! 

 The Solution 

 After some furious googling, I actually found the solution to the problem, which turns out to be on the system side rather than in the keyboard itself. Simply reconfiguring your HID Apple module fixes the issue. If you’re technically savvy and/or do not care about any further details, go use the module configuration and enjoy using your keyboard. 

 echo "options hid_apple fnmode=2" | sudo tee /etc/modprobe.d/hid_apple.conf

sudo dracut --regenerate-all --force

# reboot and enjoy your fully functioning keyboard!

 

 (courtesy of Reddit user Outrageous_Opinion84 and Joao-Peterson on GitHub) 

 The Solution Explained 

 Below, I would like to go into more details for several reasons: 

 

 

 Reddit is going to paywall some of the content , and other content might get authwalled - therefore the solution may not be readily available. 

 It took me some time to find the solution, so by writing this article, I’m hoping the more elaborate explanation will be more SEO-friendly than some of the existing answers scatted over the internet. 

 I am sure I will forget this solution next time I install a new system and writing it up will hopefully mean I will not need to re-figure it out. 

 I wanted to learn (and share) in a bit more detail what happens with the configuration files and how the fix works. 

 

 

 First, let’s take a look at how the Epomaker keyboard reports to the system when loaded up (the last but one row): 

 Now the problem is that the keyboard is being detected as an Apple wireless keyboard clone - an abomination of a keyboard discontinued in 2015 : 

 By default, the F-row keys on the Apple wireless keyboard emit the media commands, which is how those keypresses will get interpreted by the hid_apple kernel module. Luckily, the module offers configurable function modes ( fnmode ), as documented on Arch Wiki or Ubuntu Wiki . We are interested in the fnmode=2 option: 

 

 

 2 - normally function keys, switchable to media keys by holding Fn key (=auto on non-Apple keyboards) 

 

 

 You can temporarily test the parameter by sending the fnmode parameter to the kernel module via sysfs: (You likely need to do a sudo su first) echo 2 >> /sys/module/hid_apple/parameters/fnmode 

 Now your keyboard should work as God Epomaker intended! The problem is that this change will not be persisted after reboot. 

 The way to do it is through the modprobe.d configuration files, which allow to pass parameters to the modules when they are loaded during system boot. The following command will create the configuration file for the hid_apple module and pass the fnmode=2 parameter to it: echo "options hid_apple fnmode=2" | sudo tee /etc/modprobe.d/hid_apple.conf 

 Now to apply the changes to the initramfs (a mini-image with your kernel, including its modules), you need to run the dracut command to re-generate initramfs. sudo dracut --regenerate-all --force Note : Some distributions may require a different command to re-generate initramfs, such as sudo update-initramfs -u for Ubuntu or sudo mkinitcpio -P in Arch. 

 Finally, all you need to do is either reboot your machine, or reload the module: 

 sudo rmmod hdi_apple

modprobe hdi_apple

Installation

Reinstall Grub after installing windows
Boot from a Live Linux USB drive. You can use any Live Linux distribution for this, such as Ubuntu, Fedora, or Debian. 

 Once you have booted into the Live Linux environment, open a terminal. For example; Ctrl+Alt+T 

 Type the following command to list all the partitions on your system:

 sudo fdisk -l 

 

 Identify the partition where your Linux operating system is installed. Note the partition name, for example, /dev/sda1>/code>. 

 Mount the partition with the Linux operating system:

 sudo mount /dev/sdx1 /mnt 

 Replace /dev/ sdx1 with the actual partition name. 

 

 Mount other necessary partitions:

 sudo mount --bind /dev /mnt/dev 

 sudo mount --bind /proc /mnt/proc 

 sudo mount --bind /sys /mnt/sys 

 

 Chroot into the mounted Linux partition:

 sudo chroot /mnt 

 

 Install the Grub bootloader:

 sudo grub-install /dev/sdx 

 Again, replacing /dev/ sdx with the actual device name of your hard drive. If you have more than one hard drive, make sure to select the one where the Linux operating system is installed. 

 

 Update the Grub configuration:

 sudo update-grub 

 

 Exit the chroot environment:

 exit 

 

 Unmount the partitions:

 sudo umount /mnt/dev 

 sudo umount /mnt/proc 

 sudo umount /mnt/sys 

 sudo umount /mnt 

 

 Reboot your computer from the terminal.

 sudo reboot 

 

 

   

 

 My System 

 sudo fdisk -l [sudo] password for csr:   Disk /dev/sda: 465.76 GiB, 500107862016 bytes, 976773168 sectors Disk model: Samsung SSD 850   Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: 18F7B150-AD7B-4AC1-9864-0391968D7FE3 Device 

   

Start 

End 

Sectors 

Size Type /dev/sda1 

4096 

1232895 

1228800 

600M EFI System /dev/sda2 

1232896 

3330047 

2097152 

1G Linux filesystem /dev/sda3 

3330048 904714755 901384708 429.8G Linux filesystem /dev/sda4  904714756 976768064  72053309  34.4G Linux swap Disk /dev/nvme0n1: 953.87 GiB, 1024209543168 bytes, 2000409264 sectors Disk model: addlink M.2 PCIE G3x4 NVMe 

 Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: gpt Disk identifier: 753AD7B1-9E5B-4DC8-A1BF-4966F9CA2801 Device 

   

Start 

End 

Sectors 

Size Type /dev/nvme0n1p1 

2048 

206847 

204800 

100M EFI System /dev/nvme0n1p2 

206848 

239615 

32768 

16M Microsoft reserved /dev/nvme0n1p3 

239616 1999320142 1999080527 953.2G Microsoft basic data /dev/nvme0n1p4 1999321088 2000406527 

1085440 

530M Windows recovery environment Disk /dev/zram0: 8 GiB, 8589934592 bytes, 2097152 sectors Units: sectors of 1 * 4096 = 4096 bytes Sector size (logical/physical): 4096 bytes / 4096 bytes I/O size (minimum/optimal): 4096 bytes / 4096 bytes Disk /dev/sdb: 115.69 GiB, 124218507264 bytes, 242614272 sectors Disk model: Ultra Fit 

 Units: sectors of 1 * 512 = 512 bytes Sector size (logical/physical): 512 bytes / 512 bytes I/O size (minimum/optimal): 512 bytes / 512 bytes Disklabel type: dos Disk identifier: 0x0d320863 Device 

   Boot 

Start 

End 

Sectors 

Size Id Type /dev/sdb1  * 

2048 242548735 242546688 115.7G  7 HPFS/NTFS/exFAT /dev/sdb2 

242548736 242614271 

65536 

32M ef EFI (FAT-12/16/32) 

 mount | grep /dev/sda /dev/sda3 on / type btrfs (rw,relatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=400,subvol=/@) /dev/sda3 on /home type btrfs (rw,relatime,compress=zstd:1,ssd,discard=async,space_cache=v2,subvolid=399,subvol=/@home) /dev/sda2 on /boot type ext4 (rw,noatime) /dev/sda1 on /boot/efi type vfat (rw,noatime,fmask=0022,dmask=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro)